Ordinarily, poisonous critters produce their toxins to ward off potential predators. But it turns out that some tadpoles may be turning their toxins on their rivals. This is thought to be the first time that any animal has been found to use its poison to target members of its own species, and raises the curious question of why they do it.
Researchers found that the tadpoles of common toads harbored more toxins when they were raised in crowded conditions, and they suspect that it might give the little amphibians a competitive advantage over their rivals. While plants are well known to fine tune their defenses in response to different threats, this is the first time that a vertebrate has been found to ramp up its toxins in response to competition.
Many animals produce highly toxic substances, with amphibians particularly adept at it. In fact, the tiny golden poison dart frog, which claims the title of most poisonous animal in the world, produces enough toxin to kill 10 grown men in three minutes flat. It has been fairly understandably assumed that these ferocious frogs churn out the poisons to prevent other animals from snacking on them.
But the production of such molecules is, energetically speaking, expensive. So researchers wanted to investigate whether or not animals making these toxins use them for other purposes. “It would be very profitable for such animals to kill two birds with one stone by using their anti-predatory toxins as chemical weapons against their competitors, too,” explained Veronika Bókony, lead author of the study published in Functional Ecology, to Nature.
They wanted to see what it was that influenced the potency of the toxins found in toad tadpoles, which has been found to vary from pond to pond. They suspected that competition might be at play, and so raised tadpoles in three different environments, putting them in direct competition with not only other toad tadpoles, but also tadpoles from the larger and more aggressive agile frogs.
They thought that the increase in toxins might be a response to the agile frog tadpoles, as being bigger they easily outcompete them. But the researchers discovered something else curious instead. It turned out that the tadpoles made more toxins when there were more rivals of their own species, rather than of the agile frogs.
This throws into question the entire role of poisons in amphibian defense: Is it for predators, competitors, or both? How the toxins actually help the tadpoles beat their rivals is still not fully understood. One theory is that it might prevent cannibalism, but the authors think that the most likely explanation is that it could help protect them against diseases that may spread in the crowded conditions.