Arthropods vs. Cane Toads

Cane toads are toxic because their bodies are loaded with cardiac glycosides, deadly toxins that can stop a predator’s heart. Because the toads are non-native in Australia, the native Australian carnivores aren’t adapted to dealing with them. For some, this is very bad news: freshwater crocodiles, monitor lizards, and pythons have all experienced population declines since the introduction of cane toads in 1985 (Smith and Phillips 2006).


A cane toad. Photo by Sam Fraser-Smith, licensed under CC BY 4.0.

Not all is lost, however. It turns out that cardiac glycosides are only toxic to a very narrow group of animals: vertebrates. Predatory insects, arachnids, and other invertebrates have no trouble at all with the poison. Furthermore, a study published earlier this year (Cabrera-Guzmán et al. 2015) revealed that many are more than capable of tackling amphibian prey.

Cane toads start their lives as tadpoles, small and innocent, but plenty toxic enough to kill a hungry frog or fish. In Australia, some of their top predators are giant water bugs and water scorpions. Both are insects (not scorpions) that use tube-like mouthparts to inject acid and digestive enzymes into their prey, dissolving them from the inside out. When the tadpole’s innards are sufficiently liquefied, the insects slurp them up like an amphibian milkshake.


A water scorpion lying in wait for prey. Photo by N. Sloth, licensed under CC BY-NC 3.0.

A young dragonfly's mouthparts. Photo by Siga, licensed under CC BY-SA 3.0.

A young dragonfly’s mouthparts. Photo by Siga, licensed under CC BY-SA 3.0.

Dragonfly larvae are also aquatic and predatory, but instead of using acid, they have extendable mouthparts. These are built like the robotic arm on an automatic garbage truck, but less cumbersome and more of a surgical, spring-loaded instrument of death. Most dragonfly larvae eat other insects, like mosquito larvae, but the largest species can easily overpower a small fish or tadpole.

Diving beetles join in the fun. Their mouthparts are less exciting, more like a typical beetle’s with sharp, biting mandibles. What makes them special is speed: their bodies are constructed like those of sea turtles. Like sea turtles, diving beetles are hard-shelled, streamlined, and aquadynamic. Unlike sea turtles, diving beetles use this form to swim after tadpoles that aren’t quite fast enough to escape.

A diving beetle, waiting for tadpoles to swim by. Photo by N. Sloth, licensed under CC BY-NC 3.0.

A diving beetle, waiting for tadpoles to swim by. Photo by N. Sloth, licensed under CC BY-NC 3.0.

A diving beetle larva with tadpole prey. Photo by Gilles San Martin, licensed under CC BY-SA 2.0.

A diving beetle larva with tadpole prey. Photo by Gilles San Martin, licensed under CC BY-SA 2.0.

Diving beetle larvae are just as fierce, and they too have been observed feeding on cane toad tadpoles. Unlike their parents, larvae are long-bodied, with curved, needle-like jaws which they use to inject digestive enzymes into their prey (like the water bugs).

Any cane toad tadpoles that survive this massacre can metamorphose into toadlets, but until they reach their adult size (4-6 inches) they are still at the mercy of their invertebrate predators.

Experiments and observations in the field (Cabrera-Guzmán et al. 2015) have revealed that crayfish are efficient predators of eggs, tadpoles, toadlets, and even adult toads. Australia is home to 151 species of crayfish, including several of the largest species on earth. The spiny crayfish (Euastacus) in particular, some of which can grow to more than a foot in length, prey not only on toadlets but also on full-sized, adult cane toads.

A Lamington blue spiny crayfish (Euastacus sulcatus). Photo by Tatters, licensed under CC BY-SA 3.0.

A Lamington blue spiny crayfish (Euastacus sulcatus). Photo by Tatters, licensed under CC BY-SA 3.0.

There are, believe it or not, spiders that specialize in running out over the surface of the water to snatch aquatic insects, tadpoles, and small fish. They are the fishing spiders (large ones are sometimes called dock spiders). Experiments have shown that when fishing spiders inhabit a pond, up to 1 in every 4 tadpoles ultimately becomes spider food (Cabrera-Guzmán et al. 2015).

A fishing spider, ready for a meal. Photo by Patrick Coin, licensed under CC BY-NC-SA 2.0.

A fishing spider, ready for a meal. Photo by Patrick Coin, licensed under CC BY-NC-SA 2.0.

Finally, ants. In the cane toad’s native range of tropical Latin America, meat ants are a major predator. When a toad is attacked, it often stays still, relying on poison for protection. Ants take advantage of this strategy, swarming over the toad’s body and stinging it to death with poisons of their own. Meat ants (Iridomyrmex) and their relatives also live in Australia, and they have been seen dragging the dismembered remains of cane toads back to their nests.

Meat ants taking down a cicada nymph. Photo by jjron, licensed under GFDL 1.2.

Meat ants taking down a cicada nymph. Photo by jjron, licensed under GFDL 1.2.

I’m sorry to say giant centipedes did not make the list of cane toad predators in Australia, but I should mention that the Caribbean giant centipede (Scolopendra alternans) has been observed to prey on native cane toads (Carpenter and Gillingham 1984).

Whether bird-eating spiders, bat-snatching centipedes, or tadpole-chasing water bugs, invertebrates that prey on vertebrates are always fascinating. It’s more common than you might think! I’ll conclude by mentioning Epomis, an unusual genus of ground beetles. Both the beetle larvae and adults are specialist amphibian-eaters, and tackle frogs and toads many times their own size.

Epomis beetles attacking various European amphibians. Photos from Wizen and Gasith (2011), licensed under CC BY 3.0.

Epomis beetles attacking various European amphibians. Photos from Wizen and Gasith (2011), licensed under CC BY 3.0.

I won’t say any more, since Epomis expert Gil Wizen has already written a fantastic blog post about these beetles, complete with videos of predation in action! I encourage you to check it out here.


Cabrera-Guzman E., M.R. Crossland, and R. Shine. 2015. Invasive cane toads as prey for native arthropod predators in tropical Australia. Herpetological Monographs 29(1): 28-39.

Carpenter C.C. and J.C. Gillingham. 1984. Giant centipede (Scolopendra alternans) attacks marine toad (Bufo marinus). Caribbean Journal of Science 20: 71-72.

Smith J.G. and B.L. Phillips. 2006. Toxic tucker: the potential impact of cane toads on Australian reptiles. Pacific Conservation Biology 12(1): 40-49.

Wizen G. and A. Gasith. 2011. Predation of amphibians by carabid beetles of the genus Epomis found in the central coastal plain of Israel. ZooKeys 100: 181-191.

Crocodiles and Cane Toads

You can see them from a helicopter: the white, bloated bellies of dead crocodiles, limply floating down the Victoria River. Australian freshwater crocodiles live hard lives, and most hatchlings are quickly eaten by fish, herons, frogs, turtles, or adult crocodiles. By the time they reach adulthood, at more than 7 feet long, they’ve already proven themselves to be the toughest reptiles around, so finding dead ones didn’t used to be common. Starting in 2002 that began to change (Letnic et al. 2002). Bodies started turning up, floating on their backs, by the hundreds. In their stomachs, researchers found the culprit: cane toads.

Cane toads, an invasive species in Australia, are extremely toxic. Their skin and organs are filled with cardiac glycosides, molecules that induce heart failure. Pets that eat them often die. So do a few humans, who lick the toads hoping to experience the hallucinogenic effects* of toad poison.

A dead freshwater crocodile, after eating a cane toad. Photo by Adam Britton, used with permission.

A dead freshwater crocodile, after eating a cane toad. Photo by Adam Britton, used with permission.

The toads’ natural predators, to varying degrees, have evolved to handle toad poison (also called bufotoxin). Examples include certain army ants and the cat-eyed snakes, which eat the toads and their tadpoles with ease. Even outside the toads’ native range (tropical Latin America), predators are often able to tolerate them because they have already adapted to the toxins of their own local toads.

Australia has a special problem: the country has no native toads. None at all. Since the cane toads’ introduction, scientists have observed dramatic population declines in predatory reptiles, from monitor lizards to pythons to crocodiles. These reptiles are not adapted to living with toads: they don’t instinctively leave toads alone, and when they venture eat one, death by poison is often the result.

Australia is home to two crocodile species. The smaller of the two is the freshwater crocodile (Crocodylus johnstoni), which lives in ponds and the upper reaches of rivers, away from the northern coastline. At 7-10 feet in length, this species is not dangerous to humans unless provoked, instead subsisting on a diet of fish, amphibians, small mammals, and the like.

A freshwater crocodile. Photo by Richard Fisher, licensed under CC BY 2.0.

A freshwater crocodile. Photo by Richard Fisher, licensed under CC BY 2.0.

The larger is the saltwater crocodile (Crocodylus porosus), males of which are the largest crocodiles on earth, reaching lengths up to 20 feet. Although they often live alongside (and sometimes prey upon) freshwater crocodiles, saltwater crocodiles truly thrive in the more coastal habitats: estuaries, mangrove swamps, and sea-bound river deltas.

