Category Archives: Amphibians

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).

800px-Bufo_marinus_1_(1)

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.

98292_orig

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.

Cited:

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.

Advertisements

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.

Cited:

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.

Poisonous Frogs, Beetles, and Birds

by Joseph DeSisto

Meet the golden poison frog of Colombia’s coastal rain forests. This frog, one of nearly 200 species of poison frogs, is by far the most toxic. A single frog packs enough poison to kill 10,000 mice, or 10 or more humans (Myers et al. 1978).

The golden poison frog (Phyllobates terribilis). Photo by Brian Gratwicke, licensed under CC BY 2.0.

The golden poison frog (Phyllobates terribilis). Photo by Brian Gratwicke, licensed under CC BY 2.0.

For the golden poison frog and its close relatives in the genus Phyllobates, batrachotoxin is the weapon of choice. Batrachotoxin acts on the nervous system, opening up the membranes of nerve cells so they can no longer carry signals to and from the brain. Death comes from paralysis, which leads to heart failure.

The golden poison frog was only discovered in 1971 when scientists found them around an indigenous Colombian (Emberá Chocó) village (Myers et al. 1978). The Emberá use the frogs to lace poison darts, with which they hunt game in the surrounding forest. The frog-handlers were careful to cover their hands with leaves, with good reason. Scientists who touched the frog felt a strong burning sensation, and they stressed in their initial description that:

The new species is potentially dangerous to handle: One freshly caught frog may contain up to 1900 micrograms (µg) of toxins, only a fraction of which would be lethal to man if enough skin secretion came into contact with an open wound.”(Myers et al. 1978, pp. 311)

The black-legged poison frog (Phyllobates bicolor), closely related to terribilis but not quite as toxic. Photo by Drriss and Marrionn, licensed under CC BY-NC-SA 2.0.

The black-legged poison frog (Phyllobates bicolor), closely related to terribilis but not quite as toxic. Photo by Drriss and Marrionn, licensed under CC BY-NC-SA 2.0.

It is important, as the frog’s discoverers remind us, “to be cautionary, not alarmist” (Myers et al. 1978, pp. 340). Even though in theory these frogs are dangerous, there is no record of a person ever being killed by one. Although the poison can go through a person’s skin, it seldom does so in enough quantity to injure. The frogs do not bite. So if you see golden poison frogs while exploring in Colombia, do not panic, but be wary. However delicious and lemon-drop-colored they may seem, definitely don’t try eat them.

Golden poison frogs are sometimes sold in the pet trade, since they lose their poison after being taken out of the wild. This is probably because frogs get batrachotoxin from their food: soft-winged beetles that make the toxin themselves (Dumbacher et al. 2004). In captivity, frog-keepers give their pets a blander diet of crickets and fruit flies, which don’t contain batrachotoxin.

An example of a soft-winged beetle, in the same family as those eaten by poison frogs. Photo by Udo Schmidt, licensed under CC BY-SA 2.0.

An example of a soft-winged beetle, not the same species, but in the same family as those eaten by poison frogs. Photo by Udo Schmidt, licensed under CC BY-SA 2.0.

Soft-winged beetles are found all over the world, especially in the tropics, but so far only a few other animals are known to eat them and use their batrachotoxins. Three of these are poison frogs, all found in a small rain forest region of Colombia. The others are birds. Yes, there are poisonous birds, and even though this blog is explicitly not about birds or mammals, I’m going to break that rule today.

The hooded pitohui (Pitohui dichrous) -- both males and females are brightly colored. Photo by Katerina Tvardikova, licensed under CC BY-NC-SA 3.0.

The hooded pitohui (Pitohui dichrous) — both males and females are brightly colored. Photo by Katerina Tvardikova, licensed under CC BY-NC-SA 3.0.

The toxic birds are all found in New Guinean rain forests, and most belong to a group of insect-eaters called pitohius (pronounced PI-to-hooies). Pitohui birds are related to the orioles and blackbirds found in more temperate climes. Shown above is the hooded pitohui, first found to be poisonous when a bird researcher handled one and left with a tingling, burning sensation in his hand.

