Inter-birth Intervals

Life history theory predicts that inter-birth intervals (IBIs) will depend on a trade-off between maternal investment in current and future offspring/reproduction, mediated by somatic maintenance of the mother. IBIs are therefore influenced by amount of energy available to the mother and the infant. Usually IBI scales with the body size of the mother and the infant (larger relative body size of infant to mother = slower breeding). Human IBIs, however, are much shorter relative to body size than other apes (orangutans ~8y, chimpanzees and gorillas ~ 6y, gibbons ~3y, humans ~3-4 in natural fertility hunter-gatherer societies, ~2y in natural fertility, agricultural societies. One of the suggested reasons for our species’ relatively shorter IBI is the presence of cooperative breeding: help from other member(s) of the social group in provisioning the mother and/or the infant alleviates energetic constraints on the mother. Other cooperatively breeding species, like the South American callitrichids, and even ones that have little help from other members of their group other than carrying infants, show shorter IBIs than expected. Another potential factor alleviating energetic constraints on mothers in humans might have been the advent of meat-eating, which made early weaning possible starting 2.5 mya with the Homo lineage. While humans wean as early as ~2-4 years old in hunter-gatherer groups and ~1 year old in Western societies like the U.S., gorillas wean at about ~4.3 years old, chimpanzees wean at ~4 years old, orangutans at ~3 years old. Children as young as 3 and 5 years old have been found to contribute to their own caloric consumption among the Hadza, before they would have been weaned if humans had ‘normal’ weaning periods, so perhaps human children can also contribute to a reduction of maternal costs? ‘Mother-in-law’ effects have been found in various societies, leading to shorter IBIs but higher infant mortality and stillbirths. Evolutionarily, it is likely that this shift in life-history occurred in the Homo lineage, rather than in the australopithecines, who still show life-histories more similar to the other apes. E.g. age at first molar eruption in modern humans = 5-6.3 years old; ‘early modern’ humans = 6.5, same as Neanderthals; H. erectus = 4.5-5; habilis 3.8; A. africanus and afarensis and P. troglodytes = 3.