Eliciting an immune response reduces sprint speed in a lizard

Predators and pathogens can diminish organisms’ fitness, and consequently natural selection favors antipredatory and antiparasitic strategies. Nevertheless, as both kinds of strategies consume resources, animals cannot maximize investment in both strategies simultaneously, which should lead to trade-offs between the two. Accordingly, we hypothesize that there is a trade-off between sprint capacity and immune response, which are among the most important antipredatory and antiparasitic strategies, respectively, of many animals. Consequently, we predict that animals eliciting an immune response should reduce their sprint capacity. We experimentally tested this prediction in the lizard Psammodromus algirus. A group of lizards was inoculated with the antigen LPS (lipopolysaccharide of the cell wall of Escherichia coli), which activates the immune system, whereas the other group (control) was inoculated with phosphate buffer, which has no physiological effect. Before and after the inoculations, we filmed sprint capacity of lizards, estimating the maximal sprint speed. Males were faster than females before the activation of their immune system. Nonetheless, males’ sprint speed diminished after the inoculation of LPS, whereas females’ did not. Therefore, our results support the hypothesis of a trade-off between sprint speed and immune response in males. Consequently, male lizards combating a pathogen may suffer either higher risk of predation, or reduced fitness if they avoid this risk by spending more time in shelter.