Immigrant males’ memory acts to reduce ranging overlap and mating competition in wild baboons

Mechanistic models suggest that information acquired by animals
(“knowledge”) could shape home range patterns and dynamics, and how
neighbours share space. In social species this would suggest that
immigrants could bring new knowledge into social groups, potentially
affecting the dynamics of home range overlap. We tested this “immigrant
knowledge hypothesis” in a wild population of chacma baboons (Papio
ursinus). We used data collected between 2005 and 2013 on two neighbouring
troops in Namibia, comprising GPS records of daily ranges, male natal
origins, daily females’ reproductive status, and a satellite index of
vegetation growth. We found that when the ratio of fertile females to
adult males in the focal troop increased (i.e. increasing inter-troop
mating competition costs for focal troop males), the focal troop tended to
overlap less with a neighbouring troop’s home range only when the alpha
male had immigrated from that neighbouring troop and so was
“knowledgeable” about its home range. When the alpha male was “not
knowledgeable” the reverse was observed, with troops showing greater
overlap, most likely reflecting an influence of fertile females on ranging
patterns. Our findings support the immigrant knowledge hypothesis of group
ranging behaviour, and suggest that dispersal and spatial memory may
sometimes act to increase rather than decrease spatial segregation between
groups. Very few previous studies have reported an effect of mating
competition on ranging dynamics in social species, perhaps because
previous studies did not consider divergent adaptive interest among group
members and how they can be solved through collective-decision processes.