Category Archives: Reptiles

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.

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.

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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.

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.

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.”

Crocodylus_acutus_mexico_01.jpg

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_BW.jpg

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_BW.jpg

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_illustration.jpg

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.png

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_BW.jpg

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:

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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.jpg

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).

Stomatosuchus2.jpg

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.

NileCrocodile--Etiopia-Omo-River-Valley-01

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.

Cited:

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

 

The Mountain King

by Joseph DeSisto

During my trip to Arizona, I saw tarantulas, scorpions, black widows, giant centipedes, lizards, and way too many insects to name here. What I didn’t see a lot of were snakes — in fact I only saw two, but those two snakes were the most beautiful I had ever seen.

The first was in Sierra Vista where, after a long day of beating bushes for caterpillars, we pulled into a driveway to find one of the most stunning animals on earth: the Arizona mountain kingsnake.

An Arizona mountain kingnake, held by Benedict Gagliardi. Photo by Joseph DeSisto.

An Arizona mountain kingnake, held by Benedict Gagliardi. Photo by Joseph DeSisto.

The Arizona mountain kingsnake (Lampropeltis pyromelana) and its cousin, the California mountain kingsnake (L. zonata), are some of the most sought-after snakes by North American reptile-lovers. Both are incredibly beautiful, but not especially common, and they prefer high-elevation habitats that aren’t always very accessible to naturalists. Mountain kingsnakes are secretive, spending most of their time underground. They seldom bask in the sun like garter snakes or rattlesnakes, instead emerging only to track hunt their lizard and rodent prey, which they kill by constriction.

The Arizona mountain kingsnake, from Sierra Vista. Photo by Joseph DeSisto.

The Arizona mountain kingsnake, from Sierra Vista. Photo by Joseph DeSisto.

The bright red and yellow bands are warning to predators. Snake-eating birds and mammals might easily confuse the kingsnake with the extremely venomous Sonoran coralsnake, which is also found in Arizona but prefers the lower-elevation desert scrub habitats, rather than the upland pine forests favored by the mountain kingsnake.

I am on a lucky streak when it comes to snakes. I don’t see very many, but the ones I do see are special enough to make my friends jealous. During a May trip to the Appalachians, I saw only five snakes, but two of those were corn snakes and two more were eastern worm snakes. Despite both of these being great finds, I left the South feeling a bit slighted, since what I really wanted to see was a venomous snake, a timber rattlesnake or copperhead. I had never seen a venomous snake in the wild before, so when I decided to go to Arizona, known for being rattlesnake country, I was ready.

The other mountain kingsnake, L. zonata from California. Photo by James Maughn, licensed under CC BY-NC 3.0.

The other mountain kingsnake, L. zonata from California. Photo by James Maughn, licensed under CC BY-NC 3.0.

We spent a few days in Sierra Vista collecting caterpillars and setting up lights at night to attract moths and other insect curiosities. Pat Sullivan, a beetle expert who lives in the area, had several pet rattlesnakes and was eager to show me a rock pile he had set up on his property as snake habitat.

The night he took me to the rock pile, just a few yellow scales caught the beam from my flashlight. I could see perhaps an inch of snake that looped out from under a rock, and I wanted to flip the rock to see more. I also, however, didn’t want to put my hands right next to a rattlesnake who might not be as sociable as I was. So I left the snake be, and returned to the light where moths and beetles kept me busy for the rest of the evening.

The last morning before we left Sierra Vista, I returned to the rock pile. After a few minutes of leaning over for a good angle, I realized the snake was in exactly the same position as before, only a few scales visible. In daylight those few scales were truly beautiful — they yellow and tan color revealed this was a black-tailed rattlesnake (Crotalus molossus), one of the prettiest rattlesnakes around. Pat got a long stick and, very carefully, flipped the rock over:

This is what I saw -- half a black-tailed rattlesnake. Photo by Joseph DeSisto.

Half a black-tailed rattlesnake. Photo by Joseph DeSisto.

The snake made no attempt to strike or even rattle. It simply slid beneath the rock pile with the grace of an animal that knows it can hurt you, and knows that you know it can hurt you. In the end I only had a few seconds to see less than half of a rattlesnake, but I’ll take it. I saw my first and, to date, only venomous snake in the wild, and it was one of the most beautiful creatures I’ve ever had the pleasure of meeting.

