Male–female interactions in multimale groups of mountain gorillas

Abstract

Male-female social interactions may vary according to female receptivity, female parity, and male dominance rank. Such variation may be less apparent in species with one-male mating systems than those with multimale mating systems, as within-group male-male competition and female mate choice are absent. Examining variation in male-female interactions in multimale groups in species with a predominantly onemale mating system may help to shed light on plasticity in behavioral patterns and the evolution of mating systems. In this study, we investigated the effect of female receptivity (i.e., days when mating occurred), female parity, and male dominance rank on the patterns of spatial proximity, grooming, following, and aggression among 34 male-female dyads in four multi-male groups of Virunga mountain gorillas. In addition, as a preliminary investigation of potential physiological costs incurred by females in a mating context (coercion), we tested whether female receptivity and female parity explained variation in immunoreactive glucocorticoid (iGC) levels of females. The amount of time male-female dyads spent in close proximity was significantly higher for parous versus nulliparous females and for high-versus low-ranking males. The rate of male aggression to females did not vary significantly with female parity, male rank, or female receptivity. However, post hoc analysis showed that both proximity and aggression increased for the males that participated in the matings on days that females were receptive. Grooming and following by males occurred infrequently. Neither female receptivity nor parity influenced iGC levels in females, a finding that is more consistent with courtship than coercion of females by males. Overall, our results suggest that males advertise their ability to provide protection to females and their offspring, and females seek out males that can do so.

The dispersal dilemma among female mountain gorillas: Risk infanticide and gain protection

“…In August, Mitunu mated with both the silverback and the older blackback. The sexual activity recorded reflects the males` direct benefit from infanticide (Habumuremyi et al., 2018) but females encounter a dilemma related to emigration: gaining protection but incurring a loss of short‐term reproductive fitness. Although two females transferred into the same group with infants, only the younger infant was killed.…”

“…However, infanticide is rarely recorded among the gorillas in the Bwindi Impenetrable National Park (Robbins et al., 2009). The male reproductive strategy of killing an unrelated infant and hastening resumption of a female's reproductive cycle occurs alongside the female strategy of seeking a silverback for protection (Habumuremyi et al., 2018). Female immigration can result in the death of her infant, but joining a group containing a male protector is vital for her survival.…”

Group structure and individual relationships of sanctuary-living Grauer’s gorillas (Gorilla beringei graueri)

“…Male-female relationships are considered the core relationship of gorilla groups [ 24 ]. In single-male mountain and western lowland gorilla groups, females spend more time associated with the silverback than with other females and maturing males [ 26 , 27 ], and in multi-male mountain gorilla groups, females tend to associate with the highest ranking male over lower ranking males [ 28 ]. Interestingly, despite the heavy emphasis on male-female relationships, male-immature mountain gorilla dyads have stronger association patterns than male-female dyads, highlighting that immature gorillas are an integral component of gorilla social structure [ 4 ].…”

Do we need to reclassify the social systems of gregarious apes?

“…Gibbons sometimes live in groups with more than one male, or more than one female 52,53 . Up to 40% of mountain gorilla ( Gorilla beringei beringei ) groups in the Virunga Volcanoes have more than one male 54 . While there is some intraspecific variation around the modal social system of certain apes, some are also remarkably inert in light of their association with a whole spectrum of different habitats.…”