Later study showed that the hooded pitohui, along with two other related species, has feathers laced with batrachotoxins (Dumbacher et al. 1992). More than a decade later, the same scientists demonstrated that toxin-wielding pitohui birds eat soft-winged beetles, and that these same beetles are loaded with batrachotoxin (Dumbacher et al. 2004). Despite being toxic, the birds are not nearly as dangerous as golden poison frogs, and there is little risk to a careful handler.

The variable pitohui (Pitohui kirhocephalus), not as showy as its cousin, but toxic all the same. Photo by Katerina Tvardikova, licensed under CC BY-NC-SA 3.0.

The variable pitohui (Pitohui kirhocephalus), not as showy as its hooded cousin, but toxic all the same. Photo by Katerina Tvardikova, licensed under CC BY-NC-SA 3.0.

Several more birds are now known to use batrachotoxins, and all are found in New Guinea (Weldon 2000). Many of them have similar red-and-black color patterns. By having similar colors, multiple bird species can work together to “educate” predators who might not be aware of the poisonous feathers (Dumbacher and Fleischer 2001). To make matters even more interesting, the toxins in bird feathers apparently serve as a repellent to parasitic lice (Dumbacher 1999).

We may continue to learn more about these amazing birds and their lives, or we may not. Most of these birds are becoming rarer and rarer as New Guinean rain forest is slashed and burnt, tilled and grazed into nothing.

I’ve written several articles about poisons and venoms: click here to learn about brown recluse venom and here to learn about tetrodotoxin, a poison used by many fish as well as newts, snails, and blue-ringed octopuses.

Darren Naish, writer of the superb science blog Tetrapod Zoology, writes often about birds. Click here for one of his articles on a poisonous New Guinean species. Note that the article is not on his most recent blog site, which is updated regularly at the first link to Scientific American.

To learn more about the relationship between lice and toxic pitohui birds, click here to read an excellent article by Bianca Boss-Bishop on the aptly-named blog Parasite of the Day.

Cited:

Dumbacher J.P. 1999. Evolution of toxicity in Pitohuis: I. effects of homobatrachotoxin on chewing lice (order: Phthiraptera). The Auk, 116: 957-963.

Dumbacher J.P., A. Wako, S.R. Derrickson, A. Samuelson, and T.F. Spande. 2004. Melyrid beetles (Choresine): a putative source for the Batrachotoxin alkaloids found in poison-dart frogs and toxic passerine birds. Proceedings of the National Academy of Sciences U.S.A. 101(45): 15857-15860.

Dumbacher J. P., B.M. Beehler, T. F. Spande, H. M. Garra¡o, and J.W. Daly. 1992. Homobatrachotoxin in the genus Pitohui: chemical defense in birds? Science 258: 799-801.

Dumbacher J.P. and R.C. Fleischer. 2001. Phylogenetic evidence for colour pattern convergence in toxic pitohuis: Müllerian mimicry in birds? Proceedings of the Royal Society of London B 268(1480): 1971-1976.

Myers C.W., J.W. Daly, and B. Malkin. 1978. A dangerously toxic new frog (Phyllobates) used by Emberá Indians of western Colombia, with a discussion of blowgun fabrication and dart poisoning. Bulletin of the American Museum of Natural History 161(2): 311-365.

Weldon P.J. 2000. Avian chemical defense: toxic birds not of a feather. Proceedings of the National Academy of Sciences U.S.A. 97(24): 12948-12949.

Living Illusions

by Joseph DeSisto

Good camouflage requires more than just color. Millions of years of natural selection have favored birds that can easily identify a brown moth on a brown background, but some species are a little more sophisticated. Lappet moths hide on tree bark – their odd shape, combined with mottled color, helps break up their outline, so visual predators such as birds have a hard time recognizing them as moths.

A lappet moth (Phyllodesma americana) from Arkansas. Photo by Marvin Smith, licensed under CC BY-ND-NC 1.0.

A lappet moth (Phyllodesma americana) from Arkansas. Photo by Marvin Smith, licensed under CC BY-ND-NC 1.0.

