The Evolution of Parental Care and Recruitment of Juvenile Help

The Evolution of Human Parental Care and Recruitment of Juvenile Help Karen L. Kramer Abstract Provisioning juveniles over a long period of time is a defining characteristic of human life history. Most evolutionary perspectives on parental care emphasize the expensive cost to raise children and the cooperation of other adults to help raise young. While assisting juveniles is an unusual primate trait, human juveniles also are unique in exchanging resources and labor with their siblings, mothers and other adults. In reviewing data from traditional societies, this article highlights this distinctly human and twofold nature of human juvenility. Rather than juvenile dependence signifying a costly expansion of parental care, juvenile provisioning and help may develop in tandem with the broader pattern of food sharing and division of labor that characterizes human subsistence and sociality. “Immatures are usually ignored in evolutionary scenarios except to note the cost of them” (Fuentes1991:141). Today’s children are expensive to raise. In addition to food and shelter, children spend long years in schooling, instruction and training to become competitive and competent adults. Apart from a few chores, children consume parental resources and contribute little to household economics. As a society we are vigilant not only of children’s safety, health and education but legally protect them from the obligations of work. Children in postindustrial societies are costly both because of the extent to which we invest in them and their lives are unfettered by work. The cost to parents to raise children is one explanation for today’s small families. However, the childrearing conditions prevalent in traditional societies and to which humans are adapted dramatically differed in two inter-related ways. Mothers had many more children and children were relatively inexpensive, not because they lacked care – children in traditional societies have a greatly improved probability of surviving compared to great ape juveniles-- but because they gave back to their caretakers. Among the great apes, mothers nurse infants for a long period of time, on average for 4-6 years. Once fully weaned, juveniles provide their own calories. Mothers give their exclusive attention in most cases to one young at a time, and have relatively few offspring over the course of their reproductive careers. In contrast, human infancy lasts a short period of time, 2-3 years in natural fertility populations [1]. Young are dependent on others well past infancy. Short birth intervals and extending help to juveniles commit mothers to raise multiple dependents of different ages at the same time. The predominant hypotheses explaining how human mothers manage a rapid reproductive pace emphasize the cooperation of other adults to help mothers and young [2-6]. The goal of this article is to highlight the complexity of juvenile dependence. Children receive assistance from others for a long period. But humans also harness the help of their own juveniles, something no other primate does. Yet, this distinctly human and twofold nature of juvenility remains unincorporated into our current theoretic models of human life 2 history. This review focuses on an overlooked evolutionary aspect of redistributing dependency across both infancy and juvenility. In doing so, mothers not only attenuate a physiological constraint on fertility. They also recruit a dependent, closely-related corps of juveniles who benefit from continued investment and from helping to support their siblings. I first situate human juvenility and parental care, emphasizing that hominization fundamentally restructures the economics of childrearing. Then in considering the cross- cultural economic data on children’s work in traditional societies, two points are of note. Juveniles in many preindustrial societies are hard workers. While they are not economically independent, neither are their mothers, fathers, grandparents or other adults. Mothers and adults who provision children receive subsidies from others, including from the juveniles they help. This suggests that juvenile dependence may best be characterized as a mutual economic relationship with fitness advantages for both mothers and juveniles. In the final section, the ecological conditions that pull children out of productive roles are discussed. Hominization of juvenility and maternal care As a shared mammalian trait, juvenility is bracketed by weaning and sexual maturity. Because humans often are characterized as delayed reproducers and having a long juvenile period, attention has centered on life history tradeoffs affecting age at sexual maturity and first birth. But the juvenile period is also determined by weaning age, which is strikingly early in humans compared to other primates (Table 1). While comparative animal studies broadly define juvenility, because human ontogeny includes both unusually slow and rapid growth rates, it is typically more narrowly defined as the period between weaning (or childhood) and the adolescent growth spurt (Box 1). I focus on this earlier period of juvenility, and its effect on the evolution of maternal care, as a life stage distinct from transition out of juvenility and tradeoffs raised with continued growth and sexual maturity. [Table 1 & Box 1 about here] Caring for an infant vs. subsidizing a juvenile Parental investment, while good from an offspring’s perspective, is costly from a mother’s. Young benefit from investment because it increases their probability of survival. Mothers, however, have limited time and resources to invest in current offspring without foregoing future reproductive success [7-8]. How mothers balance these two fitness components -- mortality and fertility -- centrally influences how long to provide parental care. For other mammals this tradeoff is played out during infancy and determines when young are weaned. Instead of raising young to independence during infancy as other primates do, hominin mothers shorten infancy and extend dependency into juvenility. This novel maternal strategy has several key effects on the cost of parental care (Table 2). First, because pregnancy and lactation often are incompatible metabolic expenditures, shifting nutrient delivery from milk to food relaxes a basic physiological constraint on reproductive pace [9]. A short lactation period and short birth intervals are important factors accelerating the human reproductive pace [10]. Human birth rates 2-5 times higher than predicted for primates when body size is taken into account [1] and about twice that of chimpanzees [3]. Both human and chimpanzee mothers have the biological ability to reproduce at shorter birth intervals, evident from the resumption of ovulation and short 3 birth intervals following the death of an infant [11-13]. Rather than selection for a novel biological mechanism, sustaining short birth intervals requires a behavioral reorganization of parental care. [Table 2 about here] Second, subsidizing juveniles has a survival advantage. In other primate species, self- supporting juveniles are particularly vulnerable to undernutrition because of their young age, foraging inexperience and low return rates [14]. Sharing food with juveniles has the obvious effect of smoothing fluctuations in food returns and minimizing an important morbidity risk factor. Beside these benefits, a child who is fed during an illness or injury has a distinct advantage over an independent juvenile. Human young are almost twice as likely to survive to reproductive age, even when they grow up in the absence of modern medicine, health care and sanitation [15-16]. Most of this difference is due to gains in juvenile survivorship rather than lower infant mortality. This suggests that under preindustrial conditions, parental investment has a greater survival payoff for juveniles than it does for infants. Third, the energetic and opportunity costs of maternal care differ during infancy and juvenility. Supporting an infant requires an estimated 38% increase in maternal daily caloric intake, which for an average mother is on the order of an additional 700 daily calories [17]. Beside the calorie expense, nursing, holding, transporting and babysitting infants are activities mothers perform in addition to supporting themselves. In contrast, juveniles consume the same kinds of resources as adults. While subsidizing a juvenile may require some additional energetic expenditure, this is expected to be relatively low because the time and energy spent provisioning a juvenile are embedded in the same tasks mothers do to support themselves. Finally, the economic relationship between mothers and young are very different during infancy and juvenility. During infancy, lactation and other kinds of support are transferred unidirectionally from mothers to young. Mothers (and others) help young; infants consume their help and return only their potential survival. In contrast to infants, juveniles have a more nuanced economic relationship with their caretakers. Juveniles are helped, but in most preindustrial societies they also produce resources and labor that are exchanged with not only with their siblings and mothers, but also with others who help them. Although a nonhuman primate juvenile works hard to support himself, it is these bidirectional resource and labor transfers between juveniles, mothers and others that distinguish human parenting and juvenility from our primate relatives. Juveniles who give back Young juveniles (ages ~4-10) are key helpers in carrying and

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