Why be a cannibal? The benefits to cane toad, Rhinella marina [=Bufo marinus], tadpoles of consuming conspecific eggs

Unlike many species that are ‘occasional’ cannibals, the tadpoles of cane toads specifically target conspecific eggs for consumption, ignoring the eggs of sympatric frog species (at least within the toads’ current invasive range in Australia). We tested three hypotheses as to the benefits of consuming conspecific eggs: transfer of toxins from eggs (which have high toxin content) to tadpoles (which have lower toxin content), nutritional input, and reduction of future competition. We found no evidence of toxin transfer, but eggs contained sufficient nutrition for cannibalistic tadpoles to develop through to metamorphosis, and egg consumption enhanced rates of tadpole growth and differentiation through reduction of subsequent competition from younger tadpoles. Features of the cane toads’ life history (e.g. synchronized deposition and development of all eggs within a clutch; delay between hatching and onset of feeding; short larval stage relative to interclutch interval of a given adult female) mean that the cannibals are unlikely to be close relatives of the younger conspecifics they consume (either as eggs or as metamorphs). Kin selection may thus favour rather than oppose cannibalism. The end result is that cannibalistic toad tadpoles benefit through nutrition and reduced future competition, with little collateral risk of eating their own siblings. Another potential cost of cannibalism (risk of disease transmission) may be minimal in this system, because the eggs are unlikely to contain pathogens (reflecting their brief embryonic periods and protective jelly layers). The combination of these forces has favoured the evolution of targeted cannibalism by cane toad tadpoles.