Both species are opportunists, and will happily snap up a toad if given the opportunity.

A saltwater crocodile. Photo by Lip Kee Yap, licensed under CC BY-SA 2.0.

A saltwater crocodile. Photo by Lip Kee Yap, licensed under CC BY-SA 2.0.

Dr.’s James Smith and Ben Phillips (2006) wanted to find out just how dangerous cane toads were to Australia’s native predators. They harvested the toxins from cane toads and then administered them to various Australian reptiles, including predatory lizards, pythons, and both crocodile species.

When scientists want to know how deadly a toxin is, they calculate LD50. The LD50, or median lethal dose, is simply the amount of poison that will on average cause the death of 50% of victims.

The LD50 depends both on the toxin and on the animal that ingests it. A rat, for example, has a 50% chance of death if it drinks 192 milligrams of caffeine for every kilogram that the rat weighs. Rats typically weigh about 1/3 of a kilogram, so the total LD50 for caffeine is 1/3 of 192, or 64 mg. More toxic substances have lower LD50’s, since it takes less poison to cause death. Caffeine isn’t that toxic. Aren’t numbers fun?

The mangrove monitor, a predator easily poisoned by cane toads. Photo by Jebulon, in public domain.

The mangrove monitor, a predator easily poisoned by cane toads. Photo by Jebulon, in public domain.

Smith and Phillips calculated that the LD50 for bufotoxin fed to freshwater crocodiles was about 2.76 milligrams. Cane toads, which can weigh up to 2 kilograms, are perfectly capable of killing freshwater crocodiles that eat them.

Here’s the odd thing: while freshwater crocodiles often died as a result of cane toad poisoning, none of the saltwater crocodiles did. To see if the poison was affecting them in other ways, the scientists conducted athletic tests — if the crocodile couldn’t run as fast after poisoning, that was interpreted as a sign the poison was harming the reptile. While the freshwater crocodiles slowed down after ingesting bufotoxin, saltwater crocodiles were just as energetic before as after their toxic meal.

Are saltwater crocodiles immune to bufotoxin? It’s hard to say. The scientists wanted to kill as few crocodiles as possible, and they didn’t have enough crocodiles on hand to test much higher doses. Perhaps extremely high doses of bufotoxin would kill saltwater crocodiles, but the data is lacking.

What we do know is that saltwater crocodiles are much more resistant to cane toad poison than freshwater crocodiles. There are two potential reasons for this, and the most obvious is size. Saltwater crocodiles, males of which can weigh more than 2,000 pounds, are the largest crocodilians and the largest non-marine predators in the world. An adult saltwater crocodile simply cannot eat a toad large enough to reach a lethal dose.

A saltwater crocodile. Photo by fvanrenterghem, licensed under CC BY-SA 2.0.

A saltwater crocodile. Photo by fvanrenterghem, licensed under CC BY-SA 2.0.

Smaller crocodiles are more vulnerable. In 2013, an expedition to remote areas of northern Australia revealed that some populations of pygmy freshwater crocodiles, which only grow to 5 feet, have suffered declines upwards of 60% due to toad poisoning (Britton et al. 2013). The same research team, led by Dr. Adam Britton, is trying to raise money with a crowd-funding campaign to return to these remote sites, to study and help protect pygmy crocodiles. I strongly encourage you to visit the crowd-funding site here, as Britton has prepared a terrific video on pygmy crocodiles and the unique challenges they face.

A pygmy freshwater crocodile. Photo via Adam Britton, used with permission.

An adult pygmy freshwater crocodile. Photo by Adam Britton, used with permission.

The saltwater crocodiles in the Smith and Phillips study were not even close to 2,000 pounds — they were subadults, less than three feet long and closer to five pounds. So a few milligrams of cane toad poison should have killed at least some of them. Instead the walked away un-fazed, without so much as a skip in their gait.

Why? It may have to with the two crocodiles’ evolutionary history. In addition to being the largest, saltwater crocodiles are some of the widest-ranging** crocodiles, distributed from eastern India through Southeast Asia, Indonesia, and New Guinea. Because they can live in saltwater, they have been able to colonize many Pacific Islands (e.g., the Solomons) that are out of reach of other crocodilians.

A cane toad. Photo by Sam Fraser-Smith, licensed under CC BY 4.0.

A cane toad. Photo by Sam Fraser-Smith, licensed under CC BY 4.0.

Throughout their range they encounter a tremendous variety of potential prey. Saltwater crocodiles are not picky eaters, and have been observed feeding on fish (including sharks), frogs, lizards, snakes, turtles, crabs, snails, octopuses (during marine forays), deer, monkeys, pigs, cows, rats, otters, rabbits, porcupines, kangaroos, squirrels, wild cats, jackals, emus, geese, miscellaneous birds, and bats that fly just a little too close to the water.

Also, toads.

Even though Australian crocodiles never encounter toads, they have almost exactly the same DNA as their relatives in Asia and Indonesia. Perhaps they have inherited a tolerance for bufotoxin, while the freshwater crocodile, alone and isolated in Australia, has not.

Freshwater crocodiles might seem like the evolutionary dopes in this story, but there is hope for them. While some populations have been hit hard, others appear to be unaffected, perhaps because cane toads tend to avoid the habitats where freshwater crocodiles do most of their hunting (Somaweera et al. 2012). Research (like the pygmy crocodile project) is continuing to shed light on where cane toads are affecting crocodiles the most, why, and what can be done to protect them.

Finally, crocodilians are more intelligent than most reptiles. Studies with captive specimens have shown that after just a few encounters, hatchling freshwater crocodiles are able to quickly learn to avoid cane toads. Back in the field, some populations of crocodiles are already showing signs of learning, as cane toads are attacked less often and less enthusiastically than native frogs (Somaweera et al. 2011). As with humans, the best hope for freshwater crocodiles is in the next generation.

A young freshwater crocodile. Photo by Mike Peel, licensed under CC BY-SA 4.0.

A young freshwater crocodile. Photo by Mike Peel, licensed under CC BY-SA 4.0.

*Don’t even think about it.

**Saltwater crocodiles, while secure in Australia, are endangered in Southeast Asia, where many populations have gone extinct.


Britton A.R.C., E.K. Britton, and C.R. McMahon. 2013. Impact of a toxic invasive species on freshwater crocodile (Crocodylus johnstoni) populations in upstream escarpments. Wildlife Research 40: 312-317.

Letnic M., J.K. Webb, and R. Shine. 2008. Invasive cane toads (Bufo marinus) cause mass mortality of freshwater crocodiles (Crocodylus johnstoni) in tropical Australia. Biological Conservation 141: 1773-1782.

Smith J.G. and B.L. Phillips. 2006. Toxic tucker: the potential impact of cane toads on Australian reptiles. Pacific Conservation Biology 12(1): 40-49.

Somaweera R., J.K. Webb, G.P. Brown, and R. Shine. 2011. Hatchling Australian freshwater crocodiles rapidly learn to avoid toxic invasive cane toads. Behaviour 148(4): 501-517.

Somaweera R., R. Shine, J. Webb, T. Dempster, and M. Letnic. 2012. Why does vulnerability to toxic invasive cane toads vary among populations of Australian freshwater crocodiles? Animal Conservation 16(1): 86-96.

Endangered, Bird-eating Centipedes of Mauritius

Can a centipede really be endangered? Of course!

Centipedes don’t get much love, even from each other. They are solitary, irritable, fiercely cannibalistic, and arguably some of the most widely hated animals on earth. I know many biologists who would gladly handle a snake or tarantula, but shudder at the thought of a giant centipede creeping up their arm.

An Indopacific centipede, making good use of a hole in the wall. Photo by Thomas Brown, licensed under CC BY 2.0.

An Indopacific centipede, making good use of a hole in the wall. Photo by Thomas Brown, licensed under CC BY 2.0.

I never begrudge people for being scared of centipedes. They are objectively frightening: many-legged, venomous, fast-moving, and secretive. In the rural tropics, a painful bite from a giant centipede is a very real possibility. But none of this means they can’t be endangered, put at risk of extinction either by natural circumstance or by human activity.

Unsurprisingly, very few centipedes have ever been studied from a conservation-oriented perspective. Most of the time, there simply isn’t the funding, public interest, or lack of squeamishness to make that kind of research happen. There are, however, exceptions. Today I’m going to tell you about one: the giant centipedes of Mauritius and Rodrigues.

Mauritius, Rodrigues, and their satellites form a collection of tiny islands in the Indian Ocean, just a few thousand miles east of Madagascar. Like most islands they have a long, sad history of extinctions wrought by over-hunting, invasive species, and habitat destruction. The dodo bird, native to Mauritius, was one of the first victims.


Mauritius, in panoramic view. Photo by Clément Larher, licensed under CC BY-SA 3.0.

