Toad poison: a Swiss army knife against multiple enemies

Üveges, Bálint; Basson, Anna; Móricz, Ágnes; Bókony, Veronika; Hettyey, Attila

School of common toad tadpoles. Image provided by the authors. Photo by Nikolett Ujhegyi. Used with permission.
School of common toad tadpoles. Image provided by the authors. Photo by Nikolett Ujhegyi. Used with permission.

Organisms can flexibly adapt to environmental change and enhance their survival chances by displaying plastic traits. Given that such responses are usually costly (in terms of energetics and trade-offs with other traits), organisms tend to fine-tune their responses to external factors. For instance, many animals form groups in the presence of predators to increase their chances of survival. Predation risk decreases for each individual as the group becomes larger. Consequently, members of larger groups tend to respond less intensely to predators than do animals in smaller groups or lone individuals. However, being close to conspecifics can also facilitate the spread of diseases and parasites, and can lead to competition for food and to conflict with other group members. Responding to these negative effects can also weaken anti-predator responses, since the traits beneficial for coping with the challenges of group living are often disadvantageous when facing predators. But what if organisms had a single multi-tool which was effective against various foes? The use of toxins against enemies may be such a multifunctional trait, but how the intensity of toxin production changes with increasing group size in response to the presence of predators has not been investigated in animals so far. We conducted an experiment in which we reared common toad tadpoles at different densities and with or without chemical cues indicating the presence of tadpole-eating fish. We found that tadpoles produced more toxins with increasing group size and also when fish cues were present (at least at lower densities). This means that toad tadpoles respond to the presence of other toad tadpoles and predators similarly and these responses can be additive. This is in contrast with what has been found in respect to changes in other defensive traits like body shape and behaviour, where optimal responses against competitors are disadvantageous against predators. Our results also suggest that removing tadpoles of invasive toad species, such as cane toads in Australia, would benefit the native fauna not only by decreasing the number of poisonous toads, but also by decreasing their toxin content, which could help native predators adapt to these toxic invaders. 

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