Mating strategies in primates: A game theoretical approach to infanticide

Abstract

Infanticide by newly immigrated or newly dominant males is reported among a variety of taxa, such as birds, rodents, carnivores and primates. Here we present a game theoretical model to explain the presence and prevalence of infanticide in primate groups. We have formulated a three-player game involving two males and one female and show that the strategies of infanticide on the males' part and polyandrous mating on the females' part emerge as Nash equilibria that are stable under certain conditions. Moreover, we have identified all the Nash equilibria of the game and arranged them in a novel hierarchical scheme. Only in the subspace spanned by the males are the Nash equilibria found to be strict, and hence evolutionarily stable. We have therefore proposed a selection mechanism informed by adaptive dynamics to permit the females to transition to, and remain in, optimal equilibria after successive generations. Our model concludes that polyandrous mating by females is an optimal strategy for the females that minimizes infanticide and that infanticide confers advantage to the males only in certain regions of parameter space. We have shown that infanticide occurs during turbulent changes accompanying male immigration into the group. For changes in the dominance hierarchy within the group, we have shown that infanticide occurs only in primate groups where the chance for the killer to sire the next infant is high. These conclusions are confirmed by observations in the wild. This model thus has enabled us to pinpoint the fundamental processes behind the reproductive decisions of the players involved, which was not possible using earlier theoretical studies.

Male Takeover and Infanticide in Nilgiri LangursSemnopithecus johniiin the Western Ghats, India

“…Infanticide is observed in a wide variety of mammalian species [Lyon et al, 2011] where a newly immigrated male assaults and kills the infants that are dependent on their mothers. The event of taking over a group by dis-placing the old male in the group by a new immigrant male is called male takeover [Fedigan and Jack, 2004;Lyon et al, 2011].…”

“…The sexual selection hypothesis suggests that the newly immigrated male may assault and kill the infants (mostly when the male is unrelated to the infants) already sired by the previous resident male in order to potentially gain reproductive fitness by creating a chance to sire its own infant by potentially reducing the interbirth intervals of the females and gaining more time for his own reproduction Zhao et al, 2011]. This leads to intersexual conflict among males and females, with males gaining reproductive success while females suffer decreased fitness [Lyon et al, 2011]. However, in some instances, females employ countertactics by dispersing out of the group, creating paternity confusion or by abrupt weaning of infants [Zhao et al, 2011].…”