No abstract

According to life history theory, natural selection has shaped trade-offs for allocating energy among growth, reproduction and maintenance to maximize individual fitness. In social mammals body size and dominance rank are two key variables believed to influence female reproductive success. However, few studies have examined these variables together, particularly in long-lived species. Previous studies found that female dominance rank correlates with reproductive success in mountain gorillas (Gorilla beringei beringei), which is surprising given they have weak dominance relationships and experience seemingly low levels of feeding competition. It is not currently known whether this relationship is primarily driven by a positive correlation between rank and body size. We used the non-invasive parallel laser method to measure two body size variables (back breadth and body length) of 34 wild adult female mountain gorillas, together with long-term dominance and demography data to investigate the interrelationships among body size, dominance rank and two measures of female reproductive success (inter-birth interval N = 29 and infant mortality N = 64). Using linear mixed models, we found no support for body size to be significantly correlated with dominance rank or female reproductive success. Higher-ranking females had significantly shorter inter-birth intervals than lower-ranking ones, but dominance rank was not significantly correlated with infant mortality. Our results suggest that female dominance rank is primarily determined by factors other than linear body dimensions and that high rank provides benefits even in species with weak dominance relationships and abundant year-round food resources. Future studies should focus on the mechanisms behind heterogeneity in female body size in relation to trade-offs in allocating energy to growth, maintenance and lifetime reproductive success.

Variation in mating systems and in individuals’ copulation patterns can have important consequences for the genetic structure of a population and ultimately its evolution. Whilst most gorilla sub-species form single-male, polygynous reproductive groups, mountain gorillas have both single-male and multimale reproductive groups. Considerable research has investigated the mating patterns of males in these multimale groups, but comparatively little is known about females. We investigated the copulation patterns of 71 female mountain gorillas over 13 years. We found that most sexually active, fecundable females in multimale groups had multiple mating partners. Females’ copulation patterns varied based on their reproductive stage, mating most often and with the greatest number of partners when they were pregnant, and least often when they had young dependent offspring. Females copulated throughout pregnancy, with pregnant females copulating more frequently in single-male groups than in multimale groups. Mating also varied with age, with older females mating less often and with fewer partners. Our findings confirm that when females transfer between single-male and multimale groups, they predominantly switch from copulating with one male to copulating with multiple males, and that females are also flexibly adjusting their copulation patterns across their lifetimes. This highlights the considerable variability of the mountain gorillas mating system, particularly from the female perspective, and the importance of accounting for within-species variation in copulation patterns. Significance statement Patterns of mating can vary both between individuals of the same species and within individuals across their lives. These patterns influence the genetic structure of a population. By examining copulation patterns in female mountain gorillas, we demonstrate that whilst females in single-male groups have only one mating partner, females in multi-male groups predominantly have multiple partners across a year. They also alter their copulation patterns across their lifetimes, copulating less often and with fewer partners as they age, and copulating most when pregnant. Our results highlight the flexibility with which female mountain gorillas can adjust their copulation patterns across their lifetimes, as the costs and benefits of copulation shift.

Acoustic signals that reliably indicate body size, which usually determines competitive ability, are of particular interest for understanding how animals assess rivals and choose mates. Whereas body size tends to be negatively associated with formant dispersion in animal vocalizations, non-vocal signals have received little attention. Among the most emblematic sounds in the animal kingdom is the chest beat of gorillas, a non-vocal signal that is thought to be important in intra and inter-sexual competition, yet it is unclear whether it reliably indicates body size. We examined the relationship among body size (back breadth), peak frequency, and three temporal characteristics of the chest beat: duration, number of beats and beat rate from sound recordings of wild adult male mountain gorillas. Using linear mixed models, we found that larger males had significantly lower peak frequencies than smaller ones, but we found no consistent relationship between body size and the temporal characteristics measured. Taken together with earlier findings of positive correlations among male body size, dominance rank and reproductive success, we conclude that the gorilla chest beat is an honest signal of competitive ability. These results emphasize the potential of non-vocal signals to convey important information in mammal communication.