Courtship and male ornaments as honest indicators of immune function

Abstract

Having an effective immune system can be very costly, sometimes at the expense of other important life history traits, including reproduction. This trade-off can be exaggerated in males of species that have costly sexual signalling, where condition-dependent components of the signalling system reflect the health status of the bearer. It is therefore vital for a male to be able to adequately balance the costs of activating the immune system successfully while also expressing high-quality sexual signals. We examined males of the brush-legged wolf spider Schizocosa ocreata to see whether static condition-dependent components of sexual signalling in adult males are indicative of health status (immune stress response, encapsulation) and whether female preference for these traits is influenced by infection. After experimental ingestion of a bacterial pathogen (Pseudomonas aeruginosa), symmetry of male foreleg tuft (a secondary sexual trait) was found to predict the intensity of the subsequent infection, such that males with more asymmetrical tufts had higher levels of bacteria in the haemolymph. Females were equally likely to mate with infected and uninfected males in mating trials, but females that mated with infected males had bacteria in their haemolymph and on their body surface. Males that engaged in courtship had significantly lower encapsulation responses than males that did not engage in courtship, but among those males that courted, larger tuft size indicated a higher encapsulation response even after energetically costly courtship. These results indicate that females may be able to use static sexual signalling traits to examine a male's overall health, but females do not appear to discriminate against males who are actively infected, even though there is a direct cost to the female via the transfer of male infection.