Female Dispersion Is Necessary, but Not Sufficient, for Pairbonded Monogamy in Mammals

Abstract

Explanations for the evolution of social monogamy in mammals typically emphasise one of two possibilities: females are overdispersed (such that males cannot defend access to more than one female at a time) or males provide a service to the female. However, the first claim has never been formally tested. I test it directly at three levels using population-level data from primates and ungulates. First, I show that the females of monogamous genera do not have territories that are significantly larger, either absolutely or relatively, than those of polygynous genera. Second, using two indices of territorial defendability, I show that, given their typical day journey lengths, males of most monogamous species could easily defend an area large enough to allow them to monopolise as many as 5–10 females if they ranged solitarily. Finally, I use a model of male mate searching strategies to show that the opportunity cost incurred by pairbonded males is typically 5–10 times the reproductive success they actually obtain by being obligately monogamous. This suggests that the selection pressure dissuading them from pursuing a roving male strategy must be very considerable. At present, the evidence is undecided as to whether mitigating predation or infanticide risk is the primary function, but estimates of their impacts suggest that both are in fact plausible.

Four errors and a fallacy: pitfalls for the unwary in comparative brain analyses

“…A more nuanced evaluation of the differences among taxa suggests that bonded social relationships are one solution to the scalar stresses created by living in very large groups (Dunbar & Shultz, 2021 b ; Dunbar, 2023). If a taxon does not occupy habitats that require it to live in large groups, it will never exhibit any form of social brain relationship [unless it evolves pair‐bonded monogamy for other reasons (van Schaik & Dunbar, 1990; van Schaik, 2000; Opie et al ., 2013; Dunbar, 2022 a )]. But when it does, a species can choose between incurring the cognitive and neurobiological costs of bonded social groups or opting for the less costly (but less effective) strategy of casual herding (Dunbar & Shultz, 2021 b ).…”

“…In short, demanding that a hypothesis must be universally supported across taxonomic groups and environments risks obscuring the range of solutions animals have evolved to solve the problems they face. For example, monogamy appears to have evolved under different environmental pressures in bony fish (Stanbrook et al ., 2022) than in primates (Opie et al ., 2013; Dunbar, 2022 a ), while social grouping has evolved as an anti‐predator strategy in primates and ungulates, but as a hunting strategy in cooperative hunters like hyaena ( Crocuta spp. ), the African wild dog ( Lycaon pictus ) and the lion ( Panthera leo ).…”