These moths lay their eggs on trees including birch, oak, poplar, willow, and many others. The caterpillars munch on leaves by night, hiding on twigs and bark by day. They are also well-hidden, but because they have to be able to live on a variety of different trees, each of which has a differently-colored bark, lappet caterpillars don’t have a color that matches a particular background. Instead they, like their parent moths, have bodies with distorted outlines, specifically a lateral fringe of long hairs.

On bark, this helps a caterpillar “merge” with the bark on which it rests. On a twig, maybe not so much:

A resting lappet caterpillar on one of its favorite hosts, scrub oak. Photo by Joseph DeSisto.

A resting lappet caterpillar on one of its favorite hosts, scrub oak. Photo by Joseph DeSisto.

Animals that depend on camouflage have to stay very still to avoid detection, but if they are spotted, staying still quickly becomes futile. Many animals use color to startle predators as a backup plan, the best-known example being the red-eyed tree frog. At rest, the frogs appear a solid leafy-green, but if disturbed, they quickly open their eyes. The sudden appearance of two giant, bright red eyes can be enough to startle a predator, which might give the frog time enough to make a hasty escape.

The iconic red-eyed tree frog (Agalychnis callidryas). Photo by Carey James Balboa, in public domain.

The iconic red-eyed tree frog (Agalychnis callidryas). Photo by Carey James Balboa, in public domain.

Lappet caterpillars have a similar but more elaborate trick. If a potential predator (or human finger) brushes against a caterpillar, it first flexes its body so the hairs on its back part. This reveals two striking, blood-red bands, a warning this caterpillar might be toxic.

Photo by Joseph DeSisto.

The lappet caterpillar has only to flex its body to reveal these striking bands. Photo by Joseph DeSisto.

Let’s say this doesn’t work, and the bird isn’t intimidated. If a bird’s beak (or my tweezers) pinches the caterpillar, it bends its head back over its body, revealing legs surrounded by pitch-black spots:

Photo by Joseph DeSisto.

This caterpillar doesn’t like me very much. Photo by Joseph DeSisto.

Still not startled? The lappet caterpillar has one last show. In desperation, it falls from its perch in the trees and, on hitting the ground, flops upside-down, revealing a bright yellow-and-black belly:

Photo by Joseph DeSisto.

Yellow and black are universal warning signs — many toxic animals share these colors. Photo by Joseph DeSisto.

If this doesn’t work, the caterpillar is pretty much toast. For all its show, the lappet caterpillar isn’t poisonous – at worst, some of its hairs are mildly irritating to the skin. Like the red-eyed tree frog, it relies entirely on visual illusions to ward off predators. It sounds risky, but it’s worked at least some of the time for millions of years.

Tree Frogs and Hybrid Dinosaurs

by Joseph DeSisto

The Jurassic World movie really got me thinking about tree frogs (warning: spoiler minefield ahead). In the movie, park management decides to boost ticket sales by genetically engineering a brand-spanking new dinosaur: Indominus rex. Indominus combines DNA from Tyrannosaurus and Velociraptor to create a monstrous killing machine — intelligent, powerful, color-changing, and able to consciously alter its body temperature to evade heat-sensing cameras.

Wait, what? Henry Wu, Jurassic World’s top scientist and dinosaur-engineer, explains by recalling that Indominus also contains tree frog DNA. Tree frogs, he tells us, can change their body temperatures at will.

The iconic red-eyed tree frog (Agalychnis callidryas). Photo by Carey James Balboa, in public domain.

The iconic red-eyed tree frog (Agalychnis callidryas). Photo by Carey James Balboa, in public domain.

I love tree frogs, and I had to know if this was true. I spent the next few days sifting through papers, trying to find an answer. What follows is the result.

Here’s the problem: tree frogs, like all amphibians, are ectothermic or “cold-blooded.” Warm-blooded or endothermic animals, such as humans, burn calories to keep their bodies a constant, ideal temperature (for you, that’s around 98.6° F). Frogs can’t do that. Instead, they have to move from place to place, seeking out warm spots if they need to raise their body temperature, and vice versa.

