Life History Hypotheses Research Articles

Evolutionary trade-offs in dormancy phenology.

Seasonal animal dormancy is widely interpreted as a physiological response for surviving energetic challenges during the harshest times of the year (the physiological constraint hypothesis). However, there are other mutually non-exclusive hypotheses to explain the timing of animal dormancy, that is, entry into and emergence from hibernation (i.e. dormancy phenology). Survival advantages of dormancy that have been proposed are reduced risks of predation and competition (the 'life-history' hypothesis), but comparative tests across animal species are few. Using the phylogenetic comparative method applied to more than 20 hibernating mammalian species, we found support for both hypotheses as explanations for the phenology of dormancy. In accordance with the life-history hypotheses, sex differences in hibernation emergence and immergence were favored by the sex difference in reproductive effort. In addition, physiological constraint may influence the trade-off between survival and reproduction such that low temperatures and precipitation, as well as smaller body mass, influence sex differences in phenology. We also compiled initial evidence that ectotherm dormancy may be (1) less temperature dependent than previously thought and (2) associated with trade-offs consistent with the life-history hypothesis. Thus, dormancy during non-life-threatening periods that are unfavorable for reproduction may be more widespread than previously thought.

Evolutionary trade-offs in dormancy phenology

Seasonal animal dormancy is widely interpreted as a physiological response for surviving energetic challenges during the harshest times of the year (the physiological constraint hypothesis). However, there are other mutually non-exclusive hypotheses to explain the timing of animal dormancy, that is, entry into and emergence from hibernation (i.e. dormancy phenology). Survival advantages of dormancy that have been proposed are reduced risks of predation and competition (the ‘life-history’ hypothesis), but comparative tests across animal species are few. Using the phylogenetic comparative method applied to more than 20 hibernating mammalian species, we found support for both hypotheses as explanations for the phenology of dormancy. In accordance with the life-history hypotheses, sex differences in hibernation emergence and immergence were favored by the sex difference in reproductive effort. In addition, physiological constraint may influence the trade-off between survival and reproduction such that low temperatures and precipitation, as well as smaller body mass, influence sex differences in phenology. We also compiled initial evidence that ectotherm dormancy may be (1) less temperature dependent than previously thought and (2) associated with trade-offs consistent with the life-history hypothesis. Thus, dormancy during non-life-threatening periods that are unfavorable for reproduction may be more widespread than previously thought.

Tooth and long bone scaling in Sardinian ochotonids (Early Pleistocene-Holocene): Evidence for megalodontia and its palaeoecological implications

Body size is a useful character to unravel the biology of fossil taxa and, indirectly, the palaeoenvironment in which they lived. However, the reliability of size proxies is debated, particularly among insular endemics in which large teeth relative to body size have been observed. To shed light on this issue, here we compute statistical models to assess: (i) teeth and long bone scaling and (ii) dentition size evolutionary dynamics in the anagenetic lineage Prolagus figaro-P. sardus from the late Early Pleistocene-Holocene of Sardinia, Italy. Postcranial bones, particularly tibial epiphyses, are shown to be the most suitable parameters for prediction of body mass in Prolagus and other lagomorph taxa. By contrast, p3 should not be used for size reconstructions, whereas m1 width provided the highest correlation with long bones. On the other hand, the postcanine occlusal surface of Sardinian Prolagus (m1 and toothrow area) appeared significantly larger than those of extant lagomorphs, pointing to a higher resistance to tooth wear and a more durable permanent dentition. It seems likely that megalodontia might have evolved in Sardinian Prolagus to cope with the abrasive diet found on islands (food habit hypothesis). Nonetheless, we cannot exclude that other biological features, such as long longevity, could be co-driving forces in this fossil lineage (life history hypothesis). The results achieved throw light on size and scale evolution in lagomorphs, as well as refine the eco-evolutionary responses of small mammals to insular regimes.