The two main islands are home to two species of giant centipede, the blue-legged (Scolopendra morsitans) and the Indopacfic (Scolopendra subspinipes) centipedes*. Both species are incredibly efficient predators, and with body lengths of 8 inches or more, they are more than capable of tackling large prey such as mice. On Mauritius, staple fare include house geckos and cockroaches, but they also take day-old chicks from their nests when opportunity strikes (Lewis et al. 2010). The Indopacific centipede can even swim, undulating side-to-side while holding its head above the surface like a crocodile (Lewis 1980).

Despite their size, venom, and general badassness, giant centipedes are prey for many larger animals. On Mauritius, they form 80% of the diet of feral cats that roam the island by night. The cats are apparently nimble (and daring) enough to tear apart the centipedes without getting bitten.

An Indopacific centipede from China. Photo by Thomas Brown, licensed under CC BY 2.0.

An Indopacific centipede from China. Photo by Thomas Brown, licensed under CC BY 2.0.

Even in the face of predation by cats, giant centipedes remained abundant until 1997, when a new invasive species came into the picture. That species was the musk shrew (Suncus murinus), introduced from India. A smaller shrew might become prey for a centipede, but the musk shrew is the largest in the world, reaching a length of 6 inches or more.

An 8-inch-long centipede is still a formidable adversary, but the shrews were used to encountering giant centipedes in their native range (as it happens, the Indopacific centipede also lives in India). They have made short work of centipede populations, which are now greatly reduced (Lewis et al. 2010). The Indopacific centipede is now found on Rodrigues, but no longer on Mauritius, while the blue-legged centipede is still found on both islands.

Mauritius and its satellite islands. From Lewis et al. (2010), licensed under CC BY 4.0.

Mauritius and its satellite islands. From Lewis et al. (2010), licensed under CC BY 4.0.

I am not about to launch into a passionate defense of blue-legged and Indopacific centipedes. As I said before, both species are abundant in tropical habitats all over the world, from Indonesia to the Caribbean. For all we know the centipedes themselves are invasive, dancing with cats and shrews on the graves of long-gone native species. Instead this article is about another giant, a third centipede, gone from Mauritius but still clinging to life on Serpent Island.

Serpent Island is a satellite of Mauritius, uninhabited by humans and with an area less than 100 acres. There is very little vegetation or soil there, and bare rock dominates the surface. In the absence of humans or large predators, sea birds thrive, especially sooty terns which nest by the thousands on open ground.

They share the space with centipedes — not Indopacific or blue-legged, but Serpent Island giant centipedes (Scolopendra abnormis), which are found on one other satellite island (Round Island) and nowhere else on earth — not even Mauritius. The centipedes are abundant on Serpent Island, with roughly 12 individuals per square meter. If centipedes frighten you, don’t plan your next vacation here.

During the day centipedes hide beneath rocky slabs and underground, away from the light and from watchful, easily enraged mother birds. Terns are active during the day, flying from land to sea and back again, gathering fish for their hungry chicks. With all the traffic, a centipede is better off staying out of sight.

A sooty tern. Photo by Duncan Wright, in public domain.

A sooty tern. Photo by Duncan Wright, in public domain.

By night the terns are less wary. Snakes, which would normally prey on tern chicks, are absent from the island, probably driven out soon after the arrival of European explorers. Without the competition, centipedes have risen to take their place. Wandering over the rocks, a centipede uses smell and touch to locate a nest, grab hold of a chick, and sink in its venom-laden fangs. More than any so-called bird-eating tarantula, the Serpent Island centipede is a true bird-eater. In captivity, they can survive for several years on a diet of chick legs (Lewis et al. 2010).

The taste for bird meat is probably a recent acquisition — Serpent Island centipedes most likely colonized the island only a few million years ago. They would have arrived from Mauritius, suggesting the larger island had a population of Serpent Island centipedes before they were driven to extinction by the introduced shrews, cats, and perhaps larger centipedes.

The Serpent Island centipede is classified as Vulnerable by the International Union for Conservation of Nature (IUCN 2012). This means the species is  “considered to be facing a high risk of extinction in the wild.” It is one of 10 potentially threatened centipedes on the IUCN Red List (of 3,300 total centipede species worldwide). So far, none have been given legal protection.

Centipede snacks. Photo by Duncan Wright, in public domain.

Centipede food. Photo by Duncan Wright, in public domain.

The bad news is that, if shrews or cats or rats were to be introduced to Serpent Island, the entire ecosystem would collapse. Invasive predators would quickly eat both chicks and centipedes, leaving Serpent Island a bare rock in the middle of the ocean, with a few tufts of grass and the occasional cockroach.

The good news is that centipedes are abundant in their last remaining habitats, with an estimated population of 10-15,000. Serpent Island is remote and protected, and biologists are pretty much the only visitors, so it is unlikely shrews will ever get there. The future of Serpent Island’s bird-eating centipedes is secure, for now.

Reminder: there are still 6 days left to donate to Dr. Adam Britton’s crowdfunding campaign to study threatened pygmy crocodiles in Australia! I’ve donated, and I encourage you to so if you think pygmy crocodiles, which you can read about here, are awesome, which of course they are. There are some amazing prizes for donors, including crocodile-themed artwork and jewelry!

*These species normally go by the common names Tanzanian giant (blue-legged) and Vietnamese giant (Indopacific). However, both are extremely wide-ranging in tropical habitats all over the world, including Hawaii where they have been introduced by humans (Shelley et al. 2014). To reduce confusion I used alternative common names.


IUCN. 2012. IUCN Red List Categories and Criteria: Version 3.1. Second edition. Gland, Switzerland and Cambridge, UK: IUCN. iv + 32pp.

Lewis J.G.E., P. Daszak, C.G. Jones, J.D. Cottingham, E.Wenman, and A. Maljkovic. 2010. Field observations on three scolopendrid centipedes from Mauritius and Rodrigues (Indian Ocean) (Chilopoda: Scolopendromorpha). International Journal of Myriapodology 3: 123-137.

Lewis J.G.E. 1980. Swimming in the centipede Scolopendra subspinipes Leach (Chilopoda, Scolopendromorpha). Entomologists Monthly Magazine 116: 219-220.

Shelley R.M., W.D. Perreira, and D.A. Yee. 2014. The centipede Scolopendra morsitans L., 1758, new to the Hawaiian fauna, and potential representatives of the “S. subspinipes Leach, 1815, complex” (Scolopendromorpha: Scolopendridae: Scolopendrinae). Insecta Mundi 338: 1-4.

Scaly and Adorable: Australia’s Pygmy Crocodiles

Yesterday I wrote about crocodile evolution, and some of their amazing extinct relatives (here). I wrote about them partly because prehistoric crocodylomorphs are amazing, and that’s as good a reason as any. But it was also to prove a point: modern crocodilians, 23 species all with similar appearances, might seem like ancient members of a group that has hardly changed at all. This is not so. Crocodilians are instead the only survivors of a vast and hugely diverse lineage of animals, most of which looked nothing like the crocodilians alive today.

Like the finned sea-crocodiles of the Jurassic, modern crocodilians are an off-shoot, just one branch in a massive crocodylomorph tree. Unlike the sea-crocodiles, by some combination of chance and adaptation, modern crocodilians have managed to avoid extinction (so far). Unlike the sea-crocodiles, crocodilians are still evolving.


A Nile crocodile (Crocodylus niloticus). Photo by Gianfranco Gori, licensed under CC BY-SA 4.0.

Dr. Adam Britton, a world-renowned crocodilian biologist, points out that his favorite animals are far more advanced than they look. “I do see them as highly refined survivors of their ancient lineage. The analogy I use when talking about croc evolution is to compare modern crocodiles to Ferraris: they might superficially look and function similarly to a Model T Ford, but they are so much more refined.”

One of Dr. Britton’s favorite species is the Australian freshwater crocodile (Crocodylus johnstoni), the smaller of Australia’s two native crocodiles. Freshwater crocodiles typically reach 7-10 feet in length — impressive, but dwarfed by the saltwater crocodile (Crocodylus porosus), males of which can grow to over 20 feet. The two species share Australia, but because saltwater crocodiles are larger and fiercely territorial, freshwater crocodiles are often relegated to sub-optimal habitats, such as smaller rivers and ponds.

A young freshwater crocodile. Photo by Mike Peel, licensed under CC BY-SA 4.0.

A young freshwater crocodile. Photo by Mike Peel, licensed under CC BY-SA 4.0.

Such habitats include the remote, rocky upstream gorges of the Victoria and Liverpool Rivers (Webb 1985). Here, and at a few other sites in northern Australia, unique freshwater crocodiles live in relative isolation from humans and saltwater crocodiles. Unfortunately, they also live without much food — small streams mean few fish, which make for malnourished crocodiles.

So they evolved. Over time, the crocodiles became smaller to make up for a poor diet, and now they are truly tiny, with the largest reaching 5 feet in length. They became the pygmy crocodiles, small enough that you could (unadvisedly) pick one up and carry it around with you.

A pygmy freshwater crocodile. Photo via Adam Britton, used with permission.

A pygmy freshwater crocodile. Photo by Adam Britton, used with permission.

