Immune and parasite-mediated effects on host behaviour

Abstract

This thesis explores the effects of immune challenge and parasitism on the behaviour of host animals. Infection by pathogens mobilises the host’s immune system, which in turn gives rises to a suite of effects in the host known as sickness behaviours. In addition, parasites extract nutrients from their host, and in some cases, manipulate their behaviour. One of the key challenges in studying host-parasite interactions, therefore, is partitioning these effects. The use of protocols such as inoculation with sterile lipopolysaccharides (LPS) extracted from Escherichia coli, provides a means to induce an acute immune response in vertebrates without the confounding influence of parasite induced effects. In this study, I examined the response of fish to immune challenge across a range of contexts including social interactions, reproductive strategies and responses to predation threat. Subsequently, I examined the effects of parasite infection on host behaviour. My findings indicate that immune challenge produces a consistent effect in fish, approximately 48 hours following exposure to LPS. The behaviour of affected individuals was broadly consistent with our understanding of sickness behaviours, however there were clear indications that social context plays an important role in mediating the response. Specifically, the presence of conspecifics reduced the expression of sickness behaviours. In addition to these findings, the results from the two hostparasite systems examined strongly suggest that the parasites induce behaviours that likely expose their hosts to greater predation risk, influencing the likelihood of their transmission to a subsequent host. Taken together, my findings highlight an important role of infection and the associated immune response in shaping host's social and sexual behaviour, and in addition, parasite-mediated effects that alters risk taking behaviour in the host. I discuss this in the context of the evolution of social and sexual behaviour.