Abstract
Many organisms use inducible defenses as protection against predators. In animals, inducible defenses may manifest as changes in behavior, morphology, physiology, or life history, and prey species can adjust their defensive responses based on the dangerousness of predators. Analogously, prey may also change the composition and quantity of defensive chemicals when they coexist with different predators, but such predator‐induced plasticity in chemical defenses remains elusive in vertebrates. In this study, we investigated whether tadpoles of the common toad (Bufo bufo) adjust their chemical defenses to predation risk in general and specifically to the presence of different predator species; furthermore, we assessed the adaptive value of the induced defense. We reared tadpoles in the presence or absence of one of four caged predator species in a mesocosm experiment, analyzed the composition and quantity of their bufadienolide toxins, and exposed them to free‐ranging predators. We found that toad tadpoles did not respond to predation risk by upregulating their bufadienolide synthesis. Fishes and newts consumed only a small percentage of toad tadpoles, suggesting that bufadienolides provided protection against vertebrate predators, irrespective of the rearing environment. Backswimmers consumed toad tadpoles regardless of treatment. Dragonfly larvae were the most voracious predators and consumed more predator‐naïve toad tadpoles than tadpoles raised in the presence of dragonfly cues. These results suggest that tadpoles in our experiment had high enough toxin levels for an effective defense against vertebrate predators even in the absence of predator cues. The lack of predator‐induced phenotypic plasticity in bufadienolide synthesis may be due to local adaptation for constantly high chemical defense against fishes in the study population and/or due to the high density of conspecifics.
Highlights
The ability of a genotype to produce different phenotypes in response to varying environmental conditions is known as phenotypic plasticity (Futuyma, 1998; West‐Eberhard, 1989, 2003)
We investigated whether tadpoles of the common toad (Bufo bufo) adjust their chemical defenses to predation risk in general and to the presence of different predator species; we assessed the adaptive value of the induced defense
We found that toad tadpoles did not respond to predation risk by upregulating their bufadienolide synthesis
Summary
The ability of a genotype to produce different phenotypes in response to varying environmental conditions is known as phenotypic plasticity (Futuyma, 1998; West‐Eberhard, 1989, 2003). Whether predators induce toxin synthesis in vertebrate prey has remained controversial (Benard & Fordyce, 2003; Bucciarelli et al, 2017; Hagman, Hayes, Capon, & Shine, 2009; Üveges et al, 2017) It is plausible, that to other defensive traits, prey individuals might adjust the composition or quantity of their defensive chemicals to the type of predators present, because predator species may differ in their susceptibility to toxins (Gunzburger & Travis, 2005; Mohammadi et al, 2016; Ujvari et al, 2015). We predicted that tadpoles exhibiting predator‐induced phenotypes would have elevated survival probabilities compared to predator‐naïve conspecifics when facing free‐ranging predators
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