All parents who would like a sibling for their children probably wonder about the best time to enlarge their family. Today, we can chose to have children close together or spaced apart; the former approach ensures the children have playmates close in age and developmental stage and the intensive phase of childrearing may be over sooner, the latter approach may suit parents better who want to actively involve the older child in taking care of the baby and spreads the intensive care-taking over a longer period of time. Despite best planning efforts, however, nature sometimes takes a completely different course.
Is there something like a ‘normal’ interval between pregnancies for humans? Let us look at our evolutionarily closest cousins for some answers. For all primates, reproduction requires high levels of energy expenditure and resources. How these costs are distributed over various species lifetime has been investigated by several scientists. Biruté Galdikas and James Wood (1990) found a birth spacing of 92 months in wild orangutans, 66 months for chimpanzees and 45 months for gorillas. More than seven years between births in orang-utans? No wonder they are close to extinction!
Human hunter-gatherers, for example the Gainj of highland Papua New Guinea, have an average of 43 months between births. Pennington (2001) calculated 39 months for hunter-gatherers, taking the mean of four non sedentary populations. Three and a half to four years between children seems normal for prehistoric people before the Neolithic, i.e. the adoption of agriculture, animal husbandry and a sedentary lifestyle.
How is this child spacing achieved? Mothers breastfeed their babies for at least the first two years of life, and unrestricted breastfeeding suppresses ovulation, preventing further pregnancies. How exactly this mechanism works is still under debate – and do not try this at home: it has been shown that in well-fed, western civilisations with a limited nursing culture breastfeeding alone is not a reliable method of birth control. The continuous, around-the-clock suckling of infants produces hormones in the mother that suppress ovulation, but the energy balance of a lactating woman may also have something to do with it (Thompson 2013).
Supporting an infant nutritionally during pregnancy is already demanding, but lactation is the most costly part of the reproductive cycle for the mother (ibid. 294). Producing breastmilk and breastfeeding is a less efficient way of energy transmission compared to placental absorption. A growing baby requires more and more food, and its increasing weight also increases the mother’s burden when carrying the infant. Interestingly, the high costs of breastfeeding are met primarily by increased energy intake and reduction of expenditure, and, although the mothers’ body fat stores does buffer against nutritional stress and protect her own health, there is hardly any relationship between the mother’s BMI and her milk yield. Even extremely malnourished women can provide enough breastmilk to nourish their babies (as some very disturbing stories from babies born in the last days before the liberation of the NAZI concentration camps testify).
In nomadic societies, children under four are typically carried by their mothers on gathering trips or when the group moves. This is another significant factor in the mother’s energy expenditure. Among the !Kung of the Kalahari Desert, babies are carried everywhere in a small pouch on the back of the mother for the first one or two years of life. Between ages two and four, children are carried on the shoulders by their mothers. From the third year onwards, however, children increasingly spend time separated from their mothers in the camp, looked after by others while the mother is out. Up to age six or seven, children are partly carried by their fathers and partly walk some of the way (Lee 1972: 331).
If children are born in close succession, i.e. two years apart, a mother ends up carrying both children until the older one is mature enough to walk on his/her own. Close child spacing is recognized as giving the mother ‘a permanent backache’ (ibid. 332).
I found it interesting that children are carried for such a long period of time, and it explains the reluctance of my three-year-old to walk for more than a few hundred meters in a straight line when out and about. Children are probably ‘programmed’ to be carried when the group moves, and to practice moving when the group rests. Hence they bounce off the walls when at home!
After the adoption of a sedentary lifestyle with a more steady supply of high-calorie foodstuff ensured by agriculture and animal husbandry, the birth rate increased and demographics changed. Better nutrition and reduced female mobility led to shorter intervals between births, and ultimately to a significant growth of the Neolithic population. This ‘baby boom’ is also known as the Neolithic Demographic Transition. Whether a shortened period of lactation is also a factor in this development, is currently under investigation in a project led by Sofija Stefanović from the University of Belgrade, Serbia. The availability of suitable weaning foods such as cereal grains might have enabled to wean babies earlier, which led to a quicker return of mothers’ fertility.
In the typical pattern of Neolithic societies, siblings are now born in quicker succession, leaving only two to three years between births. Farming communities are known for having many children – not only because they can be supported nutritionally, but also because their labour is needed for the plentiful work in the fields. The physical toll of childbirth probably increases for the mothers, and their social position may change significantly. If they no longer go out on gathering trips as much and remain close to home, presumably with other women in the same situation, confinement and control can be one consequence. With children spaced more closely, childrearing practices likely changed, too. Taking care of children in a group is one of the strategies to mitigate the high costs of bringing up babies. Older siblings make perfect babysitters and both children and their mothers can be supported by their communities, sharing babysitting and provisioning.
At a little more that two years apart, my own children are closely spaced in prehistoric terms. After a year, they now sometimes play together. The younger one adores the older one and wants to join in all activities, which is not particularly popular with the older one. He is not yet mature enough to understand what babies need and can and cannot do, he does not understand he needs to treat him gently and thus the boys have to be watched closely all the time.
This would not have been an issue in prehistoric societies that brought babies up in a larger group. With children of all ages around, there would always be somebody to play with, to look after, and to bully.bully, and to look after. The exact spacing of siblings would hardly matter to the children if plenty of cousins and other children.
Bocquet-Appel, J.-P. 2008. “Explaining the Neolithic Demographic Transition,” in J.-P. Bocquet-Appel and O. Bar-Yosef (eds) The Neolithic Demographic Transition and its Consequences: Springer Netherlands: 35-55.
Borić, D., and S. Stefanović. 2004. Birth and death: infant burials from Vlasac and Lepenski Vir. Antiquity 78(301): 526-547.
Lee, R. B. 1972. “Population Growth and the Beginnings of Sedentary Life Among the !Kung Bushmen,” in B. Spooner (ed.) Population Growth: Anthropological implications. Cambridge, Massachusetts: MIT Press: 329-342.
Galdikas, B. M. F., and J. W. Wood. 1990. Birth spacing patterns in humans and apes. American Journal of Physical Anthropology 83(2): 185-191.
Pennington, R. 2001. “Hunter-gatherer demography,” in C. Panter-Brick, R.H. Layton, and P. Rowley-Conwy (eds) Hunter-Gatherers: An Interdisciplinary Perspective. Cambridge: Cambridge University Press: 170-204.
Thompson, M. E. 2013. Comparative Reproductive Energetics of Human and Nonhuman Primates. Annual Review of Anthropology 42(1): 287-304.