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Home , Parental investment

The Evolution of 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 , 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 .

“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 to raise children is one explanation for today’s small . However, the childrearing conditions prevalent in traditional societies and to which 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 [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 and 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 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 and juvenility from our primate relatives.

Juveniles who give back Young juveniles (ages ~4-10) are key helpers in carrying and minding their toddler siblings [18-22]. Children also forage, fish, hunt for small game, tend animals, work in gardens, and participate in a wide range of domestic tasks such as food processing and preparation, hauling water and collecting firewood, activities unique to human subsistence and occur across subsistence ecologies [23-29]. Compared to other primates, humans are impressively efficient food producers. Chimpanzees spend 50-75% of their daily time- budget foraging [30]. An adult human spends about half that time [31-32]. While efficient food producers, humans do spend considerable time processing and cooking food [33].

4 This is an important point with respect to children’s labor because juveniles typically allocate time to these camp-based, repetitive, low-skill support tasks [34-35].

Figure 1 summarizes how much time children spend in subsistence and domestic labor in traditional societies. The figure includes all known published studies that distinguish male from female labor, and that report values in daily hours or proportions that can be converted into hours. Subsistence economies other than hunter-gatherers are shown because very limited time allocation data are available for forager children, and because hunter-gatherer children as a subsistence class do not work more or less than children in other traditional economies. Although methodologies differ among studies, values for all groups include the average daily time juveniles spend in resource acquisition and domestic tasks. Two views of children’s labor are shown: raw data as hours of daily work and hours expressed as the proportion of the adult mean to account for population differences in overall levels of subsistence work.

[Figure 1 about here]

Compared with adults in the same society, most juveniles work at least 50% as much as their adult counterparts (Fig. 1b). This is significant if we consider that a ten-year-old, for example, is on average 60% of adult body mass [36]. Maya juveniles (ages 7-14), who are an ideal baseline for children’s work in traditional societies because their work level falls at the mean for this cross-cultural sample of children’s labor [37], spend on average 3.2 hours a day in subsistence activities; adults 6.5 hours [34]. This is energetically equivalent to children expending about 20% and adults 40% of their daily caloric intake performing subsistence activities. This and the proportional data suggest that juvenile are working less than proportional to body size, but in many cases only marginally so.

The few studies that report both children’s production and consumption in units of time show juveniles are relatively inexpensive and can become net producers by their early to midteens [38-39]. Where production and consumption are measured in calories, children are more expensive and do not become net producers until their early to mid 20’s [40]. Here I use time allocation as the more inclusive measure of children’s economic value [41]. In many societies, support tasks rather than calorie production constitute a substantial proportion of children’s labor. This labor support, which allows mothers to redirect time to infant care or productive tasks juveniles cannot perform, is invisible unless we consider time allocation.

Why juveniles help Primate juvenility has been described as a ‘phenotypic limbo’[42:28] during which an individual bides its time growing slowly, waiting to reach the onset of adolescence and sexual maturity. It is time well spent if a juvenile can also improve its chances of future survival or reproductive success by learning subsistence and social skills to become a more adept adult producer and [6, 14]. Human juveniles have an additional payoff to make the best of growing slowly by leveraging their nonreproductive status into a higher reproductive potential for their mothers and indirectly themselves [43].

For a reproductive-aged adult, a calorie spent helping to support a or mother is a calorie that could be spent furthering his or her own reproduction. A sexually immature

5 juvenile, who is not competing for mates or physically supporting reproduction, does not pay a comparable opportunity cost. If a juvenile contributes, either in direct calories or performing tasks that reduce a mother’s energetic expenditure, he or she receives an immediate fitness benefit rather than having to delay until fully grown. This time discounting advantage may be especially important in high mortality environments where a juvenile may not live long enough to reproduce themselves. Because juveniles are closely related to their siblings and benefit from securing their mother’s continued investment, transfers from juveniles, not surprisingly, tend to occur among related kin and in the context of sibling help [38, 44].

