Intersexual differences in response to an immune challenge in the frog Xenopus laevis

Activities such as locomotion and reproduction, as well as immune responses, require energy and metabolites, which can lead to conflicts in the allocation of these resources. Consequently, the allocation of energy and metabolites can limit immune response or activity patterns, resulting in trade-offs. In most sick animals immune responses include behavioural depression and consequently reduced activity, perhaps enhancing resource conservation and promoting activation of the immune system. These trade-offs mediated by reduction in activity may differ between males and females due to differences in energy expenditure primarily linked to reproduction. Therefore, we investigated the effect of a simulated infection on the locomotor performance and voluntary movement of males and females of the species Xenopus laevis. We analysed locomotor endurance, jump force and voluntary activity before and after a simulated infection through injections of LPS (endotoxins from the cell wall of gram-negative bacteria). Our results showed that locomotor performance and jump force decreased after simulated infection in both sexes. Furthermore, the magnitude of the responses differed between the sexes. Females of X. laevis exhibited greater reductions in performance and jump force, which may be related to differential energy allocation, including a greater investment in gamete production by females, leading individuals to be more immunocompetent than males. Unexpectedly, and independently of sex, the simulated infection had no impact on most variables pertaining to voluntary movement, suggesting that behavioural depression did not occur. Only the number of breaths increased after the infection, with this increase being more pronounced in females, suggesting a greater energy requirement in females in the face of an immune challenge. Keywords: amphibians, behavioural depression, energetic resources, immune system, locomotion