Infant survival in western lowland gorillas after voluntary dispersal by pregnant females

Marie L Manguette,Vidrige H Kandza,Thomas Breuer,Martha M Robbins,Jana Robeyst

doi:10.1007/s10329-020-00844-z

Abstract

In many social species, after the alpha male has been replaced or the group disintegrates, a female’s infant is at risk of infanticide by a male. Female gorillas have developed the rare strategy of secondary dispersal in which they transfer between reproductive groups during the limited time period between weaning an infant and conceiving the next one (voluntary dispersal). By doing so they leave a weaker silverback near the end of his tenure and join a stronger silverback at an earlier stage of his own tenure, thereby mitigating the risk of infanticide if the former dies. If females are pregnant or have unweaned offspring when the only male in the group dies, their offspring are vulnerable to infanticide by the new silverback that they join (via involuntary dispersal). In the few known cases of female gorillas transferring when pregnant (mainly after group disintegration), their offspring were killed. We report here on three adult females that transferred voluntarily while pregnant multiple times between two groups yet their offspring were not killed by the new group’s silverback. The gorillas were observed from 1995 to 2015 at the Mbeli Bai research site in northern Republic of the Congo. The females gave birth 5–6 months (gestation period 8.5 months) after their last transfer. To our knowledge, these observations are the first to show that wild female western lowland gorillas can transfer voluntarily while pregnant without incurring infanticide by a new silverback. These observations highlight the behavioural plasticity shown by female gorillas in response to sexual coercion by males.

Highlights

Social mammals exhibit a range of intersexual conflicts and compromises (Clutton-Brock 2007)
Infanticide reduces the reproductive success of both parents, but it can increase the reproductive success of the infanticidal male if three conditions are met: there is a low probability that the infanticidal male sired the infant, the mother resumes reproduction sooner if the infant is killed, and the mother is likely to mate with the infanticidal male (Hrdy 1977; Smuts and Smuts 1993)
Our findings show that in western lowland gorillas, rare with regards to total births recorded in the current study (~1.5%), offspring born to mothers that voluntary transfer when pregnant may not be killed

Summary

Introduction

Social mammals exhibit a range of intersexual conflicts and compromises (Clutton-Brock 2007). Secondary dispersal ( referred to as transfer), which involves the movement of females between reproductive groups, is found in only a few mammals [e.g. Thomas langurs (Sterck et al 2005); tropical bats (Nagy et al 2007); feral horses (Debeffe et al 2015); gorillas (Stokes et al 2003; Robbins and Robbins 2015)] and is believed to represent female choice for a highquality male where it occurs voluntarily (Harcourt and Stewart 2007; Lukas and Huchard 2014) It may occur involuntarily after group disintegration [in gorillas (Stokes et al 2003)] or female eviction [in ursine colobus (Teichroeb et al 2009)] and lead to infanticide if the females are pregnant or have unweaned offspring. If a female transfers while pregnant or with an infant, the first and third criteria for infanticide are met, and the infant is at risk

Methods

Results

Conclusion