Comparative and evolutionary dimensions of the energetics of human pregnancy and lactation

Hominin life history: reconstruction and evolution

In this review we attempt to reconstruct the evolutionary history of hominin life history from extant and fossil evidence. We utilize demographic life history theory and distinguish life history variables, traits such as weaning, age at sexual maturity, and life span, from life history-related variables such as body mass, brain growth, and dental development. The latter are either linked with, or can be used to make inferences about, life history, thus providing an opportunity for estimating life history parameters in fossil taxa. We compare the life history variables of modern great apes and identify traits that are likely to be shared by the last common ancestor of Pan-Homo and those likely to be derived in hominins. All great apes exhibit slow life histories and we infer this to be true of the last common ancestor of Pan-Homo and the stem hominin. Modern human life histories are even slower, exhibiting distinctively long post-menopausal life spans and later ages at maturity, pointing to a reduction in adult mortality since the Pan-Homo split. We suggest that lower adult mortality, distinctively short interbirth intervals, and early weaning characteristic of modern humans are derived features resulting from cooperative breeding. We evaluate the fidelity of three life history-related variables, body mass, brain growth and dental development, with the life history parameters of living great apes. We found that body mass is the best predictor of great ape life history events. Brain growth trajectories and dental development and eruption are weakly related proxies and inferences from them should be made with caution. We evaluate the evidence of life history-related variables available for extinct species and find that prior to the transitional hominins there is no evidence of any hominin taxon possessing a body size, brain size or aspects of dental development much different from what we assume to be the primitive life history pattern for the Pan-Homo clade. Data for life history-related variables among the transitional hominin grade are consistent and none agrees with a modern human pattern. Aside from mean body mass, adult brain size, crown and root formation times, and the timing and sequence of dental eruption of Homo erectus are inconsistent with that of modern humans. Homo antecessor fossil material suggests a brain size similar to that of Homo erectus s. s., and crown formation times that are not yet modern, though there is some evidence of modern human-like timing of tooth formation and eruption. The body sizes, brain sizes, and dental development of Homo heidelbergensis and Homo neanderthalensis are consistent with a modern human life history but samples are too small to be certain that they have life histories within the modern human range. As more life history-related variable information for hominin species accumulates we are discovering that they can also have distinctive life histories that do not conform to any living model. At least one extinct hominin subclade, Paranthropus, has a pattern of dental life history-related variables that most likely set it apart from the life histories of both modern humans and chimpanzees.

Evolutionary origins of obesity

Although it appeared relatively suddenly, the current obesity epidemic - largely manifest in industrialized societies but now spreading to the rest of the world - is the result of interaction between human biology and human culture over the long period of human evolution. As mammals and primates, humans have the capacity to store body fat when opportunities to consume excess energy arise. But during the millions of years of human evolution such opportunities were rare and transient. More commonly ancestral hominins and modern humans were confronted with food scarcity and had to engage in high levels of physical activity. In tandem with encephalization, humans evolved elaborate and complex genetic and physiological systems to protect against starvation and defend stored body fat. They also devised technological aids for increasing energy consumption and reducing physical effort. In the last century, industrialization provided access to great quantities of mass-produced, high-calorie foods and many labour-saving and transportation devices, virtually abolishing starvation and heavy manual work. In the modern obesogenic environment, individuals possessing the appropriate combination of ancestral energy-conserving genes are at greater risk for overweight and obesity and associated chronic diseases.