Stunted growth is one thing — any crocodile, fed a poor diet, will fail to reach its maximum size potential. Pygmy crocodiles are different. They have been growing this way for enough time that they are now genetically predisposed to small size. If you took a pygmy croc from the wild and fed it the same diet as a normal freshwater crocodile, the former would still be much smaller than its cousin.

A pygmy crocodile. Photo by Adam Britton, used with permission.

A pygmy crocodile. Photo by Adam Britton, used with permission.

Pygmy crocodiles aren’t quite distinct enough to be classified as their own species — yet. They haven’t been isolated for very long, so for now they are still considered an unusual population of freshwater crocodiles. They may interbreed with larger crocodiles, in which case they will never fully separate. A more exciting possibility is that they may continue to evolve and diverge in isolation, in which case they may someday become genetically unique enough to constitute a new species.

There is another possibility: pygmy crocodiles may go extinct before they get a chance to evolve any further. Although their habitat is isolated and relatively secure, they are threatened by invasive cane toads, introduced to Australia in 1935. A hungry crocodile will happily snap up a toad, but because cane toads are extremely toxic, they are often the crocodile’s last meal.

Curiosity killed this crocodile -- it tried to eat a poisonous cane toad. Photo by Adam Britton, used with permission.

Curiosity killed this crocodile — it tried to eat a poisonous cane toad. Photo by Adam Britton, used with permission.

Pygmy crocodiles, because they are so small, are especially vulnerable to the poison. Although some populations have been unaffected (Doody et al. 2014). others have declined in abundance by more than 60% since the toads’ introduction (Britton et al. 2013). Why some populations are more vulnerable than others is one of many crucial questions that remain unanswered (Somaweera et al. 2012).

A cane toad. Photo by Sam Fraser-Smith, licensed under CC BY 4.0.

A cane toad. Photo by Sam Fraser-Smith, licensed under CC BY 4.0.

Dr. Britton is leading an effort to study pygmy crocodiles in their natural habitat. The goals of his research are two-fold. First, he means to assess their wild populations to determine if the crocodiles might be endangered. Second, Britton and his team wish to collect DNA from the pygmy crocodiles, to better understand their evolutionary history, and their genetic relationship with larger freshwater crocodiles.

This is an achievable and worthy project, but an ambitious one. Field work is always costly, but pygmy crocodiles live in isolated, hard-to-reach places, and getting there requires use of a helicopter. Dr. Britton and his team have started a crowd-funding effort to raise funds to support pygmy crocodile research — I’ve donated, and if you think pygmy crocodiles are amazing, I strongly encourage you to do so as well. There are some great prizes for donors, including crocodile-themed artwork and jewelry!

You can learn more about the project at its crowd-funding site, here. On the website is a short video in which Dr. Britton discusses and handles pygmy crocodiles. They are positively adorable.*

If crocodiles are Ferraris, then pygmy crocodiles are Smart Cars — tiny and vulnerable, but awesome in an enticingly bizarre sort of way. Pygmy crocodiles are an evolutionary quirk, just like the prehistoric pelican-snouted crocodile Stomatosuchus, or the armadillo-backed Armadillosuchus, or the shark-tailed … you get the picture. There’s just one important difference: pygmy crocodiles are alive. We, as residents of a special time in the history of life, get to appreciate them for the amazing creatures that they are. Let’s try and keep it that way.

*The crocodiles, I mean. Although if you like listening to British scientists get super-duper excited about wildlife, it’ll be a happy three minutes for you.

A big thank you is owed to Dr. Britton, who graciously allowed me to use his images for this article. Once again, I encourage you to donate to his effort to study these amazing crocodiles. You can learn more about crocodilians at Dr. Britton’s encyclopedic and lavishly illustrated website here.


Britton A.R.C., E.K. Britton, and C.R. McMahon. 2013. Impact of a toxic invasive species on freshwater crocodile (Crocodylus johnstoni) populations in upstream escarpments. Wildlife Research 40: 312-317.

Doody J.S., P. Mayes, S. Clulow, D. Rhind, B. Green, C.M. Castellano, D. D’Amore, and C. Mchenry. 2014. Impacts of the invasive cane toad on aquatic reptiles in a highly modified ecosystem: the importance of replicating impact studies. Biological Invasions 16(11): 2303-2309.

Somaweera R., R. Shine, J. Webb, T. Dempster, and M. Letnic. 2012. Why does vulnerability to toxic invasive cane toads vary among populations of Australian freshwater crocodiles? Animal Conservation 16(1): 86-96.

Webb G.J.W. 1985. Survey of a pristine population of freshwater crocodiles in the Liverpool River, Arnhem Land, Australia. National Geographic Society Research Report 1979: 841-852

The Pelican, the Shark, and the Armadillo: Crocodile Evolution Part 1

Crocodiles are very TV-genic – scores of documentaries have focused on them, and most start off with a variant of the following: “Crocodilians are some of the most successful predators on the planet. They survived the dinosaurs, and have remained unchanged for millions of years.”


An American crocodile (Crocodylus acutus). Photo by Tomas Castelazo, licensed under CC BY-SA 2.5.

Today I wanted to write about why that’s not entirely true, and why crocodilian evolution, far from monotonous, is instead dynamic and, like crocodilians themselves, full of surprises.

There are only 23 species of non-extinct crocodilians, compared to 327 turtles and tortoises and more than 9,000 lizards and snakes. The fossil record, however, is full of crocodile relatives, the crocodylomorphs, dating back to at least 225 million years ago (Bronzati et al. 2012; Russel and Wu 1998).

The first crocodylomorphs looked nothing like the crocodiles alive today. Instead they were small, nimble creatures that ran on four long legs. Their sharp teeth reveal a predatory lifestyle, but instead of waiting in ambush at the water’s edge, they were adapted for wandering about over land in search of prey.

These animals, the sphenosuchians, appeared during the Late Triassic period when dinosaurs were just beginning to evolve. With legs pointed down under the body (rather than splayed to the sides), the sphenosuchians were probably terrific at chasing down prey. One such reptile, Hesperosuchus, was about the size of a domestic dog.


Hesperosuchus. Illustration by Nobu Tamura, licensed under CC BY 2.5.

As the dinosaurs began to diversify, so did the crocodylomorphs. Although many stayed small, some got larger, and began to experiment with different lifestyles. I say “experiment” euphemistically, but evolution is blind — circumstances and natural selection simply allowed crocodylomorphs to fill a wider range of niches. There were large, land-dwelling predators, like Protosuchus:


Protosuchus. Illustration by Nobu Tamura, licensed under CC BY 2.5.

There were also marine crocodylomorphs, the thalattosuchians. These, too, were predatory and looked vaguely like modern crocodilians in that they had long jaws, a muscular tails, and laterally flattened bodies (Young et al. 2014). Here, from Europe, are three species of Machimosaurus:


Machimosaurus species. Illustration by M.T. Young et al. (2014), licensed under CC BY 4.0.

Others looked nothing like any modern animals, and had fin-like feet and tails. Dakosaurus, also from Jurassic Europe (Young et al. 2012), had such an un-crocodile-like skull that their fossils were at first mistakenly attributed to dinosaurs.


Dakosaurus maximus. Illustration by M.T. Young et al. (2012), licensed under CC BY 2.5.

Most of the sea crocodiles lived during the Jurassic Period, when dinosaurs on land were becoming truly massive (e.g., spike-tailed Stegosaurus and long-necked Brontosaurus*). Despite an uncanny resemblance, sea crocodiles did not “evolve into” modern crocodiles. Instead they represent an off-shoot of crocodylomorph evolution, a kind of “dead-end” with no modern descendants.

There’s plenty more to say about fossil crocodylomorphs in the Jurassic, but we’ll skip to the Cretaceous, the last period of non-bird dinosaurs with Tyrannosaurus, Triceratops, and the like. Crocodiles, too, were reaching a magnificent crescendo with on of their most diverse lineages, the Notosuchia.

The Cretaceous saw crocodylomorphs occupying all sorts of unusual ecological roles. There were, of course, the usual land-dwelling predators like Baurusuchus:


Baurusuchus. Illustration by Nobu Tamura, licensed under CC BY 2.5.

Others had turned over a new leaf, so to speak, and adopted a life of plant-eating. Simosuchus, one such herbivore (or omnivore — see Buckley et al. 2000), hardly looked like anything you might call a crocodile:


Simosuchus clarki. Illustration by Smokeybjb, licensed under CC BY-SA 3.0.

There were still weirder crocodylomorphs. Armadillosuchus had armor plates over its back — it looked, and quite possibly behaved, very much like a modern armadillo, only 6 feet long and with sharp, fang-like teeth.


Armadillosuchus arrudai. Illustration by Smokeybjb, licensed under CC BY-SA 3.0.

Meanwhile, some of the predatory crocodylomorphs had returned to the water, where they began to take full advantage of the lifestyle that would someday become the trademark of crocodilians. Many could be easily mistaken for true crocodiles — they had long snouts, flattened bodies, and spent their time ambushing visitors to the water’s edge. Some were colossal, and scientists have found evidence that some of the largest species (Sarcosuchus and Deinosuchus) preyed on dinosaurs (Boyd et al. 2013), just as Nile crocodiles today pick off unwary wildebeest and zebras.