Oversummering juvenile and adult Semipalmated sandpipers in Per\xfa gain enough survival to compensate for foregone breeding opportunity

BackgroundAge at maturity and the timing of first breeding are important life history traits. Most small shorebird species mature and breed as ‘yearlings’, but have lower reproductive success than adults. In some species, yearlings may defer northward migration and remain in non-breeding regions (‘oversummering’) until they reach 2 years of age. Some adults also oversummer. Oversummering would be favoured by natural selection if survival were as a result raised sufficiently to compensate for the missed breeding opportunity. Several thousand Semipalmated Sandpipers (Calidris pusilla) spend the non-breeding period at Paracas, Perú, including individuals with long bills (likely from eastern Arctic breeding populations ~ 8000 km distant) and short bills (likely from western Arctic breeding populations, up to 11,000 km distant), with short-billed birds more likely to oversummer. We tested the prediction that oversummering birds have higher survival than migrants, and that the magnitude of this higher survival for oversummering birds is enough to compensate for their lost breeding season.MethodsWe used a Multi-State Mark-Recapture model based on 5 years of encounter data (n = 1963 marked birds, and 3229 resightings) obtained year-round at Paracas, Perú, to estimate seasonal (i.e. breeding and non-breeding) survivorship for migrant and oversummering birds. We calculated the magnitude of the oversummering survival advantage required to compensate, for both yearlings and adults, based on published measures of annual survival and reproductive success. Using bill length as a proxy for migration distance, we investigated whether migratory survival is distance-dependent.ResultsWe estimate that 28% of yearlings and 19% of adults oversummer. Survival is higher for oversummering birds than for migrants, and the oversummering survival advantage is greater for adults (0.215) than for yearlings (0.140). The theoretical thresholds predicted by the size of the missed reproductive opportunity are 0.240 for adults and 0.134 for yearlings. Migratory survival decreases and the oversummering rate increases with migration distance, as assessed by culmen length.ConclusionsOur results support the life history hypothesis that oversummering raises survival enough to compensate for the loss of a breeding opportunity. Greater migration distance lowers survival and increases the probability of oversummering.

Determinants of establishment success: Comparing alien and native freshwater fishes in Taiwan.

Many parts of Asia, including Taiwan, have suffered severely from freshwater fish invasions. However, few studies using an assemblage approach have been conducted in the region so far that would help to prioritize suitable preventive actions. In this study, we focused on the invasion process from the import stage to the establishment stage, and defined establishment success as the success during this predefined process. We used datasets of freshwater fish assemblages in Taiwan to (1) compare established versus non-established alien species to distinguish the determinants of establishment success, and (2) to use these determinants to test a life history hypothesis which predicts that the magnitudes of the determinants should be significantly different between established alien species and native species. We collated a dataset for freshwater fish species which were imported into Taiwan (n = 118) of which some successfully established (n = 26), and another dataset for freshwater fish species native to Taiwan (n = 77). For each imported species, we collected data for 17 variables, including two phylogenetic, two human-use, two invasion history, and 11 life history variables. We then used decision tree methods, which have advantages in analyzing datasets with many variables of mixed types without the need to make assumptions about data distributions and input data for missing values. Our results showed that aquaculture use and maximum body length were the most important determinants for predicting establishment success of alien freshwater fish in Taiwan. Comparing five important determinants between established alien versus native species showed that the established alien species were significantly more often used in aquaculture, were associated with a higher number of established countries, and had a larger body length and greater highest water temperature tolerance than the native species. We thus conclude that our results provided evidence to support our stated hypothesis. We suggest that aquaculture use, measures of body size, and the number of previously invaded countries may alert researchers and conservation managers to species with a high establishment potential, especially for countries with similar conditions as those in Taiwan.

