Boldness in anti-predator behaviour and immune defence in field crickets Raine Kortet, 1 * Markus J. Rantala 2 and Ann Hedrick 3 1 Behaviour, Ecology and Evolution Team (Integrative Ecology Unit), Department of Biological and Environmental Sciences, University of Helsinki, Helsinki, Finland, 2 Section of Ecology, Department of Biology, University of Turku, Turku, Finland and 3 Section of Neurobiology, Physiology and Behavior, College of Biological Sciences, University of California, Davis, CA, USA ABSTRACT Questions: Are prey animals’ behavioural responses against predation associated with their ability to resist parasites and pathogens, and if so, how is this adjusted in different populations? Methods: We studied the association between anti-predator behaviour and immune defence (encapsulation response and lytic activities of the haemolymph) using laboratory-reared male field crickets originating from two wild populations that differ in predation and parasitism risk. The anti-predator behaviours that we measured were as follows: (1) the animal’s latency to become active inside a safe refuge, (2) its latency to emerge from a safe refuge after disturbance, and (3) the animal’s freezing time following a separate alarm cue. Results: Crickets originating from a high-parasitoid, high-predation risk population (Arizona) had higher encapsulation responses than crickets from a low-parasitoid, low- predation risk population (California). Lytic activity correlated negatively with freezing time. Encapsulation response was positively correlated with the latency to become active and latency to emerge from a safe refuge in the high-predation risk population, but not in the low-predation risk population. Conclusion: Predation and parasitoids may increase the cricket’s investment in parasite resistance, despite the potential costs of anti-predator behaviour. Keywords: anti-predator response, cost, field cricket, Gryllus, immunocompetence. INTRODUCTION Responses of prey to predation risk include defences such as specialized morphological traits and behaviours that have been adjusted by natural selection over time (Endler, 1980; Lima and Dill, 1990; Chivers and Smith, 1998). Presumably, natural selection acts to favour stronger anti-predator behaviour in populations with higher predation risk (Riechert and Hedrick, 1990; Chivers et al., 2001), contributing to co-evolutionary cycles between predator and prey [cf. the ‘Red Queen’ hypothesis of Van Valen (1973)]. Numerous experimental studies have * Address all correspondence to Raine Kortet, Department of Biology, University of Oulu, PO Box 3000, FIN-90014 Oulu, Finland. e-mail: rkortet@cc.jyu.fi Consult the copyright statement on the inside front cover for non-commercial copying policies. Evolutionary Ecology Research, 2007, 9: 185–197 © 2007 Raine Kortet