Plasticity in expression clarifies the ecological drivers and evolutionary history of cannibalism in invasive cane toads (Rhinella marina)

When rapid evolutionary change occurs in a trait's expression, what role is played by proximate factors that induce variation in expression? We assessed drivers of variation in a recently evolved behaviour-targeted cannibalism by invasive cane toad tadpoles (Rhinella marina) in Australia-to better understand its evolution. Because targeted cannibalism probably evolved due to intense intraspecific competition, we first assessed effects of two proximate factors that influence resource availability-cannibal density and developmental stage-on rates of cannibalism. We then used published data to quantify evolutionary shifts in the impact of another potential driver of variation-tadpole mass. In laboratory trials, rates of cannibalism increased with cannibal development and rearing density, and cannibals reared at higher densities began consuming eggs earlier in embryonic development. Analyses of published data revealed that ancestral-range tadpoles were less cannibalistic than invasive conspecifics, and ancestral rates of cannibalism increased more slowly with larval mass. Thus, larval cannibalism is plastic in both ancestral and derived populations. Our work clarifies the ecological drivers and evolutionary history of targeted cannibalism and cannibal-induced defences in invasive Australian cane toad tadpoles and illustrates the importance of considering proximate drivers of trait variation when investigating rapid evolution.