All that moving around sounds like a lot of effort, but in fact frogs and other ectotherms spend a lot less energy regulating their body temperature than mammals do. This means frogs can get by on much less food than, say, mice can, even if they are the same size and eat the same type of food.

A White's tree frog (Litoria caerulea). Photo by Travis W. Reeder, licensed under CC BY-NC 3.0.

A White’s tree frog (Litoria caerulea). Photo by Travis W. Reeder, licensed under CC BY-NC 3.0.

But are any frogs capable of changing their body temperature at will, without moving around? As early as 1970, California herpetologist Bayard Brattstrom thought so. He studied how amphibians adapt to different environments, conducting experiments to see which species could survive at the hottest and coolest temperatures. During a study of Australian frogs, he found that one species in particular was extremely adaptable. That species was the White’s tree frog.

The White’s tree frog (or Australian green tree frog) is common across northern and eastern Australia. It lives in rain forests, deserts, farmland, and pretty much everywhere in between. This frog even makes a hardy and forgiving pet, and captive-bred specimens are sold in pet shops around the world.

Another White's tree frog. Photo by Barbara Sing, licensed under CC BY-NC 3.0.

Another White’s tree frog. Photo by Barbara Sing, licensed under CC BY-NC 3.0.

When sweat evaporates off your skin, it takes some heat with it — this is called evaporative cooling. Amphibians are subject to evaporative cooling too, not by sweating, but because of the water evaporating through their permeable skins. Brattstrom (1970) believed that the White’s tree frog could maintain a constant body temperature under rapidly changing conditions, by controlling how quickly water left its body.

An Australian red-eyed tree frog. Photo by John Sullivan, licensed under CC BY-NC 3.0.

The Australian red-eyed tree frog (Litoria chloris) — not to be confused with the Central American red-eyed tree frog, shown earlier in this article. Photo by John Sullivan, licensed under CC BY-NC 3.0.

Decades later an experiment would prove him wrong — the White’s tree frog can’t change its own rate of water loss to regulate its body temperature. But a closely related species, the Australian red-eyed tree frog, can (Buttemer 1990). Buttemer used these two species to study how Australian tree frogs are adapted to different environments.

In the wild, red-eyed tree frogs are only found in rain forests. So, Buttemer expected the more versatile White’s tree frog to be better at regulating its rate of water loss. The opposite was true. Not only was the red-eyed tree frog able to control evaporative cooling, it also had tougher skin, with a rate of water loss almost as low as that of an alligator.

The Australian red-eyed tree frog (Litoria chloris) -- not to be confused with the Central American red-eyed tree frog, shown earlier in this article. Photo by LiquidGhoul, licensed under CC BY-SA 3.0.

An Australian red-eyed tree frog. Photo by LiquidGhoul, licensed under CC BY-SA 3.0.

Could Indominus rex have gotten its temperature-changing abilities from the Australian red-eyed tree frog? Probably not. Even though these frogs can regulate their body temperature, they can only do so because of water evaporating through their skin. Dinosaurs didn’t have the breathable skins of amphibians. Related to modern-day birds, both Tyrannosaurus and Velociraptor would have had largely water-tight skins, covered with insulating … feathers.

Velociraptor, with insulating feathers that would have protected it from dehydration in the deserts of Central Asia. Illustration by Matt Martyniuk, licensed under CC BY-SA 3.0.

Velociraptor, with insulating feathers that would have protected it from dehydration in the deserts of Central Asia. Illustration by Matt Martyniuk, licensed under CC BY-SA 3.0.

Cited:

Brattstrom B. H. 1970. Amphibia. In G. C. Whittow (ed.), Comparative physiology of thermoregulation 135-166. Academic Press, New York.

Buttemer W.A. 1990. Effect of temperature on evaporative water loss of the Australian tree frogs Litoria caerulea and Litoria chloris. Physiological Zoology 63(5): 1043-1057.

How Poisons Work: Tetrodotoxin

by Joseph DeSisto

This is the first of a series of short articles, each featuring a different type of poison or venom used by animals.

