Maternal Tolerance of Allomaternal Assistance
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Hover over keys for definitions:Great Apes mothers are obsessively possessive of newborn infants, remaining in continuous physical (typically ventro-ventral) contact 100% of the time. In the wild, the earliest a chimpanzee mother has been observed to permit another group member to take her infant was 3.5 months. First transfers among gorillas and orangs were not observed before five months. By contrast, human mothers are more permissive. In all hunter gatherer societies for which information is available, the mother is the principle caretaker but allows allomaternal access. Apparently, Great Ape mother and mothers in traditional human societies perceive risks to their infants differently. Human Social Structure (MOCA ) and Childbirth Customs (see MOCA 32.01) may be factors, as well as the human capacity to foresee future benefits to a slow maturing human infant from having allomothers who will help to protect and feed it. Physiological factors may also be implicated in human post-partum permissiveness. For example, it is known that receptors to the neuropeptide oxytocin are built up during pregnancy, and oxytocin released during birth and lactation play a role in reducing maternal social inhibitions in mammals generally and in enhancing inter-individual trust in humans.
TIMING OF APPEARANCE OF THE 'DIFFERENCE' IN THE HOMINID LINEAGE:
Unknown. Given its universality in hunter-gatherer societies, post-partum infant-sharing predated the common origin and spread of modern humans. Some authors have argued that alloparental provisioning of young (O'Connell et al. 2002) and shared infant care (Hrdy 2005) evolved in the hominid line by 1.7 million years ago and long predates the emergence of Homo sapiens.
Great ape females seek seclusion when they give birth and may not rejoin others for several days afterwards. Other community members are typically very interested in the new infants, but great ape mothers are obsessively protective and possessive of newborns, scrupulously avoid or rebuff allomaternal attentions, and carrying their infants everywhere, maintaining direct physical contact 100% of the time. In the wild, the earliest a mother chimpanzee has been observed to allow another female (her daughter) to hold her infant was 3.5 months (Goodall 1969). First transfers in gorillas and orangutans are even later, after five months (Fossey 1979; Van Schaik 2004). Thereafter mothers remain the infant's main caretaker and sole provisioner, imposing a heavy metabolic load on mothers which translates into interbirth intervals around six years in the case of chimpanzees, up to eight years among some wild orangutans and bonobos (Knott 2001). Just over half of all primate species qualify as "Continuous Care and Contact" mothers. The threat of infanticide combined with the lack of availability of experienced matrilineal kin probably means that maternal possessiveness is probably warranted in the case of great apes. However, just under half of primate species do manage to delegate care of infants, either through infant-parking or shared care. Humans belong among these infant-sharers.
Unlike other apes, humans in traditional foraging societies permit post-partum access to newborns. Right from birth, and through the long period of nutritional dependence, mothers share caretaking and over time the task of provisioning their infants with others. As in other primates with shared care, one important outcome is a reduction of maternal opportunity costs and metabolic load, contributing to inter-birth intervals among foragers that are about half that of those found in other apes (more nearly 3-4 years than the 6-8 found among wild chimpanzees and orangutans. As among other apes, infants born in foraging societies are treated very indulgently, nursed nearly on demand, and constantly held by someone _often, but not always, the mother. Right from birth however, and throughout the first months of life, human mothers permit other group members access to their newborns. Even before weaning and for many years after weaning, fathers and other allomothers help the mother to provision her infant, initially with premasticated and later with solid foods.
Human societies with exclusively maternal "ape-style" attendance to newborns are the exception. According to an analysis using data from the Human Relations Area Files (Hewlett 1989), allomothers are permitted post-partum access to newborns in 92% of the world's cultures for which information is available.
UNKNOWN. As with other apes, obsessive concern for infant well-being is typical of parturient mothers from (see Leckman et al. 1994 for a sample of western mothers). Nevertheless, this maternal hyper-vigilance does not inhibit human mothers from allowing others to hold their newborns. Several explanations can be hypothesized: These include human cognitive capacities that permit mothers to consciously weigh potential benefits of allomaternal care against probability of harm to her infant,, or subscribe to cultural customs that promote sharing. Social conditions, particularly residence patterns which increase the availability of matrilineal kin, may decrease potential risks to infants and/or increase the availability of trusted allomothers. In line with this suggestion, it is worth noting that under highly unusual situations, nonhuman ape mothers giving birth in captivity do allow an especially trusted associate (e.g. a human allomother, or the ape's own mother) access to her newborn (Matsuzawa 2001; Nakamichi et al. 2004 for gorillas), while in situations when the mother feels safe she may set her infant down briefly in order to eat a favored food (Hrdy, pers. obs. for bonobos). There may also exist physiological differences that contribute to reduced levels of postpartum anxiety in women compared to other apes, increasing tolerance towards allomothers. Across mammals, including women, the neuropeptide oxytocin mediates both labor and lactation, and is implicated in post-partum reduction of social inhibitions (Carter et al. 2001), as well in enhancement of inter-individual trust (unfortunately in an all-male sample, Kosfeld et al. 2005). So far little is known about differences between humans and other apes in respect to oxytocin, oxytocin receptors, vasopressin or other neuropeptides implicated in postpartum mood changes (Carter et al.2001). (Suggestions and discussion welcome)
Although the timing is unknown, once shared care evolved in the hominid line maternal tolerance of allomaternal assistance would tend to be maintained by natural selection in hominid populations characterized by high child mortality since under those conditions, allomaternal assistance is correlated with maternal reproductive success (see Hrdy 2005 for review).
An evolutionary heritage of shared care has important implications for our understanding of child development and the human infant's evident quest for intersubjective engagement. Infants best able to monitor, engage and solicit investment from mothers and others (especially critical for cooperatively breeding species where maternal commitment to offspring tends to be socially contingent), would be most likely to survive. Mutual gazing, smiling, and other engaging traits already present in other apes, would have been more fully expressed over development in species with multiple caretakers and more contingent care, providing novel opportunities for selection to favor infants best able to interpret intentions and engage others (Hrdy 2005).
Exclusive maternal care of infants is the mammalian norm. Of 5,000 species of mammals, cooperative breeding occurs in around 3% of them. If we confine ourselves to the Order Primates, some allomaternal (including paternal) care including the first weeks and months of life occurs in 40% or more species. Cooperative breeding (with alloparental care as well as provisioning of infants) occurs in roughly 14% of primates, mostly due to the large number of cooperative breeding species in the Family Callitrichidae. (Note these are crude estimates calculated for the purpose of this very general comparison).
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