The Alligator’s Nest

by Joseph DeSisto

If you are careless in wandering along the swamps of the southeastern United States, you may hear this sound emanating from the brush:

[Recording by Adam Britton, used with permission.]

That is the hiss of an angry American alligator — if you hear it on land, you may have stumbled upon an alligator nest. If so, do not delay in your retreat. A mother alligator’s warning is no bluff.

An American alligator (Alligator mississippiensis) from South Carolina. Photo by Gareth Rasberry, licensed under CC BY-SA 3.0.

An American alligator (Alligator mississippiensis) from South Carolina. Photo by Gareth Rasberry, licensed under CC BY-SA 3.0.

If you could stay, however, you might be surprised at the tenderness with which alligators treat their offspring. When a female is ready to lay, she hauls herself on shore and finds a shaded, protected area not too far from the water. She lays her eggs in a pile of mud and leaf litter, then heaps more litter on top of them, so that the end result is a leaf-and-mud pile 2-3 feet tall and 5-7 feet wide (McIlhenny 1935).

A thick layer of insulating leaves also keeps the eggs at a more-or-less constant temperature. On a daily basis, even though the environment might go through wild changes in temperature, the inside of the nest stays within 3° F (Chabreck 1973). Alligator eggs usually take around 2 months to develop, and stable temperatures are critical.

Baby American alligators from the Okefenokee Swamp in Georgia. Photo by William Stamps Howard, licensed under CC BY-SA 3.0.

Baby American alligators from the Okefenokee Swamp in Georgia. Photo by William Stamps Howard, licensed under CC BY-SA 3.0.

In human beings, the presence or absence of a Y chromosome decides whether one develops into a male or female. In other words, human sex determination is chromosome-dependent. Alligators instead, like many reptiles, show temperature-dependent sex determination. Between days 20 and 35 of incubation, if eggs are kept between 86° and 93°F, a roughly even mixture of females and males will be the result (Ferguson and Joanen 1983). If, however, the batch stays above 93°, only males will emerge, and if below 86°, only females.

Alligator babies aren’t the only things that grow in alligator nests. The heap of dead leaves, twigs, and mud provides a haven for bacteria and other microorganisms. As bacteria digest the rotting vegetation, they produce heat — enough to keep the eggs 3-4° warmer than the habitat outside the nest (Chabreck 1973). In fact, bacteria keep alligator nests so consistently warm that the nests are also home to unique, heat-loving fungi (Tansey 1973).

More baby alligators! Photo by Ianaré Sévi, licensed under CC BY-SA 3.0.

More baby alligators! Photo by Ianaré Sévi, licensed under CC BY-SA 3.0.

Eggs, alligator or otherwise, look simple but are surprisingly complex. The “solid” shell is mostly made of calcium, but it’s far from a perfect seal — the whole surface is peppered with thousands of tiny holes, allowing the egg to take in oxygen and water, while “exhaling” carbon dioxide (Kern and Ferguson 1997). The thickness of the shell must be precise — too thin and the egg is easily crushed or infected by disease, but too thick and breathing, drinking, and hatching become difficult.

On alligator farms, where alligators are bred and raised for their skins, roughly 30-60% of eggs hatch successfully. Meanwhile more than 90% of alligator eggs hatch successfully in the wild (Kern and Ferguson 1997), as long as the nest isn’t flooded or raided by predators first. Experiments have shown that captive alligator eggs are less porous than their wild counterparts, and of the captive eggs, the least porous are doomed to die before hatching (Wink et al. 1990). Captive alligator eggs also have much thicker shells than wild eggs. So where is the difference coming from?

A baby American alligator. Photo by Ianaré Sévi, licensed under CC BY-SA 3.0.

A baby American alligator. Photo by Ianaré Sévi, licensed under CC BY-SA 3.0.

The bacteria in wild alligator nests, aside from producing heat, also produce acids. These acids aren’t strong or abundant enough to harm the developing reptiles, but over the 2 months it takes for them to develop, acids gradually erode the hard, calcium shell around each egg (Ferguson 1981). By the time the alligator is ready to hatch, its shell is significantly thinner than when the egg was first laid — just thin enough for the hatchling to easily break through.