Size comparison of two extinct and three living crocodylomorphs. Figure by Matt Martyniuk, licensed under CC BY-SA 3.0.

Size comparison of two extinct and three living crocodylomorphs. Figure by Matt Martyniuk, licensed under CC BY-SA 3.0.

Modern-day crocodilians belong to the Neosuchia (“new crocodiles”). This group also evolved in the Mesozoic and includes such strange beasts as Stomatosuchus, a 30-foot-long behemoth with a broad, spoon-like snout. Paleontologists aren’t sure exactly what it ate, but some suspect the lower jaw supported a pouch-like membrane for gulping fish, just like a pelican (Nopsca 1926).


Stomatosuchus inermis. Photo by Dmitri Bogdanov, licensed under CC BY 3.0.

Neither Sarcosuchus nor Stomatosuchus were “true” crocodilians, but crocodylians start to appear in the fossil record at around the same time. The oldest crocodilian fossils date back around 80 million years, during the Late Cretaceous (Russel and Wu 1998).

Just 15 million more years — a blink of an eye, really — until Earth’s most recent apocalypse. The same mass extinction that wiped out the dinosaurs (except birds), likely caused by a meteor, also wiped out most of the crocodylomorphs. Perhaps owing to their aquatic habits and (relatively) small size, crocodilians were one of the few survivors. A few other groups, like as the sea-faring dyrosaurids, lingered into the so-called Age of Mammals, but these too gradually fell away until only the true crocodiles, alligators, caimans and the gharial remained.

Guarinisuchus munizi, one of the few marine crocodylomorphs after the K-T extinction event. Illustration by Nobu Tamura, licensed under CC BY 3.0.

Guarinisuchus munizi, one of the few marine crocodylomorphs after the K-T extinction event. Illustration by Nobu Tamura, licensed under CC BY 3.0.

So we arrive at the present day, with 23 living species of crocodilians. Those 23 are the survivors survivors, here today because they are adaptable, tough, and a little bit lucky. They bear witness to an ancient lineage of incredibly diverse, versatile, and often bizarre animals, most of which have died. They are the ones that lived through mass extinctions, competition with dinosaurs, and climate change. They may even survive us.


A Nile crocodile (Crocodylus niloticus). Photo by Gianfranco Gori, licensed under CC BY-SA 4.0.

*Brontosaurus may or may not be the same as Apatosaurus. If they are the same, then Brontosaurus is not a valid name. I’m not a paleontologist, so I have no opinion on the matter — I used the name Brontosaurus here since I think it is more recognizable, although I may be wrong about that, too. To learn more, check out this article by zoologist and writer Darren Naish.

I’ll be writing more about crocodilian evolution tomorrow, this time with an amazing species that is alive today. If you want to read more about extinct crocodylomorphs, read this article, also by Darren Naish. If this kind of stuff interests you, I recommend following his blog Tetrapod Zoology at the same link.

Finally, if you are interested in a more technical review of crocodylomorph evolution, some good papers to read are Bronzati et al. (2012) and Russel and Wu (1998), cited below.


Boyd C.A., S.K. Drumheller, and T.A. Gates. 2013. Crocodyliform feeding traces on juvenile ornithischian dinosaurs from the Upper Cretaceous (Campanian) Kaiparowits Formation, Utah. PLOS ONE 8(2): e57605. doi: 10.1371/journal.pone.0057605

Bronzati M., F.C. Montefeltro, and M.C. Langer. 2012. A species-level supertree of Crocodyliformes. Historical Biology 24(6): 598-606.

Buckley G.A., C.A. Brochu, D.W. Krause, and D. Pol. 2000. A pug-nosed crocodyliform from the Late Cretaceous of Madagascar. Nature 405: 941-944.

Nopcsa F. 1926. Neue Beobachtungen an Stomatosuchus. Centralbl. Min. Geol. Palaontol, B212-215.

Russel A. and X.-C. Wu. 1998. The crocodylomorpha at and between geological boundaries: the Baden-Powell approach to change? Zoology 100(3): 164-182.

Young M.T., S. Hua, L. Steel, D. Foffa, S.L. Brusatte, S. Thüring, O. Mateus, J.I. Ruiz-Omeñaca, P. Havlik, T. Lepage, and M. Brandalise de Andrade. 2014. Revision of the Late Jurassic teleosaurid genus Machimosaurus (Crocodylomorpha, Thalattosuchia). Royal Society Open Science doi: 10.1098/rsos.140222

Young M.T., S.L. Brusatte, Marco Brandalise de Andrade, J.B. Desojo, B.L. Beatty, L. Steel, M.S. Fernández, M. Sakamoto, J.I. Ruiz-Omeñaca, and R.R. Schoch. 2012. The cranial osteology and feeding ecology of the mentriorhynchic crocodylomorph genera Dakosaurus and Plesiosuchus from the Late Jurassic of Europe. PLOS ONE 7(9): e44985. doi: 10.1371/journal.pone.0044985


Nicotine, a Natural Insecticide

Nicotine, the addictive agent in cigarettes, comes from the leaves of tobacco plants (Nicotiana). Plants, of course, do not manufacture nicotine as a favor to smokers, but for their own benefit: nicotine is one of the most powerful insecticides in the world, highly effective at stopping hungry, leaf-munching pests in their six-footed tracks.


Tobacco flowers and leaves. Photo by Joachim Mullerchen, licensed under CC BY 2.5.

Nicotine is a neurotoxin, and to understand how it works, you’ll need to understand a few things about the nervous system. Nerves are simply long, thin cells that run through your body, carrying signals as electrical impulses. Here’s the problem: at the junction between two nerve cells, or between a nerve and a muscle cell, there’s a space across which electrical impulses cannot travel. This space is called the synapse.

To keep the message going, the signal-sending nerve cell sends out an army of molecules called neurotransmitters, who boldly drift across the synapse like astronauts from space-ship to satellite. When a neurotransmitter arrives at the receiving nerve or muscle cell, it enters through a tube-shaped receptor molecule.

There are many kinds of neurotransmitters, each with its own unique receptor. Take acetylcholine, which carries signals from nerve cells to muscle cells. When you recoil from a hot stove, it’s acetylcholine that tells your muscles to get moving.


A synapse between two nerve cells. Figure in public domain.

If a poison kept all your acetylcholine receptors closed, you would be paralyzed: your muscles wouldn’t get any signals from the nervous system. If, on the other hand, the toxin kept receptors constantly open, your muscles would be constantly trying to move. You would go into convulsions, unable to control your body. Eventually you would exhaust yourself and die.

That’s how nicotine works (Zevin et al. 1998). In small doses, like in a cigarette, it works as a mild stimulant, keeping a few more receptors open than usual. In massive doses, like when a caterpillar eats a tobacco leaf, it works like a doorstop, keeping all acetylcholine receptors wide open. The unfortunate insect convulses, contracting all its muscles simultaneously until it runs out of energy and expires.

For many years, farmers used nicotine as a pesticide, spraying it over their crops to poison any hungry insects. However, nicotine is quite toxic to mammals, including humans. It isn’t sold as a pesticide anymore in the U.S. or Europe, replaced by neonicotinoids. The new pesticides are similar to nicotine, highly effective, and work in the same way, but are safer for people (not insects, of course).


The tobacco hornworm. Photo by Tom Murray, used with permission.

There are insects that eat tobacco leaves, and those insects have acquired an immunity to nicotine. The best-known example is the tobacco hornworm (Manduca sexta), a big, fat, bright green caterpillar that ultimately transforms into a hawkmoth. In fact, these caterpillars have evolved the ability to store nicotine in their own bodies, making themselves toxic to caterpillar-eating predators. Experiments have shown that wolf spiders normally avoid tobacco hornworms, but if the caterpillars are fed a diet lacking in nicotine, the spiders attack without hesitation (Kumar et al. 2013).


Kumar P., S.S. Pandit, A. Steppuhn, and I.T. Baldwin. 2013. Natural history-driven, plant-mediated RNAi-based study reveals CYP6B46’s role in a nicotine-mediated antipredator herbivore defense. Proceedings of the National Academy of Sciences U.S.A. 111(4): 1245-1252.

Zevin S., S.G. Gourlay, and N.L. Benowitz. 1998. Clinical pharmacology of nicotine. Clinics in Dermatology 16(5): 557-564.

Malaria, Climate Change, and the Next Top Model

by Joseph DeSisto

Both malaria and climate change are complex global problems that scientists are working hard to understand. Malaria, a mosquito-borne disease, kills roughly 600,000 people every year, mostly in Africa. Attempts to control malaria have been massive and relatively well-funded, but the disease continues to pose a serious threat to human life in the tropics. Climate change has no body count (yet), but will dramatically change our planet, altering weather patterns, sea levels, and human life.

