Tag Archives: amphibian

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).

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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.

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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.

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.