[Recording by Adam Britton, used with permission.]

As they leave their eggs, baby alligators sound an alarm to their mother, who industriously digs them out of the nest where they spent the first two months of their lives. Although these months might seem uneventful, they are in fact full of challenges, which alligator eggs, however simple and unassuming, have ways to overcome.  Those hatchlings that survive face yet another gauntlet of obstacles, including predators and ruthless competition from their siblings. It’s tough being a baby alligator, and maybe even tougher being an egg, but the toughest few have a chance to become some of the most awe-inspiring top predators in North America.

Dr. Adam Britton, a crocodile researcher at the Charles Darwin University in Northern Territory, Australia, has graciously allowed me to use the audio files in this article. More files, along with a wealth of information about crocodilian biology and conservation, can be found at his website, crocodilian.com.

Cited:

Chabreck R.H. 1973. Temperature variation in nests of the American alligator. Herpetologica 29(1): 48-51.

Ferguson M.W.J. 1981. Increased porosity of the incubating alligator eggshell caused by extrinsic microbial degradation. Experientia 37(3): 252-255.

Ferguson M.W.J. and T. Joanen. 1983. Temperature-dependent sex determination in Alligator mississippiensis. Journal of Zoology 200(2): 143-177.

Kern M.D. and M.W.J. Ferguson. 1997. Gas permeability of American alligator eggs and its anatomical basis. Physiological Zoology 70(5): 530-546.

McIlhenny E.A. 1935. The Alligator’s Life History. Christopher Publishing House, Boston. 117 pp.

Tansey M.R. 1973. Isolation of thermophilic fungi from alligator nesting material. Mycologia 65(3): 594-601.

Wink C.S., R.M. Elsey, and M. Bouvier. 1990. Porosity of eggshells from wild and captive, pen-reared alligators (Alligator mississippiensis). Journal of Morphology 203(1): 35-39.

White Sands

by Joseph DeSisto

During my trip to the Southwest I had the chance to spend a day at White Sands National Monument, one of the most ecologically bizarre places in North America. White Sands is located in the dry lowlands of Tularosa Basin, in southern New Mexico. Due to a geological oddity, the sand here is made up of sparkling white gypsum crystals.

White dunes of gypsum sand at White Sands National Memorial. Photo by Jennifer Wilbur, licensed under CC BY-SA 3.0.

White dunes of gypsum sand at White Sands National Memorial. Photo by Jennifer Wilbur, licensed under CC BY-SA 3.0.

This fact has shaped the evolution of White Sands’ animal inhabitants, the most conspicuous of which are lizards. Although few large predators can handle the debilitating heat and drought of this habitat, birds of prey pose a serious threat to reptiles that must spend part of their days basking in the sun. Against a white background, lizards should be easy for a cruising hawk to pick out.

During my walk I found two lizards, each of which had a different solution to their avian problem. The first, and most obvious, was the little striped whiptail. Whiptails are handsome but poorly camouflaged, with blue heads and tails and black spots above their forearms.

An adult little striped whiptail (Aspidoscelis inornatus). Photo by Joseph DeSisto.

An adult little striped whiptail (Aspidoscelis inornatus). Photo by Joseph DeSisto.

The whiptail above is an adult, around 8 inches in length. The juveniles are far more striking, with electric blue tails and striped bodies.

A baby striped whiptail. Photo by Joseph DeSisto.

A baby striped whiptail. Photo by Joseph DeSisto.

Little striped whiptails are obvious, but nearly impossible to catch by hand. Almost always, by the time I saw one, it was already vanishing in a flash of blue, zooming across the sand like the Road Runner cartoon. I saw perhaps forty whiptails during my time at White Sands. The two shown above are the only ones I could photograph before they disappeared into a burrow, usually at the base of a shrub or cactus.

Whiptails are extremely fast and wary -- this makes them frustrating to approach (and photograph). Photo by Joseph DeSisto.

Whiptails are extremely fast and wary — this makes them frustrating to approach (and photograph). Photo by Joseph DeSisto.

