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Cooperative Breeding and Its Significance to the Demographic AN39CH25-Kramer ARI 16 August 2010 19:4 Cooperative Breeding and its Significance to the Demographic Success of Humans Karen L. Kramer Department of Human Evolutionary Biology, Harvard University, Cambridge, Massachusetts 02138; email: [email protected] Annu. Rev. Anthropol. 2010. 39:417–36 Key Words First published online as a Review in Advance on life history, intergenerational transfers, allocare, demography June 21, 2010 The Annual Review of Anthropology is online at Abstract anthro.annualreviews.org The demographic success of humans compared with other closely re- This article’s doi: lated species can be attributed to the relatively rapid pace of reproduc- 10.1146/annurev.anthro.012809.105054 tion and improved chances of survival. The assistance that mothers re- Copyright c 2010 by Annual Reviews. ceive from others to help raise children is a common theme in explaining All rights reserved this gain in surviving fertility. Cooperative breeding in its broad defini- 0084-6570/10/1021-0417$20.00 tion describes such a social system in which nonmaternal helpers support offspring who are not their own. In traditional societies, kin and nonkin of different ages and sex contribute both to child care and to provision- ing older children. This review discusses empirical evidence for human cooperative breeding and its demographic significance and highlights the ways in which humans are similar to and different from other co- operative breeders. An emphasis is placed on cross-cultural comparison and variability in allocare strategies. Because helping in humans occurs within a subsistence pattern of food sharing and labor cooperation, both kin selection and mutualism may explain why children are often raised with nonmaternal help. Cooperative breeding is relevant to debates in anthropology concerning the evolution of human life history, social- ity, and psychology and has implications for demographic patterns in today’s world as well as in the past. 417 AN39CH25-Kramer ARI 16 August 2010 19:4 INTRODUCTION Are Humans Cooperative Breeders? The human capacity for population growth Definition of the term cooperative breeding has is one of the remarkable stories of our evo- undergone recent debate and reconsideration lutionary history. Demographically it can be (Clutton-Brock 2006, Cockburn 1998, Russell attributed to short birth intervals and high 2004, Strassman & Kurapati 2010). Because its survivorship. But it is also fundamentally usage lacks consensus, especially in its appli- shaped by features of human parenting, social- cation to humans, cooperative breeding is used ity, and economic organization. Cooperative here in its broadest historic sense as a social sys- breeding combines these features and is a tem in which nonmaternal individuals help sup- useful framework to consider child-rearing port offspring who are not their own. Another patterns characteristic of humans. term, such as social parenting, may also be suit- Cooperative breeding refers to a parent- able. In lieu of developing a new vocabulary, the ing and social system in which nonparental cooperative breeding literature provides a rich members of the social group help support theoretic and empirical background with which offspring. Cooperative breeding models were to comparatively situate human parenting. originally developed to describe the parenting Because human mothers routinely rely on behavior of certain insects, birds, and mammals the help of others to raise young, humans (Brown 1974, Emlen 1991[1978], Skutch 1987, share many features in common with other Solomon & French 1997). Turke’s (1988) sem- cooperative breeders. But human parenting inal study among Micronesian islanders first in- and reproduction are also distinct in several troduced humans as cooperative breeders into key ways. First, the formation of cooperative anthropology. Turke showed that mothers who breeding in many species of birds and mammals bore girls, who are valuable helpers to their is broadly associated with delayed dispersal. mothers, early in their reproductive careers had Sexually mature offspring may delay leaving greater completed fertility than if their first- their natal territory and initiating reproduc- born children were boys. Since then, anthropol- tion when constraints exist either in mating ogists have paid increasing attention to humans opportunities or in the availability of resources as cooperative breeders. or territory to reproduce successfully (Emlen Cooperative breeding has been documented 1995, Woolfenden & Fitzpatrick 1984). In for ∼3% of bird species and for a similar per- several studies of historic Europe, late age centage of mammals. Although uncommon, at marriage has been related to ecological cooperative breeding occurs across diverse constraints (Strassman & Clarke 1998, Voland mammalian taxa: predominantly wild canids, et al. 1991). Delayed dispersal, however, is not foxes, meerkats, rodents, and several species a necessary condition for human cooperation in of primates, including humans (Brown 1987, raising children. Second, cooperative breeding Clutton-Brock et al. 2001, Emlen 1991[1978], in many species of birds and mammals tends Nicolson 1987). Among nonhuman cooper- to be associated with female reproductive ative breeders, helpers may guard, nurse, or suppression and reproduction by one or a small transport young, help forage for food, defend group of dominant females. In contrast, human territory boundaries, or build and clean nests. mothers acquire help without suppressing The occurrence of these helping behaviors the reproductive effort of other females in across species ranges from rare to habitual. the group. This may be partly because two Reflecting this pattern, the classification of classes of helpers common in traditional so- species as cooperative breeders varies among cieties but not among nonhuman cooperative researchers depending on definitional criteria. breeders—juveniles and grandmothers—are Cooperative breeding occurs in other primates not competing for mating opportunities or but is not a parenting strategy shared by our direct reproductive help. Nor do they compro- closest relatives. mise their own reproductive effort during life 418 Kramer AN39CH25-Kramer ARI 16 August 2010 19:4 stages when they help. Third, among humans, schooling, and institutional subsidies alter childrearing help occurs within the broader the need and economic options for providing context of food sharing in which individuals help. The final section explores implications Life history: age- of all ages and sex participate. One reason co- of cooperative breeding for postdemographic related time and operative breeding may be uncommon among transition populations. energy allocations to mammalian species is because dependency growth, reproduction, of young terminates with weaning, limiting and survival across the opportunities for help (Russell 2004). Since HUMAN LIFE HISTORY AND life course human juveniles are also dependent, it intro- COOPERATIVE BREEDING Natural fertility duces the potential for helping behaviors aside Human mothers and children are unusual with population: a population in which from child care. Because juvenile provisioning respect to a number of life-history features fertility is not limited, and other forms of help occur within a general compared with our closest primate relatives regulated, or subsistence pattern of food sharing and labor (Figure 1). Children are weaned at a young controlled through cooperation, the costs, benefits, and pathways age, reach sexual maturity late, and are more conscious means to cooperative breeding in humans may be than twice as likely to survive to reproductive Subsistence very different than those for other animals. age (Kaplan et al. 2000). The common explana- population: ahunter- The goal of this overview is to discuss tion for this remarkable improvement in child gatherer, agricultural, or pastoral society with the importance of and empirical evidence for survivorship is that, unlike other primates who little involvement in human cooperative breeding and to consider are independent of their mothers once they are wage labor or market its demographic significance. The first section weaned, human children continue to be fed, economy, and in which outlines key human life-history characteristics clothed, sheltered, and otherwise assisted. Non- households generally associated with cooperative breeding. The human primate mothers may let juveniles for- consume what they produce, and produce second section identifies helpers and addresses age in close proximity, offer agonistic support, what they consume the roles that fathers, older adults, and siblings and help negotiate social position. But before Demographic play in helping raise human young. The third the birth of the next offspring, juveniles are in- transition: the trend and fourth sections discuss two important dependent food providers. In contrast, human in recent centuries cooperative breeding questions: Does help juveniles are subsidized throughout much of toward declining benefit mothers and young, and why should their growth and development. mortality and fertility partners help? The fifth section addresses Short birth intervals, a relatively high prob- the demographic implications of cooperative ability of survival, and postweaning dependency breeding in relationship to the quality/quantity commit mothers to raising children of various trade-off. The cross-cultural examples in these ages concurrently. Many mammals have litters, sections focus on modern natural-fertility,
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