Impact of male infanticide on the social structure of mountain gorillas.

Andrew M Robbins,Edwin Kagoda,Maryke Gray,Martha M Robbins,Augustin Basabose,Prosper Uwingeli,Innocent Mburanumwe,Ronald Noë

doi:10.1371/journal.pone.0078256

Abstract

Infanticide can be a major influence upon the social structure of species in which females maintain long-term associations with males. Previous studies have suggested that female mountain gorillas benefit from residing in multimale groups because infanticide occurs when one-male groups disintegrate after the dominant male dies. Here we measure the impact of infanticide on the reproductive success of female mountain gorillas, and we examine whether their dispersal patterns reflect a strategy to avoid infanticide. Using more than 40 years of data from up to 70% of the entire population, we found that only 1.7% of the infants that were born in the study had died from infanticide during group disintegrations. The rarity of such infanticide mainly reflects a low mortality rate of dominant males in one-male groups, and it does not dispel previous observations that infanticide occurs during group disintegrations. After including infanticide from causes other than group disintegrations, infanticide victims represented up to 5.5% of the offspring born during the study, and they accounted for up to 21% of infant mortality. The overall rates of infanticide were 2–3 times higher in one-male groups than multimale groups, but those differences were not statistically significant. Infant mortality, the length of interbirth intervals, and the age of first reproduction were not significantly different between one-male versus multimale groups, so we found no significant fitness benefits for females to prefer multimale groups. In addition, we found limited evidence that female dispersal patterns reflect a preference for multimale groups. If the strength of selection is modest for females to avoid group disintegrations, than any preference for multimale groups may be slow to evolve. Alternatively, variability in male strength might give some one-male groups a lower infanticide risk than some multimale groups, which could explain why both types of groups remain common.

Highlights

Infanticide by males has been observed or suspected in a wide range of taxa including birds, carnivores, ungulates, and primates [1,2,3,4,5,6]
After using Equation 1 to combine those two comparisons, we found that infants born in one-male groups had a 2.8% probability of being killed due to dominant male replacements (IR/B), which is not significantly different from the 1.5% probability for infants born in multimale groups (Fisher exact test: p = 0.46)
Polyandrous mating in multimale groups is reportedly one of the main counterstrategies for females to avoid infanticide [10,15], so why do 50–60% of adult female mountain gorillas reside in one-male groups? We discuss two hypotheses and we compare our results with other primate populations

Summary

Introduction

Infanticide by males has been observed or suspected in a wide range of taxa including birds, carnivores, ungulates, and primates [1,2,3,4,5,6]. According to the sexual selection theory, infanticide can be an adaptive strategy for males when three conditions are fulfilled [7,8]. The first condition is that the male had little or no probability of siring the infant. The second condition is that the mother will resume reproduction sooner if the infant is killed. The third condition is that the male has an increased probability of siring the mother’s offspring [7,8]. Several other hypotheses have been proposed to explain infanticide [9,10]

Methods

Results

Conclusion