While children benefit from helping to support their siblings, may not be the only explanation why human juveniles produce resources that are redistributed to others, especially in explaining why closely-related species do not. Because fitness payoffs often are time delayed in long-lived animals, researchers have recently suggested that cooperative behaviors may be motivated by more proximate mechanisms [2, 45-46]. This is particularly prescient to humans if we consider that juveniles are both dependents and helpers, as are adults.

Mutual economic benefits Juveniles have a complex economic relationship with their caretakers. They are able to produce some resources at the level of their own consumption and some in excess of their own consumption -- foraging for fruit and berries, digging small tubers, hunting for small game, fishing, collecting shellfish, fetching water and harvesting are good examples [24, 28, 35, 47]. For example, a Pumé boy living on the llanos of Venezuela is successful in catching enough fish to feed himself, his siblings and parents, but is dependant on shares of processed plant food and hunted game from others [37]. A Pumé girl is able to weave a burden basket, but is not strong enough to collect the moriche palm fronds or strip and process the fiber. Pumé girls, like Mikea girls of southwestern Madagascar, can dig small roots, but are not able to transport large quantities back to camp, a task their mothers do. Among Western Desert Australian foragers children collect grubs and fruit and successfully capture small game, such as reptiles and rodents, but depend on others for seed cakes and larger game [48].

The pattern of an individual both overproducing some resources and underproducing others characterizes juvenile subsistence behavior. But, it also describes adult subsistence behavior. Older Pumé women, for example, produce the majority of roots, a critical wet season resource, consumed by their extended families. But, they also depend on kin for shares of many other resources. Juveniles may target different resources, have different foraging strategies, and perform easier or less skilled tasks than adults [23, 35, 38, 49-50]. Depending on age, they may be less efficient and have lower return rates than adults at certain activities. Although not as productive as adults, juvenile contributions offset what is perhaps an even more important constraint on human reproduction and survival than food – time.

Juvenile labor, a difference of kind Compared to great ape subsistence, the human diet incorporates a broad diversity of plant, animal and aquatic resources most of which require extensive processing and specialized technology to access and render edible. While variable, human day foraging ranges are

6 several times greater than a chimpanzee’s [51-52], and resources are transported, often long distances, back to a central camp. Resource procurement and processing activities have different skill and strength requirements and often take place in different locations. Plant and animal foods, both of which are important to the human diet, have discrete geographic distributions, require different time investments, and search and handing costs. Although humans are efficient food producers, solving access to a wide variety of high-quality foods through sophisticated technology, food processing and large foraging ranges introduces a time allocation constraint for individuals of any age to be self-sufficient.

One solution to this time constraint is a division of labor. Explanations for the human division of labor have centered on the age-specific association between the return rate for a task and the time allocated to it [38, 53-54]. Becker’s [55] proofs demonstrate the efficiency of a division of labor when inequities in rates of return exist across tasks and individuals. Although reasons for the interdependence of males and females are debated, the sexual division of labor is widely recognized as seminal to human [33, 56- 59]. What has commanded less attention is the complementarity of the age division of labor.

Since all other primate juveniles are independent feeders, it is reasonable to infer that ancestral hominin juveniles were self-sufficient and dependency is the derived trait. This raises the questions, what benefit does an evolving hominin mother have in extending support to a juvenile? For a juvenile, if a calorie spent helping is a calorie that could be spent growing, why share resources with others?

The transition from mutual self-sufficiency to maternal-juvenile exchange may have been a zero-sum nutrient game, but a time constraint solution that benefited both. Juveniles may not have required support through additional maternal time and resource investments, but were able to overproduce some foods or resources in exchange for others added into the diet at which they were inefficient. Although juveniles incur inclusive fitness benefits {Bereczkei, 2002 #547;Kramer, 2005 #407}, juvenile help may evolve out of the mutual benefits associated with time constraints imposed by the complexity of the human diet, and an age-graded division of labor in which transfers flow bidirectionally through age classes. The timing of evolutionary changes that lead to feeding interdependence is unclear, but juvenile and adult specialization and exchange are common to all modern human subsistence strategies.

