Toxin cues affect cannibalism responses of cane toad tadpoles

In many species cannibalism is uncommon and involves non-selective
consumption of conspecifics as well as heterospecifics. However, within
their invasive Australian range cane toad larvae (Rhinella marina)
specifically target and voraciously consume the eggs and hatchlings of
conspecifics, often extirpating entire clutches. In contrast, toad larvae
rarely consume the eggs and hatchlings of native frogs. Here, we use
laboratory studies to demonstrate that this selective consumption is
triggered by species-specific chemical cues: maternally-invested
bufadienolide toxins that otherwise defend cane toad eggs and hatchlings
against predators. We find that these cues stimulate feeding behaviors in
toad tadpoles, such that the addition of bufadienolide toxins to the water
column increases predation on eggs, not only of conspecifics, but also of
native anuran species that are otherwise usually ignored. In contrast, we
find that cannibalism rates on conspecific hatchlings are high and
unaffected by the addition of bufadienolide cues. The maternally-invested
toxins present in conspecific eggs may therefore be more easily detected
post-hatching, at which point tadpole feeding behaviors are induced
whether or not additional toxin cues are present. As bufadienolide cues
have previously been found to attract toad tadpoles to vulnerable
hatchlings, our present findings demonstrate that the same toxin cues that
attract cannibalistic tadpoles also induce them to feed, thereby
facilitating cannibalism through multiple behavioral effects. Because
native fauna do not produce bufadienolide toxins, the species-specificity
of these chemical cues in the Australian landscape may have facilitated
the evolution of targeted (species-specific) cannibalism in invasive cane
toad populations. Thus, producing bufadienalide toxins confers a cost
(increased vulnerability to cannibalism in early life-stages) as well as a
benefit (reduced vulnerability to predation by other taxa).