These two problem have another thing in common: scientists use mathematical models to try and predict how they will change in the future. For example, climate scientists use past and current data on temperature, carbon emissions, and many other factors, to predict how the earth’s climate will change. Most models anticipate a global temperature increase of 2̊ to 6̊  by the year 2100 (e.g., Paaijmans et al. 2014). Those predictions may not be perfectly accurate, but that is the nature of predictions, and in the absence of time travel, it’s the best we can do.


Anopheles minimus, a malaria-transmitting mosquito. Photo by James Gathany, in public domain.

Malaria, the microbes that cause it, and the mosquitos that carry it, are all affected by temperature. Scientists have tried developing models to predict how malaria infection rates will change as the world gets hotter. In general those predictions have been bleak: models tend to show an increase in the amount of people affected by malaria (e.g. Pascual et al. 2006, Paaijmans et al. 2014), on the basis that warmer temperatures are most hospitable to malaria-carrying mosquitos. They also show malaria spreading to areas that used to be too cool, like South Africa and the southeastern United States. Yet other models show a decrease in malaria on a global scale (Gething et al. 2010), and some suggest an increase in some areas but a decrease in others (Rogers and Randolph 2000).

Before looking forward, it’s important to look back – how has malaria changed in the last few years? The maps below show malaria infection rates (the percentage of people infected) across Africa, in 2000 (left) and 2010 (right). Darker colors mean more malaria.



Photo by Noor et al. (2014), licensed under CC BY 3.0.

The two maps may not look all that different, but look closely. The red arrow points to West Africa, where the darkest area has grown smaller. Malaria is still common there, but has declined. In Kenya and Tanzania (purple arrow), malaria has declined even more. Even though efforts to completely eradicate malaria have failed, improved mosquito control programs have been successful in reducing the threat of malaria in these regions of Africa.

A model is basically an equation, and in the case of malaria it serves to calculate R0: the basic reproduction number. The basic reproduction number is the answer to the question, for each person already afflicted with malaria, to how many other people will their infection spread? This gives us an idea of how many people are likely to contract malaria in a given year.

For example, if R0 is 15, that means every person with malaria is likely to pass it on to fifteen other people – that’s about the R0 value for measles. If R0 is 0.5, then on average, half of all malaria patients will pass their infection on to another. For all diseases, when R0 is less than 1.0, the disease will decline, and if R0 is greater, the disease will increase in the population. Very high values of R0 can lead to pandemic.

Malaria is a complicated disease, affected by all three players: humans, microbes (more on them later), and mosquitos. In turn, each of these players are affected by temperature, moisture, population density, and other local conditions. As a result, malaria’s basic reproductive number varies depending on where you are. In 2007, a group of scientists attempted to calculate R0 for 121 different human populations in Africa (Smith et al. 2007). The study had two important results.

First, R0 varied wildly across Africa. Although the average value was near 115, many populations had values below 10 and many more had values over 1,000. Second, some R0 were extremely high, approaching 10,000. Remember what this means – on average, each person with malaria has the potential to transmit their malaria to 10,000 other people!

In some cases the R0 value was greater than the human population, suggesting that everyone in the population had malaria when of course this was not the case. Is there a problem with the model? The scientists in question didn’t think so. Instead they pointed out that although malaria’s R0 is much higher than for most other diseases, malaria also takes much more time to spread (on average, 200 days per generation) because of its complex life cycle.

The parasites that cause malaria are single-celled, apparently simple, but with strange and complicated lives. I say parasites, plural, because there are at least five species that cause malaria, but all are protozoans in the genus Plasmodium. The cycle, of course, has no “beginning,” but we will start with the oocyst, a kind of “egg sac” containing multiple Plasmodium cells and surrounded by a protective membrane.


A Plasmodium sporozoite (purple). Image by Ute Frevert; false color by Margaret Shear, licensed under CC BY 2.5.

The oocyst lives in the body of a female Anopheles mosquito, and by the time the mosquito lands for a blood meal, the membrane bursts. Single-celled Plasmodium parasites surge forth, down the mosquito’s mouthparts, and into the blood-stream of an unsuspecting human. The freed cells are called sporozoites; they are worm-like and active, and waste no time snaking their way through the host’s body until they reach the liver.

Here the parasites eat, grow, and change shape. They divide and transform into masses of globular cells, similar to oocysts, once again enveloped in a bag-like protective layer. As time goes on, liver becomes tiresome, and the parasites crave blood. The bag of cells bursts, and the Plasmodium cells return to the blood stream. Now they have a new mission: find a red blood cell.

Red blood cells are hollow and doughnut-shaped, like inflatable inner tubes. You use them to transport oxygen and carbon dioxide into and out of your body, with every breath. The blood cell’s membrane is thin, almost fluid, and easy for a Plasmodium cell to invade. Once inside, the parasite has two options. It can grow and divide, forming a new mass of cells (like the oocyst). If it does so, these cells will ultimately break out of their shelter to find new red blood cells of their own. More ambitious parasites, however, refrain from dividing. Instead they metamorphose, transforming into either male or female cells.

For the Plasmodium life cycle to complete, a second mosquito is required. The new mosquito lands on a malarial host, sucking up blood and with it, lots of red blood cells. Some of these are empty, but others, if Plasmodium is lucky, contain either male or female hitchhikers. In the mosquito’s digestive system, red blood cells burst open to release their passengers. The freed Plasmodium cells, male and female, meet and unite. Once they do, they are able to multiply and grow into a multi-celled oocyte, full of wriggling sporozoites ready to enter a new human host at the mosquito’s next meal.


A mosquito in the genus Anopheles, capable of transmitting malaria. Photo by Jim Gathany, in public domain.

For this system to work, mosquitos not only have to be infected with Plasmodium, but sporozoites have to be fully developed and ready to pounce when opportunity (i.e., a bite) comes along. Lots of mosquitos need to be biting people, so that at least some will slurp up the male and female cells when they are ready. Mosquitos need to have reasonable lifespans – those that meet their end in a frog’s belly or a spider’s web are of no use to Plasmodium. The mosquitos, in turn, have requirements of their own: they need optimal growing temperatures, pools of water in which to lay their eggs, and plenty of warm-bodied hosts from which to drink.

All of these factors (and many more) are variables that might appear in an equation to calculate R0. Smith and colleagues (2007) used them to calculate the R0 for all those 121 African populations. More recently, a group of scientists and mathematicians got together to predict how R0 will change with the earth’s climate (Ryan et al. 2015).

In general, both mosquitos and Plasmodium can only develop at temperatures between 63̊ and 93̊ F. That’s good, useful information, but not enough — each of Smith’s variables (mosquito life span, number of bites per person, etc.) is affected by temperature, but each in a slightly different way. Only by combining all of these factors, and considering how each will change under future climate conditions, can we accurately predict how the threat of malaria will change over time. That’s what Ryan and colleagues did, and published this month in Vector-Borne and Zoonotic Diseases.

It turns out that if you combine all those variables, you get a much more complex picture of how climate change and malaria interact. Although some areas will get warmer and more suitable for malaria, others will actually get too hot, so malaria will decline.

This particular study shows a decrease in malaria in West Africa, but an increase in East Africa. In other words, the hot-spot for malaria will shift east over the next six decades.


From Ryan et al. (2015), licensed under CC BY 4.0.

In the maps above, purple indicates the greatest increase in malaria transmission rates, while the palest tone indicates a decline in malaria.

Caution is important. When different models produce vastly different results, it usually means that some of those models are better than others. This model happens to consider more factors than many others, which suggest it may be more accurate, but as I have tried to show, malaria is a complex disease that will be affected by temperature in complex, hard-to-predict ways.

Having an idea of what the future might hold can inform us – where should we concentrate efforts to control malaria? Where will malaria pose the greatest threat to human health? This latest model suggests our focus will have to shift as Plasmodium, mosquitos, and malaria follow their optimal temperatures in an eastward march across Africa.


Gething P.W., D.L. Smith, A.P. Patil, A.J. Tatem, R.W. Snow, and S.I. Hay 2010. Climate change and the global malaria recession. Nature 465(7296): 342–346.

Noor A.M., D.K. Kinyoki, C.W. Mundia, C.W. Kabaria, J.W. Mutua, V.A. Alegana, I.S. Fall, and R.W. Snow. 2014. The changing risk of Plasmodium falciparum malaria infection in Africa: 2000-10: a spatial and temporal analysis of transmission intensity. The Lancet 383(9930): 1739-1747.

Paaijmans K.P., J.I. Blanford, R.G. Crane, M.E. Mann, L. Ning, K.V. Schreiber, and M.B. Thomas. 2014. Downscaling reveals diverse effects of anthropogenic climate warming on the potential for local environments to support malaria transmission. Climate Change 125: 479-488.

Pascual M., J.A. Ahumada, L.F. Chaves, X. Rodó, and M. Bouma. 2006. Malaria resurgence in the East African highlands: temperature trends revisited. Proceedings of the National Academy of Sciences U.S.A. 103(15): 5829–5834.

Rogers D.J. and S.E. Randolph. 2000. The global spread of malaria in a future, warmer world. Science 289: 1763–1766.

