Meet the cunning toad viper
Researchers believe a central African amphibian is impersonating a reptilian predator.
You’d reckon a story about an amphibian mimicking a snake likely would be on the small side. Something like: fingernail-sized frog avoids foot-long snake.
So the scale of the animals involved in the first study of a toad mimicking a venomous snake is at the least unexpected.
Results published in the Journal of Natural History reveal that the hamburger-sized Congolese giant toad (Sclerophrys channingi) likely imitates the formidable Gaboon viper (Bitis gabonica) in both appearance and behaviour.
At an average length in the 125-155 centimetre range, B. gabonica is one of Africa’s largest vipers. It has the longest fangs, reaching up to five centimetres, and highest yield of venom – although not especially toxic – of any known snake.
A research team led by Eli Greenbaum, from the University of Texas at El Paso, US, believes S. channingi may use its ability to mimic B. gabonica to escape predation.
“Our study is based on ten years of fieldwork and on direct observation by researchers lucky enough to see the toad’s behaviour first-hand,” says Greenbaum.
“We’re convinced that this is an example of Batesian mimicry, where a harmless species avoids predators by pretending to be a dangerous or toxic one.”
Greenbaum allows that, to fully test the hypothesis, researchers would have to demonstrate that predators are successfully deceived. This wouldn’t be easy in the wild as the toads are rarely seen.
“However, based on multiple sources of evidence provided in our study, we are confident that our mimicry hypothesis is well-supported,” Greenbaum says.
The researchers compared the appearance of the toad, found in central African rainforests, and the viper, which is more widely distributed in central, eastern and southern Africa.
Using live wild-caught and captive individuals, and preserved museum specimens, they found that the colour pattern and shape of the toad’s body is similar to that of the viper’s head.
Most striking are two dark brown spots and a dark brown stripe that extends down the toad’s back, the triangular shape of its body, a sharp demarcation between its tan back and dark brown flanks, and the species’ extraordinarily smooth skin for a toad.
The study team posits that, because B. gabonica is capable of causing deadly bites, would-be predators likely avoid the similar-looking toads to ensure they don’t make a lethal mistake.
Unlike exclusively visual mimics, the toad’s impersonation doesn’t stop there.
Gaboon vipers are renowned for their placidity – it’s said that humans must literally step on the snake to prompt a bite – and even then it’s not guaranteed; if an individual feels threatened, it will often incline its head and emit a long, loud warning hiss before resorting to a strike.
Similarly, herpetologist Chifundera Kusamba, from the Centre de Recherche en Sciences Naturelles in the Democratic Republic of Congo (DRC), observed the toad emitting a hissing noise resembling the sound of air being slowly released from a balloon. And over a century ago, US biologist James Chapin observed the toad "bow" – lowering its front limbs so they no longer propped up the viperine-shaped body, which made it look similar to the cocked head of a snake threatening to strike.
The last part of a successful impersonation is location. Even the best mimickry will only work if predators of the harmless species are familiar with the venomous one.
The researchers compared the geographical range of the toad and viper in the DRC and found that S. channingi does not seem to occur in areas where B. gabonica is absent. The researchers identified 11 locations in the eastern rainforests where the range of both species overlaps.
“Given the relatively large size and therefore calorific value of this toad compared to other species, it would make tempting prey to a large variety of generalist predators, including primates and other mammals, lizards, snakes and birds,” says Kusamba.
“Many of these predators use vision to find their prey, and because the viper is deadly venomous, they probably recognise the distinctive, contrasting markings from a considerable distance and avoid the toad because of them, receiving a threatening hiss if the appearance doesn’t put them off.”