Causes and consequences of single-male and multimale mating in free-ranging patas monkeys, Erythrocebus patas

Abstract

Patas monkeys and many forest guenon species live in one-male groups throughout the year. During some conceptive seasons, extragroup males enter the troop singly or in small groups to copulate with females, although this multimale state persists only as long as the conceptive season lasts. Our goals in this study were to (1) examine how the number of males in the group affected levels of intrasexual mate competition in males and females, (2) identify the proximate factors accounting for changes from single- to multimale mating, and (3) develop a model comparing the proximate factors leading to single- and multimale mating in patas monkeys with those for forest guenons. We studied the copulatory patterns of 23 female and 11 male patas monkeys during three breeding seasons in Laikipia, Kenya, and used rates and patterning of mate guarding, copulation harassment and wounding as indices of intrasexual competition. We observed multimale mating during the 1995 conceptive season, and single-male mating during 1997 and 1998. We found that moderately strong intrasexual competition in males led to infrequent high-intensity encounters across all mating seasons, and that males competed to control access to groups of females year round. Female intrasexual competition for mates was low, but slightly more vigorous during single-male mating seasons. Our model suggests that specific demographic and ecological conditions, combined with a single life history variable, interbirth interval, explain observed variation in mating systems of groups of patas monkeys and forest guenons.