SEX-DIFFERENCES IN PARASITE INFECTIONS - PATTERNS AND PROCESSES

Sex differences in parasite infection rates, intensities, or populatio n patterns are common in a wide range of taxa. These differences are u sually attributed to 1 of 2 causes: (1) ecological (sociological in hu mans); and (2) physiological, usually hormonal in origin. Examples of the first cause include differential exposure to pathogens because of sex-specific behavior or morphology. The second cause may stem from th e well-documented association between testosterone and the immune syst em; sexually mature male vertebrates are often more susceptible to inf ection and carry higher parasite burdens in the field. Although many r esearchers favor one explanation over the other, the requisite control led experiments to rule out confounding variables are often neglected, We suggest that sex differences in disease have evolved just as sex d ifferences in morphology and behavior, and are the result of selection acting differently on males and females. Research has often focused o n proximate mechanistic explanations for the sex difference in infecti on rates, but it is equally important to understand the generality of the patterns in an evolutionary context, Because males potentially gai n more than females by taking risks and engaging in competition, sexua l selection pressure has shaped male behavior and appearance to maximi ze competitive ability and attractiveness. Many of the classic male at tributes such as antlers on deer are testosterone-dependent, putting m ales in what appears to be a cruel bind: become vulnerable to disease by developing an attractive secondary sexual ornament, or risk towered mating success by reducing it, A variety of hypotheses have been put forward to explain why males have not circumvented this dilemma. The m ating system of the host species will influence the likelihood of sex differences in parasite infection, because males in monogamous species are subject to weaker sexual selection than males in polygynous speci es. Whether these evolutionary generalizations apply to invertebrates, which lack testosterone, remains to be seen. Copyright (C) 1996 Austr alian Society for Parasitology. Published by Elsevier Science Ltd.