Ryan S.J., A. McNally, L.R. Johnson, E.A. Mordecai, T. Ben-Horin, K. Paaijmans, and K.D. Lafferty. 2015. Mapping physiological suitability limits for malaria in Africa under climate change. Vector-Borne and Zoonotic Diseases 15(12): ahead of print.

Smith D.L., F.E. McKenzie, R.W. Snow, and S.I. Hay. 2007. Revisiting the basic reproductive number for malaria and its implications for malaria control. PLOS Biology 5(3): e42. doi: 10.1371/journal.pbio.0050042

Genetically-Modified Salmon are Safe to Eat: Here’s How We Know

by Joseph DeSisto

One of the biggest news stories of today was the approval by the Food and Drug Administration of a genetically modified salmon for human consumption. This particular fish goes by the trademarked name “AquAdvantage,” and was developed by the company AquaBounty Technologies.

The approval is a big deal because, although scientists have been genetically modifying animals for many years, the AquAdvantage salmon is the first such animal ever to be approved for sale as food in the United States. Not surprisingly, this has inspired quite the outcry from anti-GMO advocacy groups.

One of their concerns is what will happen should these fish escape into the wild. The FDA had that same concern, which is why all AquAdvantage fish are sterile females, incapable of breeding with each other or with wild Atlantic salmon.

Of more immediate concern, however, is whether AquAdvantage salmon are truly safe to eat. Although the FDA claims this is the case, I am not an especially trusting person. I studied the data behind their claim to see whether I would reach the same conclusion.

Is AquAdvantage salmon safe to eat?

The FDA consumer fact sheet claims:

“After an exhaustive and rigorous scientific review, FDA has arrived at the decision that AquAdvantage salmon is as safe to eat as any non-genetically engineered (GE) Atlantic salmon, and also as nutritious.”

Because most people don’t like graphs, tables, and any phrase starting with the word “statistical,” the fact sheet does not include the actual data. The data is, however, available under the Freedom of Information Act and you are free to study it by reading the FOI summary. It is long and tedious, so I will summarize.

The AquAdvantage salmon is the result of adding two genes into the DNA of a normal Atlantic salmon egg. One of these genes produces growth factors, hormones that cause the fish to grow twice as fast as a typical Atlantic salmon. The growth gene comes from a closely related species, the Chinook salmon, which is similar to Atlantic salmon but grows to be much larger — up to 130 pounds.


Chinook salmon can grow to 58 inches long and weigh up to 130 pounds. Photo by D. Ross Robertson, licensed under CC BY-NC-SA 3.0.

Another gene is needed to make sure the growth gene is kept turned on in AquAdvantage salmon — otherwise the salmon might fail to produce Chinook growth factors. That second gene comes from another fish, an eelpout, and it simultaneously promotes the growth gene while also producing anti-freeze proteins, which make the salmon more cold-tolerant.

When the FDA says that a food product is safe, they “mean that there is ‘a reasonable certainty in the minds of competent scientists that the substance is not harmful under the intended conditions of use.*’” In this case there are several substances: the DNA, plus the molecules and hormones the DNA is supposed to make. Fish DNA by itself is not dangerous to humans – you cannot absorb it into your own genome, and it isn’t toxic. DNA is just DNA. It’s in all living things, modified or otherwise.


An eelpout. Photo copyright: President and Fellows of Harvard College, licensed under CC BY-NC-SA 3.0.

So the new question is, are the gene products unsafe for humans to eat? The eelpout’s anti-freeze proteins are already used in many other foods, including ice cream, so we will focus on the growth factors. The factor in this case is simply growth hormone (GH).

Atlantic salmon produce their own GH, just like all fish. And you eat them, anytime you eat salmon. If you’ve eaten Chinook salmon, you’ve also eaten Chinook GH. Although Chinook salmon are endangered in the wild, they are farmed just like Atlantic salmon and sold in U.S. markets every day – approved by the FDA and safe to eat.

How do we know?

Still scientists, funded by AquaBounty, chemically analyzed the meat and skin of AquAdvantage salmon and compared their results with ordinary, farm-raised salmon. Because farm-raised salmon are already fed supplemental GH (without being genetically modified), they compared these two with farm-raised salmon that were purposefully not given any additional hormones.

The result: statistically, modified salmon do not have higher levels of GH than ordinary farm-raised fish. As an aside, both had higher levels than fish which were not fed additional GH.


Juvenile Atlantic salmon (Salmo salar). Around 2/3 of salmon consumed in the U.S. comes from this species. Photo by Perhols, licensed under CC BY-SA 3.0.

GH itself is safe, but it can trigger the production of another molecule, insulin-like growth factor (IGF1), which can be toxic at high levels. To determine if there were high enough levels of IGF1 in AquAdvantage fish to warrant concern, FDA (not AquaBounty) scientists conducted their own study, a margin of exposure (MOE) assessment. Basically they tested salmon (modified and farm-raised, Atlantic and Chinook) to determine their maximum IGF1 levels. Then they calculated how much of this stuff you would have to eat before you might suffer ill effects.

The biggest fish-lovers in the U.S. eat around 300 grams of fish every day. To be safe, the FDA assumed that all 300 grams consisted of salmon, 2/3 of which (200 grams) was expected to be Atlantic salmon. They also assumed that IGF1 was always present at its maximum known level in AquAdvantage fish.

Say you are one of these obsessive salmon-lovers, but you are wary of GMOs, so you eat 200 grams of unmodified Atlantic salmon per day. Given the chemical analysis of salmon meat and skin, you would consume roughly 2.4 micrograms (0.0000024 grams) of IGF1. If, on the other hand, you only ate AquAdvantage fish, you would find yourself taking in 3.7 micrograms (0.0000037 grams) of IGF1 every day. For comparison, IGF1 levels of 1120 grams or higher are considered unsafe.

In other words, you would be eating too much IGF1 if you ate 66 kilograms (146 pounds) of genetically modified Atlantic salmon in a single day – or 102 kilograms (225 pounds) of non-modified, farm-raised salmon. No one likes fish that much.

Why are you writing about this?

This article started as a letter to a friend who shared an ad (via Facebook) on the FDA approval. This particular ad was misleading. My friend cares very deeply about environmental ethics, food safety, and the truth. I know he did not intend to misrepresent facts, so I wanted to try and clarify the issue from a scientific perspective. As I waded through FDA reports, legal documents, and old petitions, my message to a friend grew and evolved into the article you have just read.


A misleading advertisement by GMO Free USA.

The ad above comes from GMO Free USA, an advocacy group that seeks to “harness independent science and agroecological concepts to advocate for sustainable food and ecological systems.” They also envision a world “fully protected from GMO contamination.”

Advocating caution in developing new technology is, by and large, the right thing to do. Twisting the facts to inspire fear, however, is not, and this case caution was duly exercised.

The AquAdvantage fish was developed more than two decades ago, and AquaBounty nearly went out of business while waiting for FDA approval. When the FDA finally did approve, they didn’t “think you’re too busy to notice.” Everything you just read came from the Freedom of Information report, publicly available online here. The FDA’s press release on this subject was covered by major U.S. news organizations (with varying levels of objectivity) – The New York Times, CNN, ABC News, and many others.

There is no mandatory labeling of GMOs, but non-genetically modified fish will very likely be labeled (at the discretion of the companies selling them) — and if you only want to eat those salmon, that’s a choice you are free to make. You can also choose not to eat farm-raised salmon, or not to eat Atlantic salmon, thus avoiding any contact with GMOs since they would all have to be farm-raised Atlantic salmon.

Caution and skepticism are good things, and you are free to avoid genetically modified foods for ethical, personal, philosophical or religious reasons. Yet having studied the data, I can at least tell you there are no scientific reasons to panic over the FDA’s approval of AquAdvantage salmon.

*Here the FOI report is quoting the definition of food safety by Guidance for Industry 187: Regulation of Genetically Engineered Animals with Heritable Recombinant DNA Constructs. A Guidance for Industry is sort of like a public fact sheet, but for businesses – it explains the law in a (slightly) more readable format.


North America’s Big Five Centipedes

When Halloween comes around, snakes and spiders tend to steal the show. Yet centipedes, in my experience, tend to evoke even stronger reactions from people — I have met many entomologists who would happily handle a tarantula but recoil in horror when faced with a giant centipede.

In the United States there are five species of giant centipedes in the family Scolopendridae. Today, in the spirit of Halloween, I give you the Big Five: where they are found, what they do, and why I love them.

Blue Tree Centipede (Hemiscolopendra marginata)

The blue tree centipede. Photo by Sharon Moorman.

The blue tree centipede. Photo by Sharon Moorman.

This is the smallest of the five, seldom exceeding 3 inches, but still the largest centipede throughout most of its range. It is found through much of the East, from Ohio and Pennsylvania south to Florida and west to eastern Texas. Tree centipedes are also found in Mexico south to the Yucatan Peninsula. As the name suggests, the tree centipede is often an attractive blue-green, with yellow legs and orange fangs. The brightness of the color depends on the location, however, and some are paler than others.

