Defending the Undefendable: Male Territorial Behaviour and Mating System in Monogamous Primates

It has been suggested that monogamy evolves when females forage alone and are overdispersed, such that males cannot defend more than one female at a time. I test the underlying assumption that the females of monogamous anthropoid primates are overdispersed in three different ways, and compare the results with data for several polygynous primate genera. First, I show that monogamous primates do not have per capita territories that are significantly larger than those of polygynous taxa. Second, given their day journey length and the Mitani-Rodman equation (Mitani & Rodman 1979), males of most monogamous species could easily defend areas large enough to allow them to monopolise 5-6 females. Finally, I use a model of male mate searching strategies to show that, unlike the males of polygynous species, the males of monogamous species would sire more offspring by adopting a roving male form of polygyny when females are dispersed. The opportunity cost that monogamous males incur is typically more than five times the reproductive success they have by being obligately monogamous, suggesting that the selection pressure preventing them from pursuing a roving male strategy is very considerable. Given that biparental care always follows the adoption of monogamy in primate evolution, the only viable explanation for monogamy would seem to be either high predation risk or high infanticide risk.