Tradeoffs affect the adaptive value of plasticity: Stronger cannibal-induced defenses incur greater costs in toad larvae

Adaptive developmental plasticity allows individuals to match their phenotype with their environment, which can increase fitness where threats are inconsistently present. Because adaptive traits are not ubiquitously nor infinitely plastic, tradeoffs between benefits and costs or limits are theoretically necessary to constrain plastic responses. Systems in which extreme risk can be reliably detected are ideal for investigating mechanisms that constrain plasticity, as even costly responses may be adaptive where risk is severe. Cane toads (Rhinella marina) are abundant in Australia and produce large clutches (frequently >10,000 eggs), but asynchronous breeding and rapid development result in variable larval densities within breeding pools. In the field, we found that cannibalism by older cohorts often reduces the survival of conspecific eggs and newly hatched pre-feeding larvae (“hatchlings”) by >99%, as feeding larvae (“tadpoles”) use chemical cues from the relatively immobile hatch...