Nobuo Matsumura Alamy
Snakes fed a diet of toxic toads become toxic too — and they seem to know it.
While many snakes make their own venom, not all do. Japan’s tiger keelback snake (Rhabdophis tigrinus) is one of a handful of species that can store toxins it acquires from its food.
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Tiger keelback snakes are usually less than a metre long, an ideal meal for many birds and mammals. But they eat toxic toads and store the toxins in specialised organs on the backs of their necks called nuchal glands.
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If a snake is threatened it arches its neck, making the nuchal gland area more prominent. A predator that bit the snake’s neck would probably get a jet of fluid from the glands straight in the mouth or face, which would be distasteful or even painful.
But not all keelbacks exhibit this defensive behaviour. Snakes from a toad-free island flee when attacked, rather than standing their ground. Now it seems the snakes know whether or not they are armed with toxins.
Akira Mori of Kyoto University, Japan, and Gordon Burghardt of the University of Tennessee in Knoxville reared hatchling snakes from toad-free and toad-rich Japanese islands. The snakes were fed controlled diets containing toxic toads – or not.
When snakes from the toad-free island were fed toads, they started responding to threats with nuchal gland displays, rather than slithering away.
Know thyself
“So far as I know, this is the only example in terrestrial vertebrates where there is some indication that animals act as if they are aware of when they are toxic and when they are not,” says Burghardt.
Apart from snakes, plenty of animals acquire toxins from what they eat, but Burghardt says there is no sign they change their behaviour depending on what they have eaten. “Poison dart frogs in captivity are not fed the types of food that make them toxic, but their behaviour towards predators does not seem to have changed at all,” he says.
“It is remarkable that the researchers were able to demonstrate not only a difference in behaviour between these two populations, but that if you feed toads to toxin-free snakes, they are able to adjust their behaviour in a manner consistent with being chemically defended,” says Alan Savitzky of Utah State University in Logan.
How they know is an open question. The snakes could somehow monitor the amount of toxin in their glands. Alternatively, they might detect changes in the microbial community living in their digestive system, which would be influenced by toxin levels.
Journal reference: Journal of Comparative Psychology, DOI: 10.1037/com0000075
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