Sexual and geographical divergence in head widths of invasive cane toads, Rhinella marina (Anura: Bufonidae), is driven by both rapid evolution and plasticity

Measurements of > 3700 field-collected cane toads (Rhinella marina) show that head width relative to body length differs between the sexes and has become modified during the toad's translocation from French Guiana to Hawai'i and then Australia. Relative head width was highest in the native range, and declined progressively during the invasion. In long-colonized areas (French Guiana through to Queensland) male toads have wider heads than females, but this dimorphism decreases and eventually reverses at invasion fronts in both northern and southern Australia. To explore reasons for that variation, we conducted experiments on captive toads. A toad's head width affected its maximal ingestible prey size and prey-handling ability. Head width relative to body length was highly repeatable, consistent over ontogeny, and exhibited significant heritability (h² = 0.20). Relative head widths differed between the sexes and populations-of-origin even if offspring were raised under standard conditions in captivity. Nonetheless, relative head width of a cane toad also is affected by prey availability. Captive toads raised on a diet of small prey items developed wider heads than did conspecifics raised on larger prey, partly compensating for the trophic limitations of smaller body size. Sexual and geographical divergences in relative head widths in cane toads are thus the combined result of rapid evolutionary divergence (in < 100 years) and an ability of individual toads to flexibly modify this important morphological feature depending upon local conditions.