The blue tree centipede is a habitat specialist, living under the bark of rotting trees, often before they have toppled to the ground. I have had the best luck finding them under the bark of pine logs. Because they are such good climbers, they occasionally wind up in buildings where they can cause quite a scare.

Bites from tree centipedes are painful but not much worse than a bee sting. They use their venom, as all centipedes do, to kill prey. Because they prefer to live in rotten pine logs, they may specialize in hunting beetle grubs that eat rotting wood. Like most centipedes, however, data on their feeding habits is severely lacking.

Green-striped Centipede (Scolopendra viridis)

The green-striped centipede is larger, reaching 6 inches or so, and usually pale yellow with a thick green or black stripe running down the back. Other patterns exist, however, and in parts of their range this species can appear more like a tree centipede or a tiger centipede (#4). These are adaptable centipedes, found from Florida west to Arizona, but don’t seem to venture further north than South Carolina.

The green-striped centipede. Photo by Jeff Hollenbeck, licensed under CC BY-ND-NC 1.0.

The green-striped centipede. Photo by Jeff Hollenbeck, licensed under CC BY-ND-NC 1.0.

Green-striped centipedes can live in a variety of habitats but they seem to prefer sandy forests. In Florida they can be found in scrub habitat, but like all centipedes they are not well-adapted to drought, and must stay moist by hiding underground or in rotting logs during the day.

Caribbean Giant Centipede (Scolopendra alternans)

The Caribbean giant is the only one of the Five with the russet-brown, mono-chromatic appearance of a “typical” centipede. It is probably our largest species, with a length easily exceeding 8 inches. However, the Caribbean giant is, as you might have guessed, a tropical centipede, and in the U.S. it lives only in southern Florida. It requires humid habitats, and the best place to find them is in and around the Everglades, in Dade and Monroe Counties.

A certain foreign species, the Vietnamese giant (Scolopendra subspinipes), is easily confused with the Caribbean giant at first glance. That wouldn’t be a concern, except that the Vietnamese giant has already become invasive in Hawaii and — this is just my speculating — is likely to become established in the Everglades at some point in the future. Because it is so large, often exceeding 10 inches, the Vietnamese giant is sometimes sold in the pet trade. Bites from either species are not deadly, but extremely painful.

Tiger Centipede (Scolopendra polymorpha)

A tiger centipede from Arizona. Photo by Sue Carnahan, licensed under CC BY-ND-NC 1.0.

A tiger centipede from Arizona. Photo by Sue Carnahan, licensed under CC BY-ND-NC 1.0.

Like the green-striped centipede, the tiger is a 6-inch-long animal found in a variety of habitats. Unlike the green-striped, this is a strictly western species, found from Idaho south through California into Mexico, and east all the way to Missouri. Its name comes from its color pattern: each segment is orange or yellow with a narrow, dark band.

Giant centipedes often move faster by undulating in a snake-like fashion, taking advantage of their long and muscular bodies. When a tiger centipede does this, the bands appear to “flicker,” rather like the brightly-banded milk snake and coral snake. This can make the centipede more difficult to track visually, and hence more difficult for a bird or mouse to grab.

Tiger centipedes, like their namesake, are voracious predators. They have been seen taking down prey much larger than themselves, including geckos and praying mantises. In turn, tiger centipedes are prey for scorpions, spiders, snakes, and many other predators.

A tiger centipede, fallen prey to a scorpion. Photo by Jasper Nance, licensed under CC BY-NC-ND 2.0.

A tiger centipede, fallen prey to a scorpion. Photo by Jasper Nance, licensed under CC BY-NC-ND 2.0.

Centipedes are adapted to moving fast, and their exoskeletons are thin and flexible. The drawback is that they dehydrate very easily. Although tiger centipedes are found in deserts, they still have to remain underground most of the time to conserve moisture.

Giant Desert Centipede (Scolopendra heros)

If you’ve ever seen centipedes used in a horror movie, they were probably heros*. They are big, reaching 8 inches or more. They are also brightly colored in black and orange — perfect for Halloween!

The Arizona form of the giant desert centipede. Photo by Aaron Goodwin, licensed under CC BY-ND-NC 1.0.

The Arizona form of the giant desert centipede. Photo by Aaron Goodwin, licensed under CC BY-ND-NC 1.0.

Heros are found in the desert Southwest, and color patterns vary by location. In eastern Texas and Oklahoma, they are typically jet-black with a bright orange head and yellow legs. In Arizona (above) they are usually red, with the first and last segments black. In New Mexico and western Texas the pattern is orange with black bands, much like a tiger centipede.

A giant desert centipede. Photo from NMNH Insect Zoo, licensed under CC BY-NC 2.0.

A giant desert centipede. Photo from NMNH Insect Zoo, licensed under CC BY-NC 2.0.

Why have black on just the head and the last segment? This an example of automimicry, in which one part of an animal’s body mimics the other. In this case, the tail-end of the giant desert centipede mimics its head-end. When faced with a giant centipede, predators usually attack the head, hoping to avoid a painful bite. If a predator gets confused, however, and attacks the tail instead, an unpleasant surprise awaits when the true head whips around to greet its attacker.

Centipedes, giant and otherwise, are pretty scary, and I never begrudge people who are afraid of them. Still, centipedes are amazing animals and if you see one, I encourage you to take a closer look. It will teach you, if nothing else, that just because an animal is frightening does not mean it can’t be beautiful.

*There is a centipede in one of the Human Centipede movies. People often tell me this after I tell them I study centipedes, so let me clarify a few things: I don’t know what kind of centipede the bad guy has for a pet. Not because I couldn’t identify it, but because I have never watched those movies and never will. I also don’t want to hear you describe your favorite scene with as many details as possible. Thank you.

The Scaly Crickets

by Joseph DeSisto

Among many new species named today, some of the most unusual were three new crickets from Southeast Asia (Tan et al. 2015). These crickets belong to the obscure and poorly-known family Mogoplistidae, cousins to the more recognizable (and audible) field crickets (Gryllidae). They look like field crickets too, except that their bodies are covered in scales.

A scaly cricket (Arachnocephalus vestitus). © Entomart.

A scaly cricket (Arachnocephalus vestitus). © Entomart.

When you touch a butterfly’s wings, you might notice a fine, powdery substance rubbing off on your fingers. The powder is made up of microscopic scales, which cover the wings of butterflies and moths. Scales give the wings their color, but they also provide insulation and protect the wings during flight. Perhaps most importantly, scales can fall off and make the wings slippery. This allows butterflies and moths to evade a careless hand as easily as a wet bar of soap.

The scientific name for butterflies and moths is Lepidoptera, which translates to “scaly wing” — scales are one of the most important features defining the group. However, many other groups of insects also have scales. Mosquitoes and silverfish have them, and so do scaly crickets.

The scales of a scaly cricket (Ornebius formosanus). Figure from Yang and Yen (2001), licensed under CC BY 2.0.

The scales of a scaly cricket (Ornebius formosanus). Figure from Yang and Yen (2001), licensed under CC BY 2.0.

Cricket scales, like those of butterflies and mosquitoes, are microscopic, powder-like, and easily shed. To really appreciate their beauty, a scanning electron microscope is needed. The first look came in 2001, when Yang and Yen published the first high-resolution images of cricket scales.

Aside from being scaly, scaly crickets aren’t all that unusual. They are adaptable, able to eat decaying plants as well as other insects, and they tend to live in moist sandy habitats. No scaly crickets are capable of flight, and females lack wings entirely, but the males do have small wings which they rub together to make chirping sounds (Love and Walker 1979). Click on the audio file below to listen to an amorous male scaly cricket (recorded by Thomas J. Walker).

Of the three new species, two were found in the Sakaerat Biosphere Reserve, in Thailand. This reserve consists mainly of high-altitude dry forest, with a few grasslands, and is home to many endangered species including tigers and giant black squirrels.

The third cricket is native to Pulau Ubin, an island off the coast of Singapore. Pulau Ubin is one of the last remaining wild areas in the already tiny country. Singapore’s government has been eager to develop portions of the island, but in recent years tourism has become more profitable. Fear of losing foreign visitors has encouraged officials to protect, rather than level, valuable habitat. For now, the status of the new scaly crickets appears secure, but in rapidly urbanizing Southeast Asia, nothing is certain.

A scaly cricket (Mogoplistes brunneus). © Entomart.

A scaly cricket (Mogoplistes brunneus). © Entomart.


Love R.E. and T.J. Walker. 1979. Systematics and acoustic behavior of scaly crickets (Orthoptera: Gryllidae: Mogoplistinae) of eastern United States. Transactions of the American Entomological Society 105:

Tan M.K., P. Dawwrueng, and T. Artchawakom. 2015. Contribution to the taxonomy of scaly crickets (Orthoptera: Mogoplistidae: Mogoplistinae). Zootaxa 4032(4): 381-394.

Yang J. and F. Yen. 2001. Morphology and character evaluation of scales in scaly crickets (Orthoptera: Grylloidea: Mogoplistidae). Zoological Studies 40(3): 247-253.