Intertroop relations of chacma baboon (Papio ursinus)

Male services during between-group conflict: the 'hired gun' hypothesis revisited

In many group-living mammals, philopatric females form the stable core of the group and defend food or shelter against other groups of females. Where males are larger, their participation could give their female group the edge. How can females secure the contribution of males that are neither the father of current infants, nor the dominant male expecting to sire the next generation of infants? It has been proposed that females recruit these males as 'hired guns', receiving social support and copulations in exchange for fighting, against the interests of the dominant male. We first develop the logic of this hypothesis in unprecedented detail by considering the potential pay-off consequences for females and males. We then provide empirical evidence for the existence of hired guns in this context in several primate species. The game-theoretical aspects of the phenomenon remain to be studied, as is the distribution across contexts (e.g. predation avoidance) and species of the hired gun phenomenon. This article is part of the theme issue 'Intergroup conflict across taxa'.

Comparative ecology of Guinea baboons (Papio papio)

Thorough knowledge of the ecology of a species or population is an essential prerequisite for understanding the impact of ecology on the evolution of their respective social systems. Because of their diversity of social organizations, baboons (Papio spp.) are a useful model for comparative studies. Comparative ecological information was missing for Guinea baboons (Papio papio), however. Here we provide data on the ecology of Guinea baboons in a comparative analysis on two geographical scales. First, we compare climate variables and land cover among areas of occurrence of all six baboon species. Second, we describe home range size, habitat use, ranging behaviour, and diet from a local population of Guinea baboons ranging near the Centre de Recherche de Primatologie (CRP) Simenti in the Niokolo-Koba National Park, Senegal. Home ranges and daily travel distances at Simenti varied seasonally, yet the seasonal patterns in their daily travel distance did not follow a simple dry vs. rainy season pattern. Chemical food composition falls within the range of other baboon species. Compared to other baboon species, areas occupied by Guinea baboons experience the highest variation in precipitation and the highest seasonality in precipitation. Although the Guinea baboons' multi-level social organization is superficially similar to that of hamadryas baboons (P. hamadryas), the ecologies of the two species differ markedly. Most Guinea baboon populations, including the one at Simenti, live in more productive habitats than hamadryas baboons. This difference in the ecology of the two species contradicts a simple evolutionary relation between ecology and social system and suggests that other factors have played an additional role here.

Mate guarding and risk assessment by male mountain baboons during inter-troop encounters

Aggressive herding of females is a frequent but not invariant response by male savannah baboons, Papio cynocephalus, to encounters with other troops. While males in some troops are consistently more likely to herd than those in others, not all inter-troop encounters result in herding, even within particular troops. This suggests that males assess the risk of male invasion posed by each encounter and respond accordingly. We used data from baboon troops in the Drakensberg mountains to determine the rules males follow in deciding whether to herd. Consistent differences between troops were explained only by the adult sex ratio. Males were more likely to herd if the sex ratio of their own troop was female biased, a finding that is concordant with the observation that males are more likely to immigrate into troops where the sex ratio is more female biased than the population average. Differences within troops were a consequence only of encounter distance, with herding being more likely at closer distances. We found a negative correlation between the angle of approach to the other troop and the subsequent angle of deflection. We interpret this to mean that herding functions to increase the distance between the interacting troops, thereby curtailing opportunities for strange males to inspect the troop and determine its sex ratio. In this way, possibly unlike those in other populations, the decision rules of these male baboons are geared to protecting longer-term reproductive prospects. Copyright 1998 The Association for the Study of Animal Behaviour. Copyright 1997 The Association for the Study of Animal Behaviour.

Male investment under changing conditions among chacma baboons at Suikerbosrand

Male investment in infant baboons was measured by frequency of carrying from 1978 through 1985. A series of hypotheses was generated and tested with the carrying data, based on the assumptions that: male baboons have some capacity to estimate likelihood of paternity; where paternity probability is greater, males will invest more, where potential benefits to males or infants are higher, males will invest more. Carrying was affected by probability of paternity, availability of estrous females, season of conception and season of carrying, infant age, subtrooping, and predation risk. Infants were carried by probable fathers, siblings, mothers' siblings, and unrelated "suitors." Male investment increased female reproductive fitness: carried infants were more likely to survive, and mothers of carried infants had shorter interbirth intervals. Males appeared to estimate paternity both by observed copulations by other males and by the likelihood that copulations could have occurred without being observed. Male care of infant baboons may also be affected by female choice among males, the distribution of probable infants in time, male tenure at alpha rank, the number of males per troop, the probability of infanticide, and energy demands. Subtrooping seems to be historically crucial, by initially creating a situation in which some males have high paternity certainty.

The spread of exclusive mating in a chacma baboon population

Mating has progressively tended toward pair bonding in a chacma baboon (Papio cynocephalus ursinus) population during eight years of study. Both males and females have been mating with a smaller and smaller percentage of the potential mates available to them. Troop subdivision during the dry winter in the absence of predation was the original cause of mating limitation, but most members of winter subtroops only rarely mated with adults who had not belonged to the same winter subtroop during the summers as well. Following reintroduction of potential predation, both subtroop size and frequency decreased, but mating did not become less exclusive.

Female transfer in baboons

Eight of 19 identified adult females transferred over 7 years in a study of intertroop relations among Papio ursinus at Suikerbosrand. Only one of the eight was cycling at the time of transfer. Five of the seven recent mothers who transferred had lost their infants shortly before transferring. Females who transferred received no more aggression before or after their transfer than females who did not transfer. The most important factors determining this extraordinarily high rate of female transfer include subtrooping and absence of predation.

Levels of social organization and male-female bonding in the genus Papio

Pair-bonding in the genus Papio seems to be the result of fusion of troops as well as fission. When troop segments regularly split and join again, males who permanently maintain exclusive access to a few females may have an advantage over males who compete for all females who are in estrus. When two or more troops regularly fuse, this advantage may be greatly increased.