Female dispersal patterns influenced by male tenure duration and group size in western lowland gorillas

Abstract

Evaluating the factors influencing the patterns of female dispersal in mammals is critical to understanding its importance for male and female reproductive strategies and the evolution of social systems. In western lowland gorillas, females emigrate from their natal group (natal dispersal) but also disperse between groups multiple times in their life (secondary dispersal). This strategy is uncommon in primates and may reflect females seeking to reduce feeding competition or female choice for males that can provide protection against infanticide. In this study, we test how inbreeding avoidance, group size, and male competitive ability could influence female dispersal in western gorillas, using 25 natal and 285 secondary dispersal events collected over 20 years at Mbeli Bai. We found that while all females dispersed out of their natal group, presumably to avoid inbreeding, females also left their group before having their first offspring when the dominant male was not their father, pointing towards unconditional dispersal by nulliparous females. Regarding secondary dispersal, we found that females were more likely to stay with males that were at the beginning of their tenure than transfer and more likely to disperse away from (presumably old) males that were near the end of their tenure or closer to their death. Females were also more likely to leave larger groups for smaller ones suggesting a potential effect of within-group feeding competition or avoidance of outsider males or predators. This study highlights the intersexual conflict found in gorillas: while female choice for high-quality males may influence the formation of their relatively rare social system, smaller group size may be more important for females than previously thought, which runs counter to the males' interest of having a high number of females. SignificanceDispersal is a very important life strategy for most group-living mammals. Secondary dispersal by females is a rare behavior but is believed to be a counterstrategy against sexual coercion by males, reduces feeding competition, assists in predator avoidance, and facilitates mate choice. We studied the factors influencing secondary dispersal in western lowland gorillas. We found that the duration of male tenure and the size of the group influences female transfer decisions. This study shows that female reproductive strategies such as secondary dispersal may be very powerful to counter the risk of infanticide through female choice for better protector males but also to reduce the impact of feeding competition or to avoid predators or outsider males.

Using long-term ranging patterns to assess within-group and between-group competition in wild mountain gorillas

“…We found that small gorilla groups suffered costs from between-group competition by having less exclusive home ranges and core areas as well as that larger groups experienced costs from within-group competition. However, the combined linear effects of both within- and between-group competition did not result in the expected U-shaped relationship between group size and annual home range or core area size [ 19 , 20 ]. Instead, we found evidence of a concave relationship, with the annual home range and the core area size leveling off and then declining at the larger group sizes.…”

“…Positive relationships between group size and both home range and core area size are the primary markers used to infer scramble competition within groups in the wild [16][17][18], although such scramble competition may be mitigated by dispersal of males and/or females. In particular, female dispersal allows groups to adjust their size based on food availability and feeding competition, with females being able to move to smaller groups and experience less feeding competition [19][20][21]. In female-philopatric species, however, group size cannot decrease easily when feeding competition increases, possibly leading to stronger relationships between groups size and home range size.…”

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

“…The infanticide rate after death of the silverback (leading to group disintegration) is quite high (12%) in western lowland gorillas at Mbeli (Breuer et al 2010 ; Robbins et al 2013 ). With their strategy of voluntary secondary dispersal, female gorillas may be able to choose a better male before the death of the current silverback and protect their offspring from infanticide (Palombit 2015 ; Manguette et al 2019 , 2020 ).…”

No evidence for female kin association, indications for extragroup paternity, and sex‐biased dispersal patterns in wild western gorillas

“…Moreover, it improves reproductive success by limiting intragroup feeding competition. Therefore, western gorilla females seem to show unconditionally a strategy of natal dispersal by departing from the group in which they were born (Baudouin et al., 2019; Manguette, Robbins et al., 2020; Stokes et al., 2003).…”

“…Our findings are also consistent with field observations on western gorillas. Females transfer from one group to a neighboring group during intergroup encounters which likely does not range far from their natal group (Manguette, Robbins et al., 2020; Stokes et al., 2003). Later, both voluntary (e.g., predispersal, in aging groups) and involuntary (e.g., as a consequence of silverback death or group disaggregation) secondary transfers allow them to disperse wider, increasing the likelihood that they will reside in a group with no or little kin (Manguette, Robbins et al., 2020; Stokes et al., 2003).…”