Rapid divergence in morphology and sexual dimorphism of invasive cane toads ( Rhinella marina ) on Ishigaki Island, Japan

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Abstract When a species invades new areas, novel challenges drive rapid shifts in phenotypic traits. Few studies have exploited the opportunity to examine such shifts in replicate invasions of the same species into dissimilar habitats. The spread of cane toads (Rhinella marina) from South America to Hawai’i and thence to Australia has attracted intensive research, but introductions to other countries are less well-studied. We gathered data on the morphology of 80 field-captured toads on Ishigaki Island in southern Japan, a site to which toads were translocated (from Hawai’i, via Taiwan and the Daito Islands) in 1978. Comparison of our samples with cane toads from the native range (French Guiana) and other invasive populations (Hawai’i and Australia) reveals substantial shifts in mean body size and shape. Some of the shifts are unique to Ishigaki (e.g. larger body size, relatively wider heads, greater differences between the sexes in several body-shape traits). This strong divergence, despite the small spatial scale of the colonization of Ishigaki, suggests that sustained range expansion (a key driver of toad evolution in Australia) is not essential for rapid phenotypic divergence from the native-range phenotype. Comparisons between replicate invasions of globally widespread species can clarify the consistency of evolutionary changes induced by invasion and the role of site-specific factors in driving adaptive trajectories.

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