Fitness consequences of bidirectional transfers For most mammals, including our closest relatives, mothers are the sole providers of parental care. All mothers have a limited amount of energy and resources to invest in parental care. Offspring quality, often measured as survivorship or growth, is a function of a mother’s ability to provide resources. Cooperative breeders are a notable exception (Box 2). Among cooperative breeders, extra-maternal sources of help can positively affect both offspring quality and quantity. The quality/quantity tradeoff appears altered among cooperative breeders because a unit of energy expended in the production of offspring does not necessarily diminish quality if others help provision young.

[Box 2 about here]

7 Because human juveniles are consumers of parental care, they often are overlooked as an extra-maternal source of help. While exchanges between mothers and juveniles are rare in other animals, they do occur among other cooperative breeders [60]. In studies where transfers from children have been quantified, they enable mothers to invest in continued childbearing during their mid-reproductive career and allow them to raise more children than they otherwise could, even when fathers help [21, 38, 41].

Variation in juvenile labor; implications to juvenile-adult interdependence Human cooperation is complex. It involves not only help rearing young, but also exchanges of resources and labor with many different currencies and life-long economic relationships between kin and nonkin buttressed by complex cultural rules and evolved emotional structures [5, 61]. Today many individuals help mothers – fathers, grandparents, other related kin and nonkin. Governments and institutions further extend nonmaternal sources of investment. Given these complexities, perhaps the simplest evolutionary relationship to explain is that between a mother and child. From a mother’s point of view, a juvenile as partial dependent is a dependable helper who is more likely to be alive than a grandparent or other ascendant kin [62], especially during a mother’s mid reproductive career. Juveniles benefit both from continued care and helping to support their siblings at a life stage when they have low opportunity costs and can transform their nonreproductive status into an indirect fitness benefit.

The view that children depend on adults for most of their needs historically derives from the early quantified !Kung (sub-Saharan hunter-gatherers) ethnographic case. The socio- ecological reasons why !Kung children work little are well documented [19, 24, 32, 63-64]. If we assume that juveniles were independent and hard working in the past, !Kung children, who are cross culturally the least productive (see Fig 1), may not be the best model for the evolutionary conditions under which juvenile provisioning and help develop. In contrast, Hadza juveniles (also sub-Saharan hunter-gatherers) are accomplished foragers [28]. Their work level as a proportion of an adult’s (Fig 1b), more than equals relative differences in adult-juvenile body size. No single modern population is a direct analogy to reconstruct past behavior, but variation in children’s labor offers insight into those ecological circumstances influencing children’s productive roles.

Variability in children’s participation in economic activities has been associated with the opportunity costs to spend time in competing ways, subsistence factors associated with task difficulty, return rates, how children learn and ecological factors affecting health risks [24, 38]. For example, parents tolerate lower productivity when schooling or other learning improves children’s future outcomes as parents and adults [54, 65-66]. These dynamics may explain much of the reduction in children’s economic contributions today and in past environments where access to complex resources required greater learned proficiency as a child [6].

Juveniles may be particularly challenged to maintain a stable food supply in environments where important foods require complex access, processing or storage techniques, and consequently daily variance in children’s return rates is high. The reduction in daily variance, rather than an increase in amount, may be the important factor in extending subsidies to juveniles. A reduction in daily variance is also used to generally explain food sharing, especially meat, in humans [67]. This raises the point that while juveniles are not

8 economically independent, neither are their mothers, fathers, grandparents and others. Although the feeding interdependence of adults is well appreciated, the inclusion of juveniles in this pattern is often neglected. Rather than juvenile dependence signifying a costly expansion of parental investment, juvenile provisioning and help may develop in tandem within the broader sociality of food sharing, labor exchanges and long-term economic relationships across ages and . This has been elaborated elsewhere as the pooled energy model [68]. While the result is cooperative breeding in that nonmaternal individuals help support juvenile, the pathway to human cooperative breeding through bidirectional transfers may differ from other animals.

