Does spatial sorting occur in the invasive Asian toad in Madagascar? Insights into the invasion unveiled by morphological analyses

Dispersal-relevant traits may be subjected to spatial sorting in rapidly expanding populations, driving the evolution of dispersive phenotypes. This process has been largely documented in invasive species, and ascertaining its presence can have important implications for their management. The Asian common toad was accidentally introduced to Madagascar around 2010 and is a highly problematic invasive species. Exploring the morphology of this species offers the twofold opportunity to test the hypothesis of early onset of spatial sorting and to gather information on the dynamics of this unstudied biological invasion. For these purposes, we analyze morphological data of adult toads collected across the invasive range in Madagascar, and we test the effect of the distance from the introduction point on the relative size of head, radioulna and tibiofibula, and on body size and body condition of toads, respectively. We did not detect evidence of spatial sorting in the morphological traits analyzed. Instead, morphological variation was largely dependent on sex and body size of individuals. Sexual dimorphism was limited in relation to other populations in the native range, and body size did not vary across the invasive gradient, which could indicate that both adults and juveniles are currently contributing to the dispersal in this invasive species. We provide important baseline data for the long-term assessment of morphological variation in the invasive population, and we advocate for further investigations of spatial sorting in life history and behavioral traits.