After half an hour of chasing whiptails, with little more than sunburnt skin to show for my efforts, I started to notice a second lizard in my patch of sandy scrub. These lizards were slower and easier to photograph, but a little harder to spot.

The bleached earless lizard (Holbrookia maculata). Photo by Joseph DeSisto.

The lesser earless lizard (Holbrookia maculata). Photo by Joseph DeSisto.

The lesser earless lizard is almost synonymous with White Sands — certainly it is the most famous reptile in the park. The species to which it belongs is wide-ranging, found from South Dakota west to southern California and south through central Mexico. In most places, earless lizards are some combination of brown and black — only the populations at White Sands have evolved pure white scales to help them camouflage with the gypsum sand. Their scales even shimmer, to help the lizard mimic the sand’s reflective crystal grains.

A lesser earless lizard. Photo by Joseph DeSisto.

A lesser earless lizard. Photo by Joseph DeSisto.

The White Sands earless lizards are called “bleached” earless lizards, for obvious reasons. Many animals at White Sands have evolved white coloration to match the substrate: both lizard predators (e.g., the Apace pocket mouse) and prey (insects). None are quite as effective as the bleached earless lizard, but all are powerful examples of evolution in action.

A basking earless lizard. Photo by Joseph DeSisto.

A basking earless lizard. Photo by Joseph DeSisto.

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.

The Whiptail and the Barberpole

by Joseph DeSisto

The desert is a busy place, both for animals and for biologists. Before we entered Coronado National Forest in southeastern Arizona, I stepped out of the car to look around, and see what surprises were waiting for me on the roadside. First was a whiptail lizard. I actually saw several, but only one was slow enough for me to photograph:

A whiptail lizard (genus Aspidoscelis) from Arizona. Photo by Joseph DeSisto.

A whiptail lizard (genus Aspidoscelis) from Arizona. Photo by Joseph DeSisto.

There are 11 species of whiptail lizard in Arizona alone, and many more throughout the Americas. They are insectivores, fast hunters that skate over the sand to chase down their arthropod prey. Whiptails aren’t very choosy — desert animals seldom are — but one insect they avoid was also at the roadside: the rainbow or “barberpole” grasshopper.

The grasshoppers, too, were wary of me and my camera but their wariness was half-hearted — most predators, including whiptails, know to steer well clear of the barberpole’s spectacular warning colors. The colors are a powerful advertisement that this insect is highly toxic and makes for a poor meal. Experiments have shown that whiptails will even avoid non-toxic grasshoppers if they have been painted with the barberpole’s colors (McGovern et al. 1984).

The barberpole or rainbow grasshopper (Dactylotum bicolor). Photo by Joseph DeSisto.

The barberpole or rainbow grasshopper (Dactylotum bicolor). Photo by Joseph DeSisto.

This grasshopper doesn’t have fully-developed wings, and can’t fly. When I first began observing them, I assumed the pads above the legs would develop into flight-ready wings when the hoppers reached maturity. This is the case with most grasshoppers and many other insects — wings only appear at adulthood.

But then I saw two individuals mating, without wings:

Mating barberpole grasshoppers. Photo by Joseph DeSisto.

Mating barberpole grasshoppers. Photo by Joseph DeSisto.

So the wingless grasshoppers are in fact adults. Most grasshoppers use their wings to fly away from danger, but the barberpole’s defense is so effective that the ability to fly has become redundant. Wings take a lot of energy to develop, so over time, as the grasshoppers stopped using them, the wings were reduced until they became little more than blue-and-white pads. The wing pads they have now are vestigial, like the hipbones of a whale, a reminder of their evolutionary past.

Cited:

McGovern G.M., J.C. Mitchell, and C.B. Knisley. Field experiments on prey selection by the whiptail lizard, Cnemidophorus inornatus, in Arizona. Journal of Herpetology 18(3): 347-349.

The Milk Adder

by Joseph DeSisto

Many times I was told, growing up in Maine, that if I looked hard enough I could find adders. People alleged to find them in parks, along rock walls, even in their homes. The word “adder” confused me, because this is the “true” adder:

The European adder, Vipera berus. Photo by Zdeněk Fric, licensed under GFDL.

