Differences in standard metabolic rate at the range edge versus the center of an expanding invasive population of Xenopus laevis in the West of France

Empirical and theoretical studies have investigated the trade-offs between reproduction, dispersal and/or survival that may arise between individuals at the range edge of an expanding population. The differential allocation of resources to these life-history traits may then be constrained by the metabolism of an individual. Previous studies of an invasive expanding population of the frog Xenopus laevis in the West of France showed an increase in allocation of resources to dispersal and a decrease in allocation to reproduction in individuals from the range front. The present study focuses on physiological variation through an analysis of the standard metabolic rate (SMR). SMR was analyzed through measurements of oxygen consumption and carbon dioxide production under resting conditions. Our results show a positive correlation between SMR and individual size and weight, but no difference in body condition is observed between individuals from the center and those from the periphery of the range. Moreover, a decrease in SMR at the periphery was observed in both sexes. Yet, a significant interaction effect between sex and site was observed. Males had a higher SMR than females at the center while females from the periphery showed a higher SMR than males. A lower SMR may allow animals from the periphery to allocate more resources to other functions, such as locomotion, and therefore dispersal. In contrast, the relatively higher SMR in females from the periphery (compared to males) is likely to be driven by the important energetic cost of reproduction in addition to the environmental and demographic constraints at the edge of an expanding population. From the perspective of conservation biology, these results attest to the importance of understanding the biological and evolutionary mechanisms underpinning the selection of traits to estimate the invasive potential of exotic species.