Poisons and venoms are some of the most complex substances in nature, often containing hundreds of different chemicals, each with a particular purpose. As technology advances, scientists have begun to look at some of these chemicals, usually proteins, and try to figure out what they do.

The deadly southern blue-ringed octopus (Hapalochlaena maculosa). Photo by Bernard Dupont, licensed under CC BY-SA 2.0.

The deadly southern blue-ringed octopus (Hapalochlaena maculosa). Photo by Bernard Dupont, licensed under CC BY-SA 2.0.

In some cases, the results give us a new perspective on an animal’s biology: rattlesnake venom, for example, contains a unique protein that allows the snake to track its prey after the initial strike. In other cases, we discover potentially useful surprises: giant centipede venom contains a single protein that inhibits pain in mice, using the same chemistry as morphine, but with greater efficiency. However, regardless of whether the results are useful to us, studying nature’s chemical weapons gives us a whole new appreciation and understanding of the creatures that wield them.

Tetrodotoxin is the poison of choice for a variety of animals, especially in the ocean. These include blue-ringed octopuses, cone snails, moon snails, certain angelfish, some ribbon worms, a handful of amphibians, and the puffer/triggerfish order Tetraodontiformes, for whom the toxin is named. Many of the animals that use tetrodotoxin are brightly colored, a warning to passers-by. Would-be predators, save the immune and the unlucky, heed this warning well.

An inflated pufferfish (Diodon holocanthus). Photo from Williams et al. (2010), licensed under CC BY 2.5.

An inflated pufferfish (Diodon holocanthus). Photo from Williams et al. (2010), licensed under CC BY 2.5.

Tetrodotoxin, sometimes abbreviated TTX, is a complex molecule that prevents its victims’ nerves from functioning properly. This eventually leads to paralysis, and death comes when the muscles that control breathing no longer receive signals from the nervous system. Basically, you suffocate. Humans can get TTX poisoning when they eat improperly-prepared pufferfish, but stings from blue-ringed octopuses and cone snails (both from the southwestern Pacific) are also a possibility if you are foolish enough to pick them up.

Although many animals use TTX, none of them actually manufacture the toxin themselves. Instead bacteria generate the toxin, to their host’s benefit. In exchange, the bacteria are allowed to live safely within their host’s body — the bacterium Pseudoalteromonas tetraodonis, for example, makes its home within the livers and skins of pufferfish.

A toxic moon snail (Naticarius orientalis) from East Timor. Photo by Nick Hobgood, licensed under CC BY-SA 3.0.

A toxic moon snail (Naticarius orientalis) from East Timor. Photo by Nick Hobgood, licensed under CC BY-SA 3.0.

The rough-skinned newt, from British Colombia and the western United States, has the bacteria needed to make TTX for itself — as a result, this newt has few natural predators. Two animals, however, have managed to work around and even benefit from the rough-skinned newt’s toxins, all without toxin-producing bacteria of their own.

The only animal capable of eating an adult rough-skinned newt is the garter snake. After ingesting the newt, any other predator would almost certainly die, but a few populations of garter snakes have evolved an immunity to TTX. What’s more, the snakes are able to sequester the toxins within their own bodies, so that the garter snakes themselves become poisonous (Williams et al. 2004).

The extremely toxic rough-skinned newt (Taricha granulosa). Photo by Rennett Stowe, licensed under CC BY 2.0.

The extremely toxic rough-skinned newt (Taricha granulosa). Photo by Rennett Stowe, licensed under CC BY 2.0.

Although many snakes inject toxins with their fangs, and so are venomous, garter snakes that eat rough-skinned newts are the only snakes that can truly be considered poisonous. The difference is that venomous animals have to inject their poisons into predators or prey, via fangs or stingers, while poisonous animals are laced with their chemical weapons and are dangerous to eat.

The other TTX-robber is, unexpectedly, a caddisfly. Caddisflies are insects whose larvae resemble caterpillars, but live underwater and surround themselves with cases made from pebbles, twigs, and other debris. Most are scavengers, eating decomposing plant matter and tiny invertebrates, while the flying adults are short-lived and do not feed.

