Some expectant mothers prepare for the arrival of their babies by reading books of parenting tips, painting nurseries, and buying a pram. The tiger keelback snake takes a different approach. When females get pregnant, they slither into the forest to eat as many poisonous toads as they can find.
The tiger keelback is a beautiful orange, olive, and black creature found in Japan. It defends itself with two glands on its neck, which contain poisons called bufadienolides. These irritate the airways and harm the hearts of any would-be predator. But the snake doesn’t make these poisons itself. Instead, it gets them from the toads it eats. It is immune to these poisons and shunts them into its own glands, defending itself with the pilfered defences of its own prey.
Deborah Hutchinson from Old Dominion University, Virginia discovered the tiger keelback’s thievery back in 2008. She showed that baby keelbacks that are raised in captivity are born without poisons, but quickly build up a supply if they can eat some toads. A wild-born snake doesn’t have this problem. Its mother laces her eggs and yolks with her own stolen poisons, arming her babies with chemical defences even before they hatch.
To do that, pregnant females need to find toads. After all, they’re eating poison for two.
Yosuke Kojima and Akira Mori from Kyoto University tracked the movements of 24 tiger keelbacks and found that females noticeably change their behaviour when they get pregnant.
These snakes live in a diverse area that includes forests, grasslands, riverbanks, and rice fields, all of which are teeming with amphibians. For the most part, the snakes stick to grasslands, where their favoured prey—two non-toxic species of frog—can be found in huge numbers. These frogs account for 89 percent of their food. By contrast, the Japanese common toad—the only local species that makes bufadienolides—is rarer, makes up just 1 percent of the snakes’ diet, and lives only in the forests.
But in early summer, while males are still sticking to grass, pregnant females spend a third of their time in the forests. There, they are unusually active and they hunt a lot of toads, which Kojima and Mori confirmed by checking their stomach contents.
The duo also placed several snakes in a Y-shaped maze. One arm was smeared with wet paper that had been rubbed on a toxic toad, and the other was daubed with the essence of a non-toxic frog. The male snakes always headed towards the path that smelled of poison-free prey. The females usually did the same but the pregnant ones flipped their preferences and went after Eau de Toad instead.
All of this strongly suggests that the pregnant snakes deliberately seek out poisonous prey. That’s not a trivial thing to do. The toads are much rarer than the snakes’ usual prey, so it costs more energy to find them. But the effort is presumably worth it.
When baby tiger keelbacks hatch in late summer, their jaws are too small to swallow toads. They have no way of building up their own bufadienolides until the next spring, when smaller, younger toads appear. By then, a predator could easily have killed them. But their mothers, by going the extra mile to stock up on toxins, provide them with defences to see them through this vulnerable window. These snakes practice a kind of toxic nepotism: the females that amass the greatest chemical wealth can give their children the greatest start in life.
Reference: Kojima & Mori. 2014. Active foraging for toxic prey during gestation in a snake with maternal provisioning of sequestered chemical defences. Proceedings of the Royal Society B. http://dx.doi.org/10.1098/rspb.2014.2137