Skin morphology in cane toads

The structure of the skin may evolve rapidly during a biological invasion,
for two reasons. First, novel abiotic challenges such as hydric conditions
may modify selection on traits (such as skin thickness) that determine
rates of evaporative water loss. Second, invaders might benefit from
enhanced rates of dispersal, with locomotion possibly facilitated by
thinner (and hence more flexible) skin. We quantified thickness of layers
of the skin in cane toads (Rhinella marina) from the native range
(Brazil), a stepping-stone population (Hawai’i), and the invaded range in
Australia. Overall, the skin is thinner in cane toads in Australia than in
the native range, consistent with selection on mobility. However, layers
that regulate water exchange (epidermal stratum corneum and dermal Ground
Substance layer) are thicker in Australia, retarding water loss in hot dry
conditions. Within Australia, epidermal thickness increased as the toads
colonised more arid regions, but then decreased in the arid Kimberley
region. That curvilinearity might reflect spatial sorting, whereby mobile
(thin-skinned) individuals dominate the invasion front; or the toads’
restriction to moist sites in this arid landscape may reduce the
importance of water-conservation. Further work is needed to clarify the
roles of adaptation versus phenotypic plasticity in generating the strong
geographic variation in skin structure among populations of cane toads.