A Limnephilus caddisfly larva, in its protective case made of twigs. Photo by Tom Murray, used with permission.

A Limnephilus caddisfly larva, in its protective case made of twigs. Photo by Tom Murray, used with permission.

A few predatory species in the genus Limnephilus, however, have developed an unusual appetite for rough-skinned newt eggs, which also develop in the water. These eggs are loaded with TTX, and the toxin-resistant Limnephilus larvae manage to eat so many that even the caddisfly adults are toxic (Gall et al. 2012).

Even though many marine invertebrates contain tetrodotoxin, we still don’t know how many contain the bacteria that make it, versus how many, like the caddisfly, “steal” the toxin from their TTX-laced prey. Certain sea slugs, for example, are often washed on shore where they are eaten by beach-combing scavengers such as dogs. In New Zealand, several dogs have died after eating sea slugs that contained TTX (McNabb et al. 2009), and in Argentina, a population of the same slugs appears to have become invasive (Farias et al. 2015). Yet we still do not know whether they have their own TTX-generating bacteria.

A toxic sea slug, Pleurobranchaea meckelii. Photo from Wägele and Klussmann-Kolb (2005), licensed under CC BY 2.0.

A toxic sea slug, Pleurobranchaea meckelii. Photo from Wägele and Klussmann-Kolb (2005), licensed under CC BY 2.0.

Experiments have shown that certain populations of sea slugs have TTX, while others do not (Khor et al. 2014) — just like the garter snakes of North America. Does this mean the sea slugs are obtaining TTX from some unknown, toxic prey item? The same researchers conducted a survey of all the marine invertebrates that these slugs might be eating, testing everything for TTX. The only positive result was a toxic species of sand dollar, but the sand dollars didn’t produce nearly enough TTX to explain the huge amounts found in sea slugs.

Ignorance has consequences, and there is still plenty of exploring left to do. The slugs may in fact make tetrodotoxin themselves (using bacteria). A more enticing possibility, and just as likely, is that there are still more toxic animals on the sea floor, waiting to be discovered.

One of the very first articles I wrote for this site featured the same sea slugs mentioned in this one, but that was when my only readers were a few sympathetic family members. If you’re curious about sea slugs, and/or you want to know the difference between a nudibranch and a pleurobranch, you can read that article here.

I also recently wrote about how rattlesnake’s use their venom to track once-bitten prey (mentioned in the second paragraph of this article). To learn more about that, click here. It’s amazing, I promise.

Cited:

Farias, N.E., S. Obenat, and A.B. Goya. 2015. Outbreaks of a neurotoxic side-gilled sea slug (Pleurobranchaea sp.) in Argentinian coasts. New Zealand Journal of Zoology, published online: DOI: 10.1080/ 03014223.2014.990045.

Gall, B.G., A.N. Stokes, S.S. French, E.D. Brodie III, and E.D. Brodie Jr. 2012. Predatory caddisfly larvae sequester tetrodotoxin from their prey, eggs of the rough-skinned newt (Taricha granulosa). Journal of Chemical Ecology 38(11): 1351-1357.

Khor, S., S.A. Wood, L. Salvitti, D.I. Taylor, J. Adamson, P. McNabb, and S.C. Cary. 2014. Investigating diet as the source of tetrodotoxin in Pleurobranchaea maculata. Marine Drugs 12(1): 1-16.

McNabb, P., L. Mackenzie, A. Selwood, L. Rhodes, D. Taylor, and C. Cornelison. 2009. Review of tetrodotoxins in the sea slug Pleurobranchaea maculata and coincidence of dog deaths along Auckland Beaches. Prepared by Cawthron Institute for the Auckland Regional Council Technical Report 2009/108.

Williams, B.L., E.D. Brodie Jr., and E.D. Brodie III. 2004. A resistant predator and its toxic prey: persistence of newt toxin leads to poisonous (not venomous) snakes. Journal of Chemical Ecology 30(10): 1901-1919.