In the great menagerie of poisonous creatures, there are some animals that don’t make their weapons on their own. Instead, they sequester toxins from other animals that they eat. Many poison dart frogs, for example, get their toxins from insects in their diet. Sometimes, this is just a fringe benefit. The poison pilferer is probably going to eat the poison producer because that’s the prey that’s available, or that’s what they evolved to eat or that’s what they prefer, and the poison they get to use for their own protection is icing on the cake.

But every once in a while, there’s a case where toxic prey is not normally on the menu, and an animal who needs a little poison deliberately seeks it out to boost its defenses. In Japan, zoologists Yosuke Kojima and Akira Mori have found one of these, a snake that switches up its diet when it gets pregnant in order to arm its babies before they’re even born. 

The Ashiu Forest is a mountainous area in Kyoto with diverse habitats like grasslands, forests and rice paddy fields. The area is home to the tiger keelback (Rhabdophis tigrinus), and for most of the year, these snakes split their time evenly between the grasslands and the forests. They usually eat frogs that are plentiful in both places—just two species make up 89 percent of their diet—but will occasionally also eat poisonous toads. From these, they sequester toxins that they use to defend themselves and, by depositing the poison in their eggs, their young.* The toxins, called bufadienolides, find their way into fluid that oozes from glands on the snakes’ necks and irritates the skin and eyes and alters the heart rhythms of would-be predators. Toads that produce bufadienolides are rarer in Ashiu Forest than other prey (an amphibian census taken by Kojima and Mori turned up just 41 of them) and live only in the wooded areas, so they’re a rare treat for the snakes and normally account for only 0.9 percent of their diet. 

By tracking radio-collared snakes for several months and checking their stomach contents, Kojima and Mori found that these patterns in diet and habitat change come May and June. While male keelbacks shun the forests and spend almost all of their time in the grass during these months, pregnant females continue to use the forests, do most of their hunting there and dine mainly on poisonous toads while ignoring their usual prey. 

After ruling out other factors that might explain the change in behavior, Kojima and Mori thought that the females might be seeking out toads so they could provision their eggs with bufadienolides. In late spring and early summer, snakes that mated the previous fall would be about ready to lay their eggs, so the timing made sense. To test that idea, the pair caught male, pregnant female, and non-pregnant female snakes and put them in a Y-shaped maze. Each arm of the “Y” was baited with paper that had been rubbed on either a frog that the snakes typically eat or a toxic toad. When the snakes reached the fork in the path and caught a whiff of the bait, they went into hunting mode and investigated, smelled, and bit the papers. The males and non-pregnant females showed a strong preference for the scent of frogs, but the pregnant females went after the toad-rubbed papers around three times more often than the froggy ones. 

All this points to a shake up in the keelbacks’ diet based on whether or not they’ve got buns in the oven. The snakes appear to deliberately switch their behavior and seek out toxic prey when they’re pregnant, most likely so they could give some of the poison to their young. 

The toads are relatively hard to come by in Ashiu Forest, and the grassland is a much better place for a pregnant snake to be so she can regulate her temperature and that of her developing eggs, so going on a toad-heavy diet isn’t easy. But when keelbacks hatch, they’re highly vulnerable, and unable to steal toxins on their own until they’re big enough to hunt juvenile toads some six months later. Until then, a supply of poison from mom is essential to their survival, so the effort of hunting down toads is well worth it to make sure the kids are alright. 

*The snakes are actually a toxic double threat. They’re not only poisonous thanks to the toads, but also venomous (if you’re curious about the difference, see here). They produce the venom on their own and use it to incapacitate prey. It’s rarely used defensively against predators or bothersome humans because the keelback’s fangs are far back in its mouth and not much use for striking large animals.