Mounting a specific immune response increases energy expenditure of the subterranean rodent Ctenomys talarum (tuco-tuco): implications for intraspecific and interspecific variation in immunological traits.

It was recently hypothesised that specific induced defences, which require substantial time and resources and are mostly beneficial against repeated infections, are more likely to be favoured in 'slow-living-pace' species. Therefore, understanding how different types of immune defences might vary with life history requires knowledge of the costs and benefits of defence components. Studies that have explored the energetic costs of immunity in vertebrates have done so with a focus primarily on birds and less so on mammals, particularly surface-dwelling rodents. In this study, we evaluated whether an experimental induction of the immune system with a non-pathogenic antigen elevates the energetic expenditure of a subterranean rodent: Ctenomys talarum (tuco-tucos). In both seasons studied, a significant increase in oxygen consumption was verified in immune-challenged tuco-tucos injected with sheep red blood cells (SRBC) compared with control animals. The increase in oxygen consumption 10 days after the exposure to SRBC was lower for female tuco-tucos monitored in the breeding season compared with females in the non-breeding season. Interestingly, antibody titres of female tuco-tucos did not decrease during the breeding season. Our results add new insight into the role of other factors such as basal metabolic rate or degree of parasite exposure besides 'pace of life' in modulating the interspecific immunological variation observed in natural populations of mammals.