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).
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.
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.
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.
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).
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.
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.
I won’t say any more, since Epomis expert Gil Wizen has already written a fantastic blog post about these beetles, complete with videos of predation in action! I encourage you to check it out here.
Cabrera-Guzman E., M.R. Crossland, and R. Shine. 2015. Invasive cane toads as prey for native arthropod predators in tropical Australia. Herpetological Monographs 29(1): 28-39.
Carpenter C.C. and J.C. Gillingham. 1984. Giant centipede (Scolopendra alternans) attacks marine toad (Bufo marinus). Caribbean Journal of Science 20: 71-72.
Smith J.G. and B.L. Phillips. 2006. Toxic tucker: the potential impact of cane toads on Australian reptiles. Pacific Conservation Biology 12(1): 40-49.
Wizen G. and A. Gasith. 2011. Predation of amphibians by carabid beetles of the genus Epomis found in the central coastal plain of Israel. ZooKeys 100: 181-191.