The biology of the colonizing ape

Hominin evolutionary history is characterized by regular dispersals, cycles of colonization, and entry into novel environments. This article considers the relationship between such colonizing capacity and hominin biology. In general, colonizing strategy favors rapid rates of reproduction and generalized rather than specialized biology. Physiological viability across diverse environments favors a high degree of phenotypic plasticity, which buffers the genome from selective pressures. Colonizing also favors the capacity to access and process information about environmental variability. We propose that early hominin adaptive radiations were based upon the development of such capacities as adaptations to unstable Pliocene environments. These components came together, along with fundamental changes in morphology, behavior, and cognition in the genus Homo, who exploited them in subsequent wider dispersals. Middle Pleistocene hominins and modern humans also show development of further traits, which correspond with successful probing of, and dispersals into, stressful environments. These traits have their precursors in primate or ape biology, but have become more pronounced during hominin evolution. First, short interbirth intervals and slow childhood growth allow human females to provision several offspring simultaneously, increasing the rate of reproduction in favorable conditions. This allows rapid recovery from population crashes, or rapid population growth in new habitats. Second, despite high geographical phenotypic variability, humans have high genetic unity. This is achieved by a variety of levels of plasticity, including physiology, behavior, and technology, which reduce the need to commit to genetic adaptation. Hominin behavior may increasingly have shaped both the ecological niches occupied and the selective pressures acting back on the genome. Such selective pressures may have been exacerbated by population dynamics, predicted to both derive from, and favor, the colonizing strategy. Exposure to ecological variability is likely to have generated particular selective pressures on female biology, favoring increasing steering of offspring ontogeny by maternal phenotype. We propose that the concept of hominins as "colonizing apes" offers a novel unified model for interpreting the suite of traits characteristic of our genus.

Oscillations in total body fat content through life: an evolutionary perspective

Adipose tissue is considered an efficient system in which to store energy. Throughout life, the total amount of body fat exhibits some oscillations. Typically, there are three specific periods in which there are notable increases in fat mass, specifically early in life, during pregnancy and lactation, and with ageing. The existence of the first two peaks in fat mass has been interpreted, from an evolutionary point of view, as a beneficial manoeuvre to protect against the scarcity of energy to the offspring of the species. Nevertheless, the role of increasing body fat with ageing is more dubious. However, recent evidence suggests that the gain in adiposity in senescence may also be interpreted in the same evolutionary context. The aim of this review is to focus on the age-related changes in fat depots. In addition, an evolutionary explanation to the observed changes has been emphasized.

Evolution of Infant and Young Child Feeding: Implications for Contemporary Public Health

Evolutionary anthropological and ethnographic studies are used to develop a general conceptual framework for understanding prehistoric, historic, and contemporary variation in human lactation and complementary feeding patterns. Comparison of similarities and differences in human and nonhuman primate lactation biology suggests humans have evolved an unusually flexible strategy for feeding young. Several lines of indirect evidence are consistent with a hypothesis that complementary feeding evolved as a facultative strategy that provided a unique adaptation for resolving tradeoffs between maternal costs of lactation and risk of poor infant outcomes. This evolved flexibility may have been adaptive in the environments in which humans evolved, but it creates potential for mismatch between optimal and actual feeding practices in many contemporary populations.

Infant carrying: The role of increased locomotory costs in early tool development

Among the costs of reproduction, carrying one's infant incurs one of the greatest drains on maternal energy, simply because of the added mass alone. Because of the dearth of archaeological evidence, however, how early bipeds dealt with the additional cost of having to carry infants who were less able to support their body weight against gravity is not particularly well understood. This article presents evidence on the caloric drain of carrying an infant in one's arms versus having a tool with which to sling the infant and carry her passively. The burden of carrying an infant in one's arms is on average 16% greater than having a tool to support the baby's mass and seems to have the potential to be a greater energetic burden even than lactation. In addition, carrying a baby in one's arms shortens and quickens the stride. An anthropometric trait that seems to offset some of the increased cost of carrying a baby in the arms is a wider bi-trochanteric width.

Changes in body fat distribution in relation to parity in American women: a covert form of maternal depletion

Using data from the Third National Health and Nutrition Examination Survey (NHANES III), conducted from 1988-1994, we investigated the effect of reproduction on the distribution of body fat in well-nourished American women. While women tend to gain weight and fat with succeeding pregnancies, if age and body mass index are controlled, increasing parity is associated with a decrease in hip and thigh circumferences, suprailiac and thigh skinfolds, and body fat estimated from skinfolds, while waist circumference increases, resulting in a relative decrease in lower-body fat. The mobilization of fat stores in the lower body during late pregnancy and lactation may help to meet the special needs of the developing brain for essential fatty acids and energy during the time of peak growth. When fat is regained after the postpartum period, relatively more is stored in central vs. peripheral depots, resulting in a patterned change in body shape with parity.