The European adder, Vipera berus. Photo by Zdeněk Fric, licensed under GFDL.

That’s the European adder, a viper found in Europe and northern Asia. It is venomous, like all vipers, although not especially prone to biting. In New England, we only have two vipers, the copperhead and the timber rattlesnake. Neither are known to occur in Maine. Presumably copperheads never occurred here, prohibited by the cold winters.

The timber rattlesnake, meanwhile, is absent from Maine not because of the climate, but because the species was systematically exterminated through rattlesnake hunts. In Maine as in much of the United States, the timber rattlesnake has been stoned, burnt, and bulldozed out of our lives. Now it has become something of a legend, a name whispered when a rustle in the leaves is heard, or when a large snake slithers out of sight, just quickly enough to evade scrutiny.

A timber rattlesnake from Iowa. Photo by Psychotic Nature, licensed under CC BY-SA 3.0.

A timber rattlesnake (Crotalus horridus) from Iowa. Photo by Psychotic Nature, licensed under CC BY-SA 3.0.

Yet the adder of Maine is neither copperhead nor rattlesnake, but the harmless eastern milk snake. Why they should be called adders, I haven’t a clue. Despite looking nothing like a viper, milk snakes are sometimes confused with rattlesnakes and copperheads because they

a) shake their tails in dead leaves to mimic the sound made by a rattle,

b) aren’t afraid to hold their ground and defend themselves, and

c) are snakes.

That they should be killed is shameful, because milk snakes happen to be some of the most beautiful snakes in New England. As hatchlings they are creamy-white, with brick-red spots running down the back, each outlined in black as if traced with a fountain pen. With each year the snake loses a bit of its color — by adulthood most milk snakes are a pale gray, with red spots faded to a duller, but still handsome, chestnut brown.

A young milk snake or "milk adder" from Iowa. Photo by Psychotic Nature, licensed under CC BY-SA 3.0.

A young milk snake (Lampropeltis triangulum) or “milk adder” from Iowa. Photo by Psychotic Nature, licensed under CC BY-SA 3.0.

For most of my childhood, the eastern milk snake was my herpetological holy grail. I spent hours wandering through fields, digging in wood piles, and painstakingly tracing the edges of rock walls. Finally I tried my luck at a car junkyard, where I was told that snakes liked to hide under the heat-soaked, mouse-infested hoods and spare parts. When I asked the manager if he had seen any snakes with red spots, he told me that yes, many of the snakes had red spots, but only after he killed them and nailed them to trees like the Old Testament bronze serpent.

Milk snakes are not uncommon, but they are very secretive. Unlike the sun-loving garter snakes and racers, milk snakes are mostly nocturnal and subterranean, emerging from hiding only when necessary to track their rodent prey. When they are uncovered, milk snakes often defend themselves violently, shaking their tail, rearing up, and striking. The six-inch-long hatchlings are just as tenacious, although their bite amounts to little more than a soft pinch.

An adult eastern milk snake -- less striking than a hatchling, but still a handsome snake. Photo by Trisha Shears, licensed under CC BY-SA 3.0.

An adult eastern milk snake — less striking than a hatchling, but still a handsome snake. Photo by Trisha Shears, licensed under CC BY-SA 3.0.

I never did find my milk snake, but I like to think those years taught me something about nature. Milk snakes, like so many wonders of the natural world, are not found – they appear, like angels or shooting stars or gusts of wind. I don’t look for milk snakes any more, but I still go for long walks, flip logs, and scrutinize rock walls. Without fail, something amazing is there, and with luck one day that amazing thing might be a milk snake. Until then I look forward to a few moments reveling in its beauty before, with a flick of its muscular body, it vanishes.

This blog is mostly focused on invertebrates, but you can expect me to write more about reptiles as well, because I love them. My last article on snakes focused on vipers, and the chemicals they use to track their prey.

Happy World Snake Day!

Death by Disintegrin

by Joseph DeSisto

Disintegrins. Definitely the coolest-sounding proteins, manufactured by some of the coolest animals, the vipers.

