The evolution of shorter inter-birth intervals in humans
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 the amount of energy available to the mother and the infant. Usually, IBI scales with the body size of the mother and the infant, with a larger relative body size of infant to mother correlating to slower breeding. Human IBIs, however, are much shorter relative to body size than other apes. 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.