RETROVIRUSES AND SEXUAL SIZE DIMORPHISM IN DOMESTIC CATS (FELIS-CATUSL.)

Hochberg and co-workers have predicted that an increase in host adult mortality due to parasites is balanced by an earlier age at first repr oduction. In polygynous species we hypothesize that such a pattern wou ld lead to diverging selection pressure on body size between sexes and increased sexual size dimorphism. In polygynous mammals, male body si ze is considered to be an important factor for reproductive success. T hus, under the pressure of a virulent infection, males should be selec ted for rapid growth and/or higher body size to be able to compete suc cessfully as soon as possible with opponents. In contrast, under the s ame selection pressure, females should be selected for lighter adult b ody size or rapid growth to reach sexual maturity earlier. We investig ated this hypothesis in the domestic cat Felis catus. Orange cats have greater body size dimorphism than Iron-orange cats. Orange females ar e lighter than non-orange females, and orange males are heavier than n on-orange males. Here, we report the extent to which orange and non-or ange individuals differ in infection prevalence for two retroviruses, feline immunodeficiency virus (FIV) and feline leukaemia virus (FeLV). FIV is thought to be transmitted almost exclusively through aggressiv e contacts between individuals, whereas FeLV transmission occurs mainl y through social contacts. The pattern of infection of both diseases i s consistent with the higher aggressiveness of orange cats. In both se xes, orange cats are significantly more infected by FIV, and tend io b e less infected by FeLV than other cats. The pattern of infection is a lso consistent with an earlier age at first reproduction in orange tha n in non-orange cats, at least for females. These results suggest that microparasitism may have played an important role in the evolution of sexual size dimorphism of domestic cats.