Snake venom contains thousands of different proteins, making it one of the most complex substances in the natural world. Not all of these proteins are toxic, but those that are belong to four major categories, depending on their effects.

Neurotoxins act on your nervous system, which can make for a bad time. Your nerve cells are needed to control muscle movement, and since muscle movement allows you to breath, neurotoxic venom can cause death by paralyzing your ability to do so. Neurotoxins are found in cobras, sea snakes, and their relatives, but seldom in vipers.

Cytotoxins are the good old-fashioned cell-killers, found in many snake venoms. If you get bitten by a venomous snake, these are the proteins that start killing the tissue around the bite. If enough tissue dies on a limb, amputation might be necessary. More often, cytotoxins just make venomous snake bites really, really painful.

A Russell's viper (Daboia russelli) from Bangalore, one of the deadliest vipers in the world. Photo by Sandilya Theuerkauf, licensed under CC BY-SA 2.5.

A Russell’s viper (Daboia russelli) from Bangalore, one of the deadliest vipers in the world. Photo by Sandilya Theuerkauf, licensed under CC BY-SA 2.5.

Hemotoxins are more a viper’s purview, and act on the cardiovascular system. They can destroy red blood cells, prevent the blood from clotting, or just destroy cardiovascular tissue. While neurotoxins are relatively fast-acting, death by hemotoxin is slow. When a mouse or other prey animal is bitten by a viper, it typically runs away for some distance before it succumbs to shock. This presents vipers with a problem — how to track down prey after it has gone off to die? But before we answer that question …

The fourth category, proteases, are proteins that specialize in breaking down other proteins, and viper venom is positively loaded with them. Disintegrins are just one class of proteases, but have many functions. Their primary purpose is to break apart integrin, the stuff animals use to keep their cells stuck together. Disintegrins can also prevent blood from clotting, and have been used in medical research (McLane et al. 2004). Finally, two special disintegrins have a third purpose: they serve as tracking signals that can help the predator find its poisoned prey.

The western diamondback rattlesnake (Crotalus atrox). Photo by Holger Krisp, licensed under CC BY 3.0.

The western diamondback rattlesnake (Crotalus atrox). Photo by Holger Krisp, licensed under CC BY 3.0.

It was only a decade ago that Parker and Kardong (2005) first demonstrated, through a set of experiments, that rattlesnakes (a subfamily of vipers) use airborne scents to relocate their prey after injecting venom. Prior to that, it was thought that rattlesnakes followed a scent trail on the ground. Although experiments had since suggested that rattlesnakes are attracted to prey containing venom, it was unclear just how venom was being used to track prey.

Modern chemistry was the key. Saviola and colleagues (2013) extracted venom from diamondback rattlesnakes, then separated it into some of its major chemical components. With the venom divided, the authors injected each mouse with a different component, then offered these mice to the snakes, as well as mice that had been injected with un-divided venom. Snakes were by far the most responsive to mice that had either been injected with whole venom or with two proteins: crotatroxins 1 and 2, both of which are disintegrins.

So, rattlesnakes don’t merely “smell their own venom” — they smell to a particular pair of compounds, without which they would be unable to find prey after it had been bitten. Vipers very well might not have evolved the bodies, venoms, and life strategies we see today if it had not been for this tiny but crucial adaptation.

The western diamondback rattlesnake (Crotalus atrox) Photo by Clinton & Charles Robertson, licensed under CC BY 2.0.

The western diamondback rattlesnake (Crotalus atrox). Photo by Clinton & Charles Robertson, licensed under CC BY 2.0.

Cited:

McLane M.A., E.E. Sanchez, A. Wong, C. Paquette-Straub, J.C. Perez. 2004. Disintegrins. Current Drug Targets: Cardiovascular and Haematological Disorders 4(4): 327-355.

Parker M.R. and K.V. Kardong. 2005. Rattlesnakes can use airborne cues during post-strike prey relocation. In Mason R.T. et al. (Eds.), Chemical Signals in Vertebrates 10 (397-402). Springer.

Saviola A.J., D. Chiszar, C. Busch, and S.P. Mackessy. 2013. Molecular basis for prey relocation in viperid snakes. BMC Biology 11(20) doi: 10.1186/1741-7007-11-20