Seasonality, body condition, and timing of reproduction in Propithecus verreauxi verreauxi in the Kirindy Forest

Mammals that live in seasonal environments may adjust their reproductive cycles to cope with fluctuations in food availability. Because lemurs in Madagascar experience highly seasonal variation in food availability, we examined the effects of fluctuating food availability on body condition and reproduction in one of the larger living species, Verreaux's sifaka (Propithecus verreauxi verreauxi), in the Kirindy Forest of western Madagascar. Seven years of demographic data were combined with an intensive study of 25 individuals over the course of 18 months. In contrast to other populations of Verreaux's sifaka, females were found to have greater body mass than males. Both male and female sifaka exhibited significant losses of body mass and fat during the dry season. Females were more likely to give birth and successfully wean an infant when they had higher body mass during the mating season. They mated during the periods of high and declining food availability, gave birth during the lean season, and then timed mid/late lactation with the period of increasing food availability. Thus, we conclude that sifaka follow the "classic" reproductive strategy (sensu van Schaik and van Noordwijk [Journal of Zoology (London) 206:533-549, 1985]).

Costs of twins in free-ranging white-bellied spider monkeys (Ateles belzebuth belzebuth) at Tinigua National Park, Colombia

A female spider monkey (Ateles belzebuth belzebuth) that gave birth to twins was studied during 13 months, and her activity budget and diet were compared to those of females with single offspring and females with no offspring to assess selective pressures that could influence litter size. We recorded qualitative information on the development and social interactions of the twins and three other single infants. Emi, the female that had the twins, had the highest proportion of resting time and the lowest proportion of feeding and moving time compared to other adult females and males. Emi also had the lowest average daily travel distance and relied more heavily on flowers and leaves than other group members who included a higher proportion of fruits in their diets. These results suggest that twins are energetically costly to spider monkeys because of the direct energy allocated into raising and carrying the twins and, also, because these costs have direct effects in the ability of a mother to obtain high-quality resources (e.g., fruits). The twins had a slower rate of physical and social development as well as a smaller body size compared to same-aged singletons by the end of the study. Thus, the extended periods of maternal care and the costs associated with rearing and carrying offspring are some of the factors that influence the evolution and maintenance of a litter size of one in most anthropoid primates.

Sex differences in scent marking in Propithecus edwardsi of Ranomafana National Park, Madagascar

In this study we compared the scent-marking rates of females with those of males. Specifically, we examined the ability of season, dominance status, and natal status to explain the frequency of scent marking in female sifakas living wild in Ranomafana National Park, Madagascar, and compared the results with those published for males [Pochron et al., American Journal of Primatology, in press]. We also sought to determine whether vulnerability to infanticide affects marking frequency in adults of either sex, and whether female reproductive status affects female marking behavior. We found that males marked at twice the rate of females, and like males, females in single-female groups marked at the highest rates. Dominant females and non-natal females marked at higher rates than did subordinate females and natal females, a pattern also seen in males. This suggests that scent marks may convey important information about status. Neither females nor males varied their marking frequency with the presence of vulnerable infants. Females did not alter marking frequency with reproductive state, and like males, they marked at higher rates in the period prior to the mating season than they did in the mating season itself. This implies that females may use scent marks more for intrasexual aggression or territoriality than for mate attraction.

Fetal origins of developmental plasticity: are fetal cues reliable predictors of future nutritional environments?

