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Social grooming

The Primate Socioecological Model was developed in an attempt to explain variations in size and structure exhibited within primate groups through contrasts within food distributions across varying environments. The traditional model directly opposes the explanation of predation pressure as the driving force in the formation of social networks within primates, although this has been reassessed by later researchers. The approach differed from previous models in that it selected to examine strictly female distribution and behavior, under the assumption that variables in males are governed primarily by the distribution of available mating partners. At its core, the primate socioecology model proposes that females form groups in order to defend unevenly distributed resources. The benefit of between-group competition then outweighs the costs of within-group competition. The selection for individuals who are willing to work cooperatively then promulgates the building of strong social and kin-ties.

Theory and Approach

Background and Framework

The primate socioeconomic model was born out of advances in studies conducted in the 1970s which undertook comparative analyses of a variety of mammalian species, indicating that the distribution of resources could greatly affect the levels of competition when feeding, as well as affecting the costs of sociability. These conclusions indicate that resource availability could be a driving factor in determining the size of social groups. In 1980 this research was linked with research into variable group compositions and resource competition strategies among primates by Richard Wrangham. Although Wrangham did not use the term within his paper, the model which he developed would become known as the primate socioeconomic model. Key to Wrangham’s model was the suggestion that in cases in which food availability was distributed in patches across the landscape selection would favor the formation of dominance hierarchies in which groups of related, natal, females could better defend access. In contrast to this, in environments in which food resources are spread evenly throughout, a reduction in competition would result in less stable kin groups and dominance hierarchies would be subdued, resulting in the dispersal of females.

Criteria for defining female-bonded groups

One of the prime assumptions of the primate socioecological model is that it is the structure of the females within the group which will be most greatly affected by food resource availability, and it is because of this that females are of the greatest interest within the model. The logic behind this being that male strategy will be driven first and foremost by access to fertile females. Because of this, the social of females with other females becomes a key to understanding the development of sociality and its relationship to resource availability. The term ‘female-bonded’ was thus employed to make distinctions between species in which this sociality has developed. Three criteria were established to properly categorize female-bonded species:

• Female breeding within natal groups
• Systematic intragroup movement by males with restricted movement of females
• For groups in which transfers and residence patterns are unknown, differentiated and consistent female relationships based upon evidence of grooming, aiding, huddling, or interactions of dominance are employed as evidence

Analysis of primate species under this criteria indicates that female-bonded groups (Table 1), being represented by 24 known species, are much more common that non-female-bonded groups (Table 2), being represented by only 4 known species.

Ring-tailed Lemur Baby

Japanese Monkies at Jigokudani hotspring in Nagano Japan

Papio anubis

Table 1- Female Bonded Species

Species	Common Name
Lemur catta	Ringtailed lemur
Propithecus verreauxi	White sifaka
Alouatta villosa	Mantled howler
Alouatta seniculus	Red howler
Cebus capucinus	White-throated capuchin
Saimiri sciureus	Squirrel monkey
Cercopithecus aethiops	Vervet monkey
Ceropithecus ascanius	Redtail monkey
Cercopithecus mitis	Blue monkey
Cercocebus albigena	Grey-cheeked mangabey
Erythrocebus patas	Patas
Papio anubis	Olive baboon
Papio cynecephalus	Yellow baboon
Papio ursinus	Chacma baboon
Theropithecus gelada	Gelada
Macaca fuscata	Japanese monkey
Macaca mulatta	Rhesus monkey
Macaca radiala	Bonnet macaque
Macaca sinica	Toque macaque
Macaca sylvanus	Barbary macaque
Macaca fascicularis	Crab-eating monkey
Presbytis entellus	Hanuman langur
Presbytis senex	Purple-faced leaf monkey
Colobus guereza	Black-and-white colobus

Table 2- Non-Female Bonded Species

Species	Common Name
Colobus babius	Red colobus
Papio hamadryas	Hamadryas baboon
Pan gorilla bergei	Mountain gorilla
Pan troglodytes	Chimpanzee

Competition and cooperation

Wrangham identified that the primary advantage to cooperation and sociability within female primates is that of environments which contain patchy resources. In these conditions female-bonded individuals are more likely to succeed in defending the limited resources than those which do not cooperate. This promotes competitors to also adopt cooperative strategies in order to displace cooperative defenders. Selection thereby will favor those who are able to form alliances, specifically those who can form alliances larger than their adversaries. The control factor limiting the uncontrolled growth of alliances then becomes the environment which the groups resides within. If the feeding sites within a given patch are lower than the number of individuals within the alliance, groups will fragment. By this then the maximum group size will be dictated by the sustainability of the environment.

Later researchers further refined this to incorporate varying intragroup and intergroup competitions. These include within-group scrambles, within-group contests, between-group scrambles, and between-group contests. A scramble, or scramble competition, is defined as a situation in which a given patch of resources is finite and open to all competitors. A contest, alternatively is an altercation in which one group has an initial monopoly over the given resource.

One of the great paradoxes left unanswered within Wrangham’s original development of the model was the mechanisms which caused female-bonding to occur in groups which do not defend resources. This was also addressed by later researchers who postulated that the selective pressures caused by predation were more intrical than appreciated by Wrangham. It was postulated that predation would have played a sufficient role in primates to explain why groups are formed, however it is the level of food dispersion which dictates whether alliances are selected for or not. Several of these new concepts, developed by Bernard Thierry 2008, were incorporated within Wranham’s original model.

