Group size is a function of relative neocortical volume in nonhuman primates. Extrapolation from this regression equation yields a predicted group size for modern humans very similar to that of certain hunter-gatherer and traditional horticulturalist societies. Groups of similar size are also found in other large-scale forms of contemporary and historical society. Among primates, the cohesion of groups is maintained by social grooming; the time devoted to social grooming is linearly related to group size among the Old World monkeys and apes. To maintain the stability of the large groups characteristic of humans by grooming alone would place intolerable demands on time budgets. It is suggested that (1) the evolution of large groups in the human lineage depended on the development of a more efficient method for time-sharing the processes of social bonding and that (2) language uniquely fulfills this requirement. Data on the size of conversational and other small interacting groups of humans are in line with the predictions for the relative efficiency of conversation compared to grooming as a bonding process. Analysis of a sample of human conversations shows that about 60% of time is spent gossiping about relationships and personal experiences. It is suggested that language evolved to allow individuals to learn about the behavioural characteristics of other group members more rapidly than is possible by direct observation alone.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0140525X00032325

Coevolution of neocortical size, group size and language in humans

Following Darwin's original insights regarding sexual selection, studies of intrasexual competition have mainly focused on male competition for mates; by contrast, female reproductive competition has received less attention. Here, we review evidence that female mammals compete for both resources and mates in order to secure reproductive benefits. We describe how females compete for resources such as food, nest sites, and protection by means of dominance relationships, territoriality and inter-group aggression, and by inhibiting the reproduction of other females. We also describe evidence that female mammals compete for mates and consider the ultimate causes of such behaviour, including competition for access to resources provided by mates, sperm limitation and prevention of future resource competition. Our review reveals female competition to be a potentially widespread and significant evolutionary selection pressure among mammals, particularly competition for resources among social species for which most evidence is currently available.

We report that female competition is associated with many diverse adaptations, from overtly aggressive behaviour, weaponry, and conspicuous sexual signals to subtle and often complex social behaviour involving olfactory signalling, alliance formation, altruism and spite, and even cases where individuals appear to inhibit their own reproduction. Overall, despite some obvious parallels with male phenotypic traits favoured under sexual selection, it appears that fundamental differences in the reproductive strategies of the sexes (ultimately related to parental investment) commonly lead to contrasting competitive goals and adaptations. Because female adaptations for intrasexual competition are often less conspicuous than those of males, they are generally more challenging to study. In particular, since females often employ competitive strategies that directly influence not only the number but also the quality (survival and reproductive success) of their own offspring, as well as the relative reproductive success of others, a multigenerational view ideally is required to quantify the full extent of variation in female fitness resulting from intrasexual competition. Nonetheless, current evidence indicates that the reproductive success of female mammals can also be highly variable over shorter time scales, with significant reproductive skew related to competitive ability.

Whether we choose to describe the outcome of female reproductive competition (competition for mates, for mates controlling resources, or for resources per se) as sexual selection depends on how sexual selection is defined. Considering sexual selection strictly as resulting from differential mating or fertilisation success, the role of female competition for the sperm of preferred (or competitively successful) males appears particularly worthy of more detailed investigation. Broader definitions of sexual selection have recently been proposed to encompass the impact on reproduction of competition for resources other than mates. Although the merits of such definitions are a matter of ongoing debate, our review highlights that understanding the evolutionary causes and consequences of female reproductive competition indeed requires a broader perspective than has traditionally been assumed. We conclude that future research in this field offers much exciting potential to address new and fundamentally important questions relating to social and mating-system evolution.

/pdf/female-competition-and-its-evolutionary-consequences-in-3z0dv5szr1.pdf

Female competition and its evolutionary consequences in mammals

We hypothesise that foraging group size (FGS) and population group size (PGS) in primates and carnivores are related to quantifiable variables indexing the intensity of exploitative competition. Group size is predicted to increase with both food density and travel capabilities, as estimated by the “constraint-free day-range” (DRs), i.e. the day-range of a solitary individual uninfluenced by competition from conspecifics. We test this “exploitation competition” hypothesis among primates and carnivores, using data on populations, species and genera. Food density was indexed by population density. Where DRs could be estimated by regression it was found to be correlated with observed day-range (DRm). DRs was therefore indexed by DRm in all species. Population density and DRm were associated with each other, but in a multiple regression each variable contributed independently to explaining variation in group size. PGS was predicted better than FGS, but regressions involving either measure of group size were significant in all analyses. The multiple regression analyses were validated by the method of linear contrasts, which accounts for possible lack of independence among taxa. We conclude that species differences in group size are influenced by both food density and DRs. This suggests that variation in the intensity of exploitation competition is partly responsible for differences in group size.

Constraints on group size in primates and carnivores: population density and day-range as assays of exploitation competition

Primates, Secondary compounds, Feeding behavior, Nutrients, ABSTRACT The recent literature on plant secondary compounds and their influence on primate feeding behavior is reviewed. Many studies of nonhuman primates document the extreme selectivity that primates, particularly herbivo- rous species, demonstrate in their food choice. Until quite recently investigators interpreted this to mean that herbivorous primates were not food limited. This view has been challenged in the past 10 years by researchers concentrating on the primate-plant interaction. Chemical analyses have demonstrated that plant parts are of varying quality due to differences in nutrient and secondary compound content. The assumption that all leaves (or fruits, flowers, and insects) are po- tential foods of equal value to the primates eating them is refuted. The observed selectivity and preferences of primates for specific plant or insect species and parts are now viewed as strategies for dealing with the nutrient and secondary compound content variation in these foods. The field of plant-herbivore interaction is a rapidly expanding one that includes the phytochemical relationship between plants and herbivores. It has long been acknowl- edged that plants and insects profoundly influenced each others' evolutionary courses, but only recently have the biological and ecological roles of plant-produced secondary compounds been recognized. Stahl (1888) was the first to suggest that plants use chemical defenses in addition to the more familiar morphological and mechanical means of protecting themselves. Despite this early observation, plant produced chemicals were considered nothing more than waste products of plant metabolism until Fraenkel (1959) again suggested a defensive function for these compounds. Since then impressive evidence, which dem- onstrates that plants do use secondary compounds for defensive purposes, has accu- mulated (Ehrlich and Raven, 1965; Feeny, 1968; Janzen, 1969; Fthoades and Cates, 1976; Rosenthal and Janzen, 1979; and included references). Mothes (1976) went so far to suggest that secondary substances may be primarily defensive in nature and are neither essential nor of any physiological importance to the plants that produce them. He stated that the loss of these chemicals does not reduce the plant's viability.

/pdf/the-impact-of-plant-secondary-compounds-on-primate-feeding-35fa8yttw9.pdf

R. W. Wrangham

Papers

Ecological and Sociological Studies of Gelada Baboons