Evidence that fetal nutrition triggers permanent adjustments in a wide range of systems and health outcomes is stimulating interest in the evolutionary significance of these responses. This review evaluates the postnatal adaptive significance of fetal developmental plasticity from the perspective of life history theory and evolutionary models of energy partitioning. Birthweight is positively related to multiple metabolically costly postnatal functions, suggesting that the fetus has the capacity to distribute the burden of energy insufficiency when faced with a nutritionally challenging environment. Lowering total requirements may reduce the risk of negative energy balance, which disproportionately impacts functions that are not essential for survival but that are crucial for reproductive success. The long-term benefit of these metabolic adjustments is contingent upon the fetus having access to a cue that is predictive of its future nutritional environment, a problem complicated in a long-lived species by short-term ecologic fluctuations like seasonality. Evidence is reviewed suggesting that the flow of nutrients reaching the fetus provides an integrated signal of nutrition as experienced by recent matrilineal ancestors, which effectively limits the responsiveness to short-term ecologic fluctuations during any given pregnancy. This capacity for fetal nutrition to minimize the growth response to transient ecologic fluctuations is defined here as intergenerational "phenotypic inertia," and is hypothesized to allow the fetus to cut through the "noise" of seasonal or other stochastic influences to read the "signal" of longer-term ecologic trends. As a mode of adaptation, phenotypic inertia may help the organism cope with ecologic trends too gradual to be tracked by conventional developmental plasticity, but too rapid to be tracked by natural selection. From an applied perspective, if a trait like fetal growth is designed to minimize the effects of short-term fluctuations by integrating information across generations, public health interventions may be most effective if focused not on the individual but on the matriline.

Examining the terminal investment hypothesis in humans and chimpanzees: Associations among maternal age, parity, and birth weight

The terminal investment hypothesis (Williams [1966] Adaptation and Natural Selection; Princeton, NJ: Princeton University Press) holds that reproductive effort should increase over time in iteroparous species in which reproductive value declines with age. Attempts to model this hypothesis and test it in various species have produced mixed results. Clutton-Brock ([ 1984] Am. Nat. 123:212-229) argued that simply testing for changes in propagule size with age fails to recognize that the costs of producing offspring of a given size may increase over the lifespan, hence absence of a positive correlation does not defeat the hypothesis. However, this interpretation is weakened by evidence of sequential increases in propagule size independent of age, as such changes reveal a capacity to increase absolute investment over time. Humans and chimpanzees meet the preconditions of the terminal investment hypothesis. Surveying the obstetrics literature, we show that the majority of published studies indicate that parity has a positive effect on birth weight, but age has no effect. Analyzing 436 captive chimpanzee births, we document a positive influence of parity and a negative influence of age. We therefore conclude that, though it is yet to be replaced by a more compelling alternative, the terminal investment hypothesis is not supported in these two species, as absence of a positive effect of age on birth weight cannot be interpreted in a manner congruent with the hypothesis.

Body composition, age and fertility among free-ranging female rhesus macaques (Macaca mulatta)

The relationship of morphometric measures and birth status among 93 females on Cayo Santiago trapped during the 2001 and 2002 seasons was assessed. The proportion of females giving birth differed between the two seasons (0.58 vs. 0.38; P = 0.006) with a prominent decline among older females. Most morphometric measures increased from adolescent to adult groups, but bicep circumference showed a significant decrease among adults. When controlled for age differences, females with infants during the 2002 season exhibited greater bicep circumference, but no difference in abdominal fat than those without. Members of the socially dominant group did not have a higher rate of birth in either 2001 or 2002, despite being significantly longer and weighing more than those of the subordinate group. Abdominal skinfold and bicep circumference were significant predictors of birth status during the 2002 season, controlling for age group, social group membership, and parity in the previous year. Bicep circumference was also a significant predictor of birth status for the 2001 birth season. These findings suggest that individual variation in body composition among females of Cayo Santiago is associated with differences in fertility.

Why are human newborns so fat? Relationship between fatness and brain size at birth [retracted article]

The plumpness of the human newborn has long been recognized as a trait in need of explanation among researchers. Using a linear regression analysis, we find that head circumference is significantly and positively associated with BMI at birth, after gestational age and birthlength were controlled for, in a sample of 1,069 healthy liveborn routinely delivered at the University Hospital of Coimbra (partial correlation r = 0.409, P < 0.0001). This significant association is consistent with the idea that newborn fatness is related to the higher need of lipids in newborn humans as an energetic and plastic substrate during its accelerated brain growth period. As birthweight and birth head size are associated with head size and cognitive abilities in childhood and adult life, it could be postulated that these cognitive abilities could have acted as selective pressure responsible for the newborn fatness increase in our lineage.