The role of males

While the role of the male in the primate socioeconomic model is subdued, it is not absent. The male's function within the model is dictated by the specific defensive strategy developed to address specific environmental conditions. Within environments in which food density is high, the establishment of territories often occurs. The maximum group size is thus established by the finite food supply within the territory. Immigrant males are at a sever disadvantage under these circumstances due to pre-established, natal alliance systems among resident females. Females are then likely to capitalize on male-male competitions by supporting one male against another. This has resulted in the development of single-male groups within territorial species. The scenario is quite different for groups within environments containing patchily distributed resources. These environments promote the development of non-territorial groups. In this scenario there is no finite food limit beyond that which can be taken from neighboring groups. This promulgates the development of larger groups which can then displace neighboring groups, resulting in a need for multiple males. Beyond this the model provides very little by way of prediction or expectation for males within female-bonded groups.

Implications

Wrangham acknowledged the qualitative nature of the primate socioeconomic model which he developed stating, "Caution is appropriate in interpreting the tests of the model. given the qualitative nature of the comparisons, the small number of non-[female-bonded] species and the lack of competing hypotheses the fit between theory and data may be more apparent than real. Further and more exacting tests are therefore required." Several novel approaches were used within the study which had not been attempted before in developing explanations for the development of primate sociability. Primary among these was the examination of the roles and importance of females in establishing social systems. Additionally, the examination of feeding competition differentiated Wrangham's model from its predecessors.

The primary implications of the primate socioecology model are that:

• Female sociability within primates has evolved out of competition for food resources
• The formation of groups does not explicitly benefit the species as a whole. It develops due to the fact that cooperation is evolutionarily stable and unbeatable by alternative strategies
• The optimal size of one group can be affected by neighboring groups. Larger neighboring groups will select for individuals who are capable of forming and working within an alliance system
• Contrary to previously held beliefs, cooperative behavior is beneficial to all members of a group
• While ecological pressures may select for cooperation, it is genetic consideration which favors kin as these allies

Critique

Several shortcomings of the primate socioeconomic model have been recognized since its original inception by Wrangham in 1980. These critiques have primarily focused upon the inherent assumptions of the theory, such as the negligible role of predation and a lack of consideration regarding male reproductive strategies and life histories upon the evolution of sociality in primates. Additionally it failed to incorporate any theory concerning the evolution of sociability within other species as it may pertain or contrast with that of primates. Mechanically the model fails to allow for the implementation of standardized quantitative measures, remaining primarily a subjective, qualitative approach. Clutton-Brock and Janson took a critical approach to the model in their 2012 publication, outlining several challenges within the model and providing potential solutions in an attempt to update and guide further study. These challenges and potential solutions are listed below

Challenge: Lack of consistent relationships between diet and feeding competition. Potential Solution: Improve measures of food distribution and feeding competition to provide a reliable basis for measuring continuous variation between populations and species. Manipulate food distribution and observed effects on conspecifics.

Challenge: Lack of consistent association between different aspects of social behavior across species. Potential Solution: Test degree of association between traits; where necessary, focus on the distribution of particular traits rather than syndromes.

Challenge: Lack of consistent association between species differences in feeding competition and social organization. Potential Sollution: Focus on particular traits; control for effects of phylogenetic variation; explore the impact of variation in life-history parameters and reproductive strategies in both sexes; reconstruct evolutionary sequences to identify adaptive constraints.

Challenge: Correlation between ecological parameters and specific differences in social behavior differ from those between ecological parameters and intraspecific differences. Potential Solution: Distinguish between questions concerning the evolution of adaptation and questions concerning the immediate consequences of ecological variation. In general, to investigate evolutionary questions, explore the correlates of interspecific differences or selection operating on individuals within populations; to assess the immediate consequences of contrast in feeding ecology or population density, compare the same population at different times or different populations. But remember that interpopulation differences can also be a consequence of contrasts in selection, while interspecific differences can be a consequence of evolutionary constraints.

Challenge: Effects of variation in food distribution on interspecific differences cannot be separated from those of differences in population density or demographic structure. Potential Solution: To identify effects of population density, examine the effects of temporal changes in density on social behavior in the same population; to assess consequences of interpopulation differences, control for density effects or manipulate food distribution.

Challenge: Lack of consistent relationships between ecological parameters and dominance relations. Potential Solution: Improve indices of the consistency of outcome in interactions to provide indices that can be used to compare species, populations, and individuals; consider effects of reproductive competition on dominance relations; explore the costs as well as the benefits of establishing dominance.

Challenge: Constraints in the effects of variation in kinship on social relationships. Potential Solution: Improve measurement of kinship; incorporate realistic estimates of benefit and cost; focus on circumstances or periods where competition is likely; recognize that benefits of discrimination are likely to decline as average relatedness increases.

Challenge: Lack of obvious pattern in the distribution of female dispersal. Potential Solution: Focus on the distribution of philopatric breeding and female immigration; distinguish between singular and plural breeders; separate analyses of the origin of breeders, the probability of dispersal, and dispersal distance; incorporate the role of male tenure length and mating strategies on female dispersal decisions

Challenge: Taxonomic variation in behavioral parameters. Potential Solution: Investigate evolutionary transitions and the circumstances in which they occur as well as the distribution of traits among contemporary species; where the number of transitions is low, extend analyses to include related taxonomic groups.

While Clutton-Brock and Janson believe that reassessment and a critical analysis of the primate socioeconomic model can breathe new life into it, a more pessimistic appraisal has been lodged by Thierry, who in his 2008 publication stated, “Significant progress was made through the efforts of those who conceived and tested it, but further ‘synthesis’ would turn the model into a theory of everything, liable to explain any social organization through a combination of ad hoc arguments. My own falsifiable prediction is that the next-generation model is stillborn; there will be no future version of the synthetic model able to bring fresh insights into the causal relations responsible for primate social diversity.”