In sum, prolonging parental investment is traditionally viewed as costly. This is particularly evident in modern developed societies. However, the reproductive conditions to which humans are adapted are quite different. Extending subsidies to juveniles is recouped through shorter birth intervals, gains in survivorship and recruitment of a closely related, dependent labor corps. Restructuring parental investment to enlist juveniles as helpers is part of a derived complex that gave Homo a distinct fitness advantage by being able to support multiple-aged dependents.

3673 words

Acknowledgments I extend much appreciation to the Maya and Pumé for the years spent living and working in their communities recording interactions between children and their caretakers. Funding for this research was provided by the National Science Foundation (0349963) and the National Institutes of Health (AG19044-01).

9 Table 1. Life history parameters affecting the juvenile period

Weaning IBI Age at Survival Age at age to independence first birth maturity Chimpanzees 5.3 5.9 14.3 .37 weaning Human 2.5 3.1 19.7 .57 variable a foragers

Weaning age (years): chimpanzees, combined male/females mean for Gombe chimpanzees[69:365]; humans, median across 112 preindustrial populations[70:189].

Interbirth intervals (years): chimpanzees, median across six wild populations when previous offspring survives[13:Table 1]; humans, mean across four natural fertility populations[1:450].

Age at first birth: chimpanzees, mean for 3 wild populations[6:158]; humans, mean for 3 groups of foragers [6:158].

Probability that offspring will survival to maturity (l 15 ): chimpanzees, wild populations male/female combined [15:326]; humans, for five groups of hunter-gatherers, male/female combined [15:326]. a While adults may be net producers [29, 40], among human foragers they are also dependent on others for many resources.

10 Table 2. Relative maternal costs to support an infant vs. subsidizing a juvenile

Infants Juveniles Reproductive cost High Low Lactation negatively No comparable cost affects probability of conception

Energetic cost High Low 38% of maternal body Extra energy spent weight, or ~ 760 kcal/day subsidizing a juvenile for a mother of average expected to be low since it size [17] is embedded in foraging activities mothers do to support themselves

Opportunity cost a High Low Infants consume nonadult Juveniles consume adult resources resources

Transfers between mother Unidirectional Bidirectional and child a Because infant care involves tasks mother do not otherwise do to support themselves (lactation, carrying, holding), mothers relinquish time they could spent in other ways [21, 71]. Since juveniles consume adult resources, mothers do not pay a comparable opportunity cost.

11 Fig 1. Children’s work in traditional subsistence societies. (a)

10 Ariaal 9 8 Bangladesh 7 Mayo Hadza Nepal 6 Java Kipsigis 5 India 4 Machiguenga MayaDominica 3 Java Pumé Mikea Miskito HoursDailyof Work 2 Piro-Machi 1 Kung 0 Hunter-gatherers Mixed Horticulturalists/ Agriculturalists Pastoralists Males Female

(b) Hadza Mayo Ariaal 1.0 Kipsigis Proportion of Adult Mean Bangladesh 0.8 Nepal Pumé Java 0.6 Mikea Machiguenga Maya Miskito Java 0.4 Piro-Machi

0.2

0.0 Hunter-gatherers Mixed Horticulturalists/ Agriculturalists Pastoralists Males Female

(a) Daily hours that male (blue) and female (white) children allocate to subsistence activities. Subsistence activities include time spent foraging, hunting, fishing, gardening, tending animals, hauling water, collecting firewood, food processing, preparation and other domestic tasks. Values do not include time spent in childcare because it is not consistently reported. Figure includes all published studies that distinguish male from female labor and that report values in daily hours or proportions that can be converted to hours of work through the length of the observation day. Age categories reported in published sources vary from group to group, but generally include juveniles ages 6-14. (b) Daily hours expressed as a proportion of the adult mean. Values given for those groups for which adult data are provided in the same study. Error bars are not shown because most studies report aggregate data.

