Evol Ecol Res 4: 109-131 (2002)     Full PDF if your library subscribes.

Spleen size, disease risk and sexual selection: a comparative study in primates

Charles L. Nunn*

Department of Biology, University of Virginia, Charlottesville, VA 22904, USA

Address all correspondence to Charles L. Nunn, Section of Evolution and Ecology, University of California, One Shields Avenue, Davis, CA 95616, USA.
e-mail: cnunn@ucdavis.edu


If individuals of different species vary in their risk of acquiring infectious disease, this variation is expected to result in systematic differences in immune defence structures across species. I used phylogenetic comparative methods to examine the correlates of spleen mass in primates, based on a priori hypotheses involving disease risk, sexual selection and correlations among organ systems. All comparative tests controlled for body mass. Contrary to predictions that more social species experience greater risk of acquiring infectious disease and should therefore exhibit increased immune defence, spleen mass was not associated with measures of sociality. Species with slower life histories had larger spleens, as expected if such species come into contact with a greater number of parasites throughout life. However, spleen mass was unrelated to use of the ground or increased mating promiscuity, both of which are thought to increase transmission of parasites. In contrast to patterns documented in previous research on birds, primate spleen mass showed no association with sexual selection involving male–male competition. The comparative tests found only one correlation among the spleen and other organs, involving the liver, which has some immune defence functions early in life. Several factors may explain the general absence of support for patterns in primates, as compared to patterns documented previously in birds, including differences in the expression of sexual selection and the involvement of the mammalian spleen in bodily functions unrelated to immune defence. These analyses suggest that spleen mass is not a useful predictor of disease risk in primates, which is important for future comparative research on the correlates of parasitism in mammals.

Keywords: comparative study, disease risk, immune system, phylogeny, primates, sexual selection, spleen.

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