We assume a proportionate mixing law for the incidence function of FIV between males, which is more appropriate in social species [22, 23] . Transmission between males of the same population occurs at a rate b M /M, where M is the total number of males in the population, and susceptible females are infected by infected males from their population at a rate b F /M. The constants b M and b F are proportional to the rate at which males are involved in fights and to the rate at which females mate, respectively. We assume constant numbers of males (M) and females (F) within each population, whereby dead cats are instantaneously replaced by newborn cats. Since vertical transmission is very unlikely in the field [7, [24] [25] [26] , all newborns are classified as susceptible to infection. Infected cats die at a rate a. Susceptible cats also die, but since they are instantaneously replaced by susceptible (and thus equivalent) newborn cats, their death is not explicitly modeled.