In roughly 3 percent of mammals and 9 percent of birds, group members other than parents (alloparents) help to protect, care for and provision young. Known as “cooperative breeding”, such alloparental care and provisioning allows mothers to breed at a faster pace or under adverse, variable or unpredictable ecological conditions without sacrificing offspring survival. Among primates, maternal reliance on others to care for and provision offspring is especially well developed among New World monkeys belonging to the Family Callitrichidae (tamarins and marmosets) and in one species of ape, humans. In addition to helping to explain the extraordinary demographic expansion of the human species, cooperative breeding has important implications for understanding the evolution of human life history traits such as early weaning, longer periods of post-weaning dependence and longer maternal lifespans, as well as such characteristically human reproductive strategies as contingent maternal commitment and variable paternal investment. The developmental context provided by cooperative breeding may also help explains such peculiarly human psychological attributes as enhanced perspective taking, intention-reading and impulses to share.
TIMING OF APPEARANCE OF THE 'DIFFERENCE' IN THE HOMININ LINEAGE:
Obligate reliance on alloparental care and provisioning evolved in the hominin lineage after the split with the lineage leading to chimpanzees ca. 7 million years ago. However based on observations of allomaternal provisioning of infants in response to begging among bonobos of various ages and both sexes, we can not rule out the possibility of incipient shared provisioning among the common ancestors of chimpanzees, bonobos and humans (Kano 1992; Hohmann and Fruth 1996). Larger body and brain size probably accompanied by longer periods of dependence in Homo erectus suggests that cooperative breeding may have been ongoing by 1.7 million years ago (O’Connell et al. 2002)
Definite Occurrence: Cooperative breeding was part of the human repertoire by the emergence of Homo sapiens.
Cooperative breeding encompasses a range of unusually flexible monogamous, polygynous and polyandrous mating systems in which individuals of either sex may mate sequentially or at the same time with one more partners, with one common feature: group members other than genetic parents (alloparents) help to care for and provision young. This mode of infant-rearing occurs in some 9% of ten thousand bird species and roughly 3% of all mammals. Alloparental care and provisioning of infants is estimated to occur in around 20% of primate species, is especially extensive among New World monkeys belonging to the Callitrichidae and in humans, but occurs in no other ape (Hrdy 2009). Many prosimian and monkey mothers park infants or when safe, take advantage of the willingness of allomothers (typically fathers, possible fathers or mother’s kin) to protect or carry their babies, resulting in various degrees of shared care without provisioning. However neither post-partum sharing of infants nor allomaternal provisioning in any of the apes except humans. Other apes (along with such Old World cercopithecine monkeys as baboons and rhesus macaques) exhibit exclusive maternal care (see MOCA ENTRY XXX)
Most primates find infants fascinating and are to various degrees attracted to them. In roughly half of primate species, mothers permit other group members to take and carry infants in the days and weeks after birth, and in some prosimians and monkeys, allomothers may provision (often nothing more than one lactating female allowing another female's young to briefly suckle). However, of the extant apes, humans are the only ones that routinely allow other group members post-partum access to their babies and the only apes to rely on shared provisioning to rear their young. This said, occasional sharing of food with young by group members of both sexes and various ages has been seen in bonobos which may exhibit a very rudimentary, incipient form of shared provisioning (Hohmann and Fruth 1996). As a consequence of allomaternal provisioning around the time of weaning, humans can wean their young at an earlier age than do other apes without sacrificing child survival. This means that humans can breed at a faster and persist in a wider range of habitats than would be possible if youngsters had to entirely provision themselves.
Shared care of infants right from birth and shared provisioning of immatures from just prior to weaning to nutritional independence has been observed in all African hunter-gatherers where childcare has been studied. Furthermore, across traditional societies with high rates of infant and child mortality ("high" here means on the order of 40% or more of infants born die before maturity) shared care and provisioning turn out to be essential for maternal reproductive success.
Because primate infants are slow-maturing and energetically costly to rear, mothers can almost always benefit from allomaternal assistance, provided that the allomother is willing, competent, benign and likely to return the infant to the lactating mother. The limiting factor on maternal willingness to share care of her young is whether or not she is confident in her surroundings and trusts potential allomothers, a luxury females in primarily patrilocal species such as chimpanzees do not have. Great Ape mothers have only been observed to share care of neonates under highly unusual conditions either in captivity or when an inexperienced new mother chimpanzee in the wild happened to be in the company of her own mother (Wroblewski 2008). In contrast to other apes, maternal tolerance of allomaternal attentions (See MOCA entry xxxx) is virtually universal in hunter-gatherer societies, as is shared provisioning of young at all ages. Proximate causes are likely to involve to subsistence conditions, environmental risks and residence patterns (Hrdy 2009).
Provided alloparents (often kin) are benevolent and either competent or supervised, infants obviously benefit. From the perspective of the mother, alloparental assistance is correlated with shorter birth intervals and/or higher infant survival. With greater freedom to forage unencumbered, the mother is both better fed and often safer from predation or accident. From the father’s perspective, his investment can vary opportunistically, permitting the highly facultative paternal care typical of the human species, without necessarily jeopardizing survival of young he sires. However, alloparents benefit only to the extent that they a) are related to the parents; b) gain vital experience or c) benefit from enhanced reputation and reciprocal favors including continued group membership, and d) alloparental care does not decrease their own opportunities to reproduce. This last condition is one reason so much attention focuses on provisioning by post-menopausal kin and the hypothesis that unusually long lived human life-spans evolved as a corollary of cooperative breeding (Robson et al. 2006).
Various life history traits (such as earlier weaning, longer post-weaning dependence, longer maternal lifespans, and larger brains), reproductive strategies (contingent commitment in mothers and facultative investment by fathers), and psychological attributes (enhanced perspective-taking mental attribution skills) are correlated with and may have co-evolved with shared care and provisioning of young, although there is considerable debate about the processes involved, and their sequences. Once cooperative breeding emerges, longer childhoods become feasible and selection can also favor longer lifespans among those (like post-menopausal women) who contribute to the survival of kin (see MOCA ENTRY xxxx). There may also have been selection for enhanced mental attribution capacities and intersubjective engagement in developmental contexts where infants rely on mutiple caretakers and had to chronically monitor both mothers and others and where infants and children better at reading intentions and soliciting care were better cared for and better fed (Burkart et al. in press; Hrdy 2009 and references therein).
Although rare in primates, cooperative breeding has evolved in other social mammals with delayed dispersal, particularly in social carnivores such as African wild dogs, wolves and meerkats and in mammals living under extremely harsh conditions such as mole rats. Necessary preconditions include phenotypic flexibility and benefits from remaining in one’s natal group long enough for potential helpers to be exposed to infant signals and solicitations. Facilitating conditions include unpredictable resources or otherwise challenging habitats and high post-weaning mortality, benefits to philopatry (or group membership) including social or material resources controlled by the group, and high degrees of relatedness within the group.
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