Sources (left to right): Hadza [72:556]; !Kung children’s work [63:349], adult work [32:Table 9.12, 64:90-91]; Pumé [73-74] ; Mikea [35:155]; Machiguenga [75:305]; Piro-Machiguenga [54:40], value given for age 10 and age 40); Miskito/Mayangna (Koster 2007:220); Maya [34:308]; Dominica [76:475]; Mayo [77:330]; Java [78:295]; Nepal [78:296]; Java [79:141]; India [80:340]; Bangladesh [27:216]; Ariaal [81:434]; Kipsigis [82:43] .

12 Box 1. Hominization of juvenility Humans have a particularly sinuous growth trajectory of increasing and decreasing growth velocities compared to other primates [83-84]. Consequently, human ontogeny is often further partitioned into early juvenility (childhood), juvenility and adolescence. The origin, antiquity and significance of childhood and adolescence are debated [85-89], and remain opaque because few children are represented in the fossil record. Well-dated dental formation and eruption patterns suggest that the human pattern of juvenility begins to take a modern form as early as two million years ago, but does not assume a fully modern form until early Homo sapiens {Dean, 2006 #526;Smith, 1995 #129;Smith, 2010 #697}. During this time frame, a number of behavioral and life history features emerge that affect the economic relationship between mothers and young and have implications for the human reproductive potential.

Childrearing traits that distinguish modern human mothers and children from other apes. Mothers Children Wean young early Short infancy, weaned well before eruption of Short birth intervals M1 Rapid reproductive pace Higher probability of surviving juvenility Raise multiple dependent young of Not independent foragers different ages at the same time Exchange resources and labor with others Provide help to juveniles Share parental care with other dependents Receive help from others to raise young

13 Box 2. Offspring quality vs. quantity and cooperative breeding The quality/quantity tradeoff models optimal reproductive solutions and is a framework used to compare species differences in parental investment strategies [8, 93-95]. It predicts that offspring survival either will plateau or decline at higher levels of fertility. This intuitive expectation follows from the principle of allocation that because time and resources are limited, a mother cannot both have more children and produce higher quality offspring, often measured by survivorship.

(a) In the classic quality/quantity tradeoff model offspring quality is a function of parental care, and in most mammalian species, specifically maternal investment. Energy available to an individual to allocate either to offspring quantity or quality is fixed (solid ellipse), often modeled as a function of maternal body size. (b) This modification, applicable to humans and other cooperative breeders, accounts for nonparental sources of reproductive energy. The dotted ellipse signifies that resources available to invest in either offspring quality or quantity are not limited to maternal production or body size. Extra-parental investment can affect offspring quantity and quality in two ways. Extra-parental inputs can be allocated directly to offspring quality (A), such as provisioning, allonursing, childcare, monetary or other transfers; or they can be allocated to increasing reproductive output (B) by reducing the energy mothers expend foraging and in other activities, leaving a greater metabolic balance available for the production of offspring. While offspring production is phylogenetically and physiologically constrained, for cooperative breeders offspring quality is not necessarily a direct function of offspring quantity.

(a) (b)

growth growth Individual Individual offspring quantity offspring quantity reproduction reproduction offspring quality offspring quality B A

Nonparental investment and energy sources

If transfers from children are important to the human reproductive pattern, two predictions can be made about offspring quantity. 1) Humans should have a different fertility trajectory in phylogenetic comparisons with other cooperative breeders. Within related taxa, cooperative breeders are predicted to have higher fertility compared to noncooperative breeders because breeding females have access to extra-maternal resources. Human mothers are expected to have an even greater fertility advantage because of the scale of transfers from juveniles. 2) In cross-cultural comparisons, fertility is expected to increase with the strength of transfers from juveniles, adjusting for interactions with transfers from other classes of helpers.

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