Nursing frequency differs among mammal species. Some mammals, including chimpanzees and most other primate species, bats, and kangaroos, nurse continuously. Other mammals, of which rabbits and antelopes are prime examples, nurse discontinuously: a mother rabbit or antelope leaves her infant hidden in the grass or in a den while she goes out to forage, then she returns after a long interval and suckles the infant only a few times per day. Human hunter-gatherers resemble chimpanzees and Old World monkeys in being continuous nursers. But that pattern, which we inherited from our primate ancestors and presumably maintained for the millions of years of human evolution separate from the evolution of chimpanzees, changed only in the thousands of years since the origins of farming, when we developed lifestyles involving mother-infant separations. Modern human mothers have acquired the suckling habits of rabbits, while retaining the lactational physiology of chimpanzees and monkeys.
Infant-adult contact
Associated with those mammalian species differences in nursing frequency are differences in the percentage of an infant’s time spent in contact with an adult (especially with the mother). In the discontinuously nursing species the infant is in contact with the mother just for brief bouts of nursing and care. In the continuously nursing species the mother carries the infant while she forages: a mother kangaroo keeps the infant in her pouch, a mother bat holds the infant on her stomach even while she is flying, and chimpanzee and Old World monkey mothers carry the infant on their back.
In modern industrial societies today, we follow the rabbit-antelope pattern: the mother or someone else occasionally picks up and holds the infant in order to feed it or play with it, but does not carry the infant constantly; the infant spends much or most of the time during the day in a crib or playpen; and at night the infant sleeps by itself, usually in a separate room from the parents. However, we probably continued to follow our ancestral ape-monkey model throughout almost all of human history, until within the last few thousand years. Studies of modern hunter-gatherers show that an infant is held almost constantly throughout the day, either by the mother or by someone else. When the mother is walking, the infant is held in carrying devices, such as the slings of the !Kung, string bags in New Guinea, and cradle boards in the north temperate zones. Most hunter-gatherers, especially in mild climates, have constant skin-to-skin contact between the infant and its care-giver. In every known society of human hunter-gatherers and of higher primates, mother and infant sleep immediately nearby, usually in the same bed or on the same mat. A cross-cultural sample of 90 traditional human societies identified not a single one with mother and infant sleeping in separate rooms: that current Western practice is a recent invention responsible for the struggles at putting kids to bed that torment modern Western parents. American pediatricians now recommend not having an infant sleep in the same bed with its parents, because of occasional cases of the infant ending up crushed or else overheating; but virtually all infants in human history until the last few thousand years did sleep in the same bed with the mother and usually also with the father, without widespread reports of the dire consequences feared by pediatricians. That may be because hunter-gatherers sleep on the hard ground or on hard mats; a parent is more likely to roll over onto an infant in our modern soft beds.
For example, !Kung infants spend their first year of life in skin-to-skin contact with the mother or another care-giver for 90% of the time. !Kung infants are carried by the mother wherever she goes, interrupted only when the infant is passed from the mother to other care-givers. A !Kung child begins to separate more frequently from its mother after the age of one and a half, but those separations are initiated almost entirely by the child itself, in order to play with other children. The daily contact time between the !Kung child and care-givers other than the mother exceeds all contact time (including contact with the mother) for modern Western children.
One of the commonest Western devices for transporting a child is the stroller, which provides no physical contact between the baby and the care-giver (Plate 39). In many strollers, the infant is nearly horizontal, and sometimes facing backwards. Hence the infant does not see the world as its care-giver sees the world. In recent decades in the United States, devices for transporting children in a vertical (upright) position have been more common, such as baby carriers, backpacks, and chest pouches, but many of those devices have the child facing backwards. In contrast, traditional carrying devices, such as slings or holding a child on one’s shoulders, usually place the child vertically upright, facing forwards, and seeing the same world that the care-giver sees (Plates 21, 38). The constant contact even when the caretaker is walking, the constant sharing of the care-giver’s field of view, and transport in the vertical position may contribute to !Kung infants being advanced (compared to American infants) in some aspects of their neuromotor development.
In warm climates, it is practical to have constant skin-to-skin contact between a naked baby and a mostly naked mother. That is more difficult in cold climates. Hence about half of traditional societies, mostly those in the temperate zones, swaddle their infants, i.e., wrap the infant in warm fabric. The swaddled infant is often strapped to a board called a cradle board. That practice was formerly widespread around the world, mainly in societies at high latitudes. The basic idea of swaddling and of the cradle board is to wrap the baby as protection against the cold, and to restrict the baby’s ability to move its body and limbs. Navajo Indian mothers who use cradle boards explain that the purpose is to induce the child to go to sleep, or to keep the child asleep if the child is put on the cradle board when it is already asleep. The Navajo mother usually adds that the cradle board prevents the infant from suddenly jerking while asleep and thereby waking itself up. A Navajo infant spends 60%–70% of its time on a cradle board for the first six months of life. Cradle boards were formerly also common practice in Europe but began to disappear there a few centuries ago.