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

Evolution of Multilevel Social Systems in Nonhuman Primates and Humans

Multilevel (or modular) societies are a distinct type of primate social system whose key features are single-male-multifemale, core units nested within larger social bands. They are not equivalent to fission-fusion societies, with the latter referring to routine variability in associations, either on an individual or subunit level. The purpose of this review is to characterize and operationalize multilevel societies and to outline their putative evolutionary origins. Multilevel societies are prevalent in three primate clades: papionins, Asian colobines, and hominins. For each clade, we portray the most parsimonious phylogenetic pathway leading to a modular system and then review and discuss likely socioecological conditions promoting the establishment and maintenance of these societies. The multilevel system in colobines (most notably Rhinopithecus and Nasalis) has likely evolved as single-male harem systems coalesced, whereas the multilevel system of papionins (Papio hamadryas, Theropithecus gelada) and hominins most likely arose as multimale-multifemale groups split into smaller units. We hypothesize that, although ecological conditions acted as preconditions for the origin of multilevel systems in all three clades, a potentially important catalyst was intraspecific social threat, predominantly bachelor threat in colobines and female coercion/infanticide in papionins and humans. We emphasize that female transfers within bands or genetic relationships among leader males help to maintain modular societies by facilitating interunit tolerance. We still lack a good or even basic understanding of many facets of multilevel sociality. Key remaining questions are how the genetic structure of a multilevel society matches the observed social effort of its members, to what degree cooperation of males of different units is manifest and contributes to band cohesion, and how group coordination, communication, and decision making are achieved. Affiliative and cooperative interunit relations are a hallmark of human societies, and studying the precursors of intergroup pacification in other multilevel primates may provide insights into the evolution of human uniqueness.

"Friendships" between new mothers and adult males: adaptive benefits and determinants in wild baboons (Papio cynocephalus)

Close associations between adult males and lactating females and their dependent infants are not commonly described in non-monogamous mammals. However, such associations [sometimes called "friendships" (Smuts 1985)] are regularly observed in several primate species in which females mate with multiple males during the fertile period. The absence of mating exclusivity among "friends" suggests that males should invest little in infant care, raising questions about the adaptive significance of friendship bonds. Using data from genetic paternity analyses, patterns of behavior, and long-term demographic and reproductive records, we evaluated the extent to which friendships in four multi-male, multi-female yellow baboon (Papio cynocephalus) groups in Amboseli, Kenya represent joint parental care of offspring or male mating effort. We found evidence that mothers and infants benefited directly from friendships; friendships provided mother-infant dyads protection from harassment from other adult and immature females. In addition, nearly half of all male friends were the genetic fathers of offspring and had been observed mating with mothers during the days of most likely conception for those offspring. In contrast, nearly all friends who were not fathers were also not observed to consort with the mother during the days of most likely conception, suggesting that friendships between mothers and non-fathers did not result from paternity confusion. Finally, we found no evidence that prior friendship increased a male's chances of mating with a female in future reproductive cycles. Our results suggest that, for many male-female pairs at Amboseli, friendships represented a form of biparental care of offspring. Males in the remaining friendship dyads may be trading protection of infants in exchange for some resources or services not yet identified. Our study is the first to find evidence that female primates gain social benefits from their early associations with adult males.

Mitochondrial phylogeny and systematics of baboons (Papio)

Baboons (Papio, s.s.) comprise a series of parapatric allotaxa (subspecies or closely related species) widely distributed in sub-Saharan Africa. Despite extensive studies of their ecology, morphology, and behavior, disagreement about their phylogenetic relationships continues, as expressed in the current coexistence of at least three major, competing taxonomic treatments. To help resolve this situation, we sequenced approximately 900 bases of mitochondrial DNA of 40 individuals from five of the widely recognized "major" allotaxa. Total sequence diversity (>5%) is high compared to most primate species. Major mitochondrial clades correspond to recognized allotaxa, with the important exception that haplotypes from yellow and olive baboons form a single, monophyletic clade within which the two allotaxa do not comprise mutually exclusive clusters. The major clades fall unambiguously into the pattern: (chacma (Guinea (hamadryas (yellow + olive)))). This phylogeny does not support taxonomies that oppose hamadryas to all other baboons ("desert" vs. "savanna"), but is compatible with the view that all definable allotaxa should be recognized as coordinates, either as "phylogenetic" species or "biological" subspecies. The close relationship and unsegregated distribution of haplotypes from Kenyan and Tanzanian yellow and olive baboons are unexplained, but may reflect introgression across the documented hybrid zone. The overall phylogeny, when combined with paleontological data, suggests a southern African origin for extant Papio baboons, with all extant lineages sharing a common mitochondrial ancestor at approximately 1.8 Ma.

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.

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.