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Some cross-cultural evidence suggests lethal coalitionary aggression in humans is the product of residence and descent rules that promote fraternal interest groups, i.e., power groups of coresident males bonded by kinship. As such, human lethal coalitions are hypothesized to be homologous to chimpanzee (Pan troglodytes) border patrols. However, humans demonstrate a unique metagroup social structure in which strategic alliances allow individuals toform coalitions transcending local community boundaries. We test predictions derived from the fraternal interest group and strategic alliance models using lethal coalition data from a lowland South American population, the Yanomamö. Yanomamö men who kill an enemy acquire a special status, termed unokai. We examine the social characteristics of co-unokais or men who jointly kill others. Analyses indicate co-unokais generally are (i) from the same population but from different villages and patrilines, (ii) close age mates, and (iii) maternal half-first cousins. Furthermore, the incident rate for co-unokai killings increases if men are similar in age, from the same population, and from different natal communities. Co-unokais who have killed more times in the past and who are more genetically related to each other have a higher probability of coresidence in adulthood. Last, a relationship exists between lethal coalition formation and marriage exchange. In this population, internal warfare unites multiple communities, and co-unokais strategically form new residential groups and marriage alliances. These results support the strategic alliance model of coalitionary aggression, demonstrate the complexities of human alliance formation, and illuminate key differences in social structure distinguishing humans from other primates. [ABSTRACT FROM AUTHOR]

Partible paternity, the conception belief that more than one man can contribute to the formation of a fetus, is common in lowland South America and characterized by nonexclusive mating relationships and various institutionalized forms of recognition and investment by multiple cofathers. Previous work has emphasized the fitness benefits for women where partible paternity beliefs facilitate paternal investment from multiple men and may reduce the risk of infanticide. In this comparative study of 128 lowland South American societies, the prevalence of partible paternity beliefs may be as much as two times as common as biologically correct beliefs in singular paternity. Partible paternity beliefs are nearly ubiquitous in four large language farnilies—Carib, Pano, Tupi, and Macro-Je. Phylogenetic re- construction suggests that partible paternity evolved deep in Amazonian prehistory at the root of a tentative Je-Carib-Tupi dade. Partible paternity often occurs with uxorilocal postmarital residence (males transfer), although there are exceptions. Partible paternity may have benefits for both sexes, especially in societies where essentially all offspring are said to have multiple fathers. Despite a decrease in paternity certainty, at least some men probably benefit (or mitigate costs) by increasing their number of extramarital partners, using sexual access to their wives to formalize male alliances, and/or sharing paternity with close kin. [ABSTRACT FROM AUTHOR]

Use of space by both humans and other mammals should reflect underlying physiological, ecological, and behavioral processes. In particular, the space used by an individual for its normal activities should reflect the interplay of three constraints: (i) metabolic resource demand, (ii) environmental resource supply, and (iii) social behaviors that determine the extent to which space is used exclusively or shared with other individuals. In wild mammals, there is an allometric scaling relation between the home range of an individual and its body size: Larger mammals require more space per individual, but this relation is additionally modified by productivity of the environment, trophic niche, sociality, and ability to defend a territory [Kelt DA, Van Vuren D (1999) Ecology 80: 337-340; Kelt DA, Van Vuren D (2001) Am Nat 157:637-645; Haskell JP, Ritchie ME, Olff H (2002) Nature 418:527-530; Damuth J (1987) Biol J Linn Soc 31:193-246; Damuth J (1981) Nature 290:699-700; and other previously published work]. In this paper we show how similar factors affect use of space by human hunter-gatherers, resulting in a nonlinear scaling relation between area used per individual and population size. The scaling exponent is less than one, so the area required by an average individual decreases with increasing population size, because social networks of material and information exchange introduce an economy of scale. [ABSTRACT FROM AUTHOR]

The biogeographic expansion of modern humans out of Africa began ≈50,000 years ago. This expansion resulted in the colonization of most of the land area and habitats throughout the globe and in the replacement of preexisting hominid species. However, such rapid population growth and geographic spread is somewhat unexpected for a large primate with a slow, density-dependent life history. Here, we suggest a mechanism for these outcomes by modifying a simple density-dependent population model to allow varying levels of intra-specific competition for finite resources. Reducing intra-specific competition increases carrying capacities, growth rates, and stability, including persistence times and speed of recovery from perturbations. Our model suggests that the energetic benefits of cooperation in modern humans may have outweighed the slow rate of human population growth, effectively ensuring that once modern humans colonized a region long-term population persistence was near inevitable. Our model also provides insight into the interplay of structural complexity and stability in social species. [ABSTRACT FROM AUTHOR]