Eli Greenbaum

One of these is a deadly viper. The other is a harmless toad. Can you tell the difference?

The Congolese giant toad, which grows to the size of a small hand, would be a hearty meal for any predator. But it escapes being eaten by birds, lizards, and snakes with a trick never seen anywhere else in the world: It looks and acts just like the Gaboon viper, one of the most venomous snakes in Central Africa.

Many animals imitate dangerous ones to avoid being eaten. Viceroy butterflies are colored like the toxic monarch, for example, and many harmless snakes mimic venomous ones. But this is the first time a toad has been found to mimic a snake.

To make sure researchers weren’t seeing things, a team of herpetologists spent 10 years comparing dead toads from museums with live toads from 11 sites throughout the Democratic Republic of the Congo to the deadly viper. First, the researchers noticed that the toad’s body has a triangular shape similar to that of the viper’s head (above, left and right, respectively). Then, they observed a striking—and consistent—color pattern: Just like the viper, the toad has two dark brown spots and a dark brown stripe that extend down its back. Finally, when the toad senses danger, it lets out a long, low hissing noise, similar to the warning hiss a Gaboon viper might make before it strikes.

Taken together, these similarities suggest the toad is a near-perfect mimic for the viper, the researchers write today in the Journal of Natural History. Further, given their close evolutionary history (both evolved between 4 million and 5 million years ago) and the fact that the toad is found only in locations where the viper is present, it’s likely that the toads and vipers coevolved together, the authors write.

The toad’s color and shape are not an exact match. But most predators are likely to avoid anything that looks similar to the snake—because a single mistake could be deadly.