Genetic analysis and satellite tracking reveal origin of the green turtles in San Diego Bay

Understanding population structure and migration patterns is critical for conservation of marine species that undertake seasonal migrations, often spanning entire oceans, between breeding grounds and distant feeding areas. To examine the stock origin of the green turtle foraging aggregation in San Diego Bay, California, USA (32.6°N, 117.1°W) and evaluate current life history hypotheses, 770 bp sequences of the mitochondrial DNA control region from 121 green turtles captured in San Diego Bay were compared to potential source (nesting) populations across the Pacific. Mixed stock analysis indicated that the San Diego Bay foraging population originates from eastern Pacific nesting sites, primarily the Revillagigedo Archipelago and the coast of Michoacan, Mexico. To further understand migratory pathways and breeding destinations, three mature female turtles were satellite tracked from the San Diego Bay foraging ground (FG), including one turtle tracked for 364 days that nested at Socorro Island in Revillagigedo and returned to San Diego Bay (total distance approx. 3200 km), one of the first continuous satellite tracks of a sea turtle from a FG to its nesting site and back. Tracks from another turtle indicated possible nesting at Tres Marias Islands, approximately 100 km off the Mexican mainland coast. All three turtles have subsequently been recaptured in San Diego Bay, indicating a high degree of FG philopatry. These results rule out previous speculation that Hawaiian green turtles were also using San Diego Bay, provide insights into the distribution of the Revillagigedo Islands and Michoacan breeding populations and draw attention to the Tres Marias Islands as a potential source of green turtles in the temperate North Pacific.

Inventory and Assessment of theGorilla gorilla(Savage, 1847) Skeletal Collection Housed at the Yale Peabody Museum of Natural History

Museum collections are critical resources for examination of comparative anatomy, developmental biology, and life history hypotheses. Skeletal collections provide insight into spatiotemporal, species, population, and individual variation associated with environmental, social, and epidemiological history. For endangered species such as great apes, these collections provide data that are nearly impossible to replicate today. In this second in a series of articles reviewing the great ape holdings of the Peabody Museum of Natural History at Yale University, we describe the Gorilla gorilla skeletal collection, comprising 16 individuals: 13 males, 2 females, and 1 of indeterminate sex. Evidence of socioecology, interindividual violence, hunting, captive management, predation, senescence, and infectious disease are present within the skeletal material. The collection exhibits possible taxonomic diversity as well. Our evaluation of the Yale Peabody Museum collection provides a baseline for future research and testable hypotheses for alternate techniques, such as isotopic analyses of dental calculus and noninvasive genetic testing. Museum collections continue to provide new insights into taxonomic and individual variation and environmental context, and ultimately allow for comparisons between modern and historical environmental and behavioral variables.

Variation in the social organization of gorillas: Life history and socioecological perspectives.

A focus of socioecological research is to understand how ecological, social, and life history factors influence the variability of social organization within and between species. The genus Gorilla exhibits variability in social organization with western gorilla groups being almost exclusively one-male, yet approximately 40% of mountain gorilla groups are multimale. We review five ultimate causes for the variability in social organization within and among gorilla populations: human disturbance, ecological constraints on group size, risk of infanticide, life history patterns, and population density. We find the most evidence for the ecological constraints and life history hypotheses, but an over-riding explanation remains elusive. The variability may hinge on variation in female dispersal patterns, as females seek a group of optimal size and with a good protector male. Our review illustrates the challenges of understanding why the social organization of closely related species may deviate from predictions based on socioecological and life history theory.

Revealing life-history traits by contrasting genetic estimations with predictions of effective population size.

Effective population size, a central concept in conservation biology, is now routinely estimated from genetic surveys and can also be theoretically predicted from demographic, life-history, and mating-system data. By evaluating the consistency of theoretical predictions with empirically estimated effective size, insights can be gained regarding life-history characteristics and the relative impact of different life-history traits on genetic drift. These insights can be used to design and inform management strategies aimed at increasing effective population size. We demonstrated this approach by addressing the conservation of a reintroduced population of Asiatic wild ass (Equus hemionus). We estimated the variance effective size (Nev ) from genetic data (N ev =24.3) and formulated predictions for the impacts on Nev of demography, polygyny, female variance in lifetime reproductive success (RS), and heritability of female RS. By contrasting the genetic estimation with theoretical predictions, we found that polygyny was the strongest factor affecting genetic drift because only when accounting for polygyny were predictions consistent with the genetically measured Nev . The comparison of effective-size estimation and predictions indicated that 10.6% of the males mated per generation when heritability of female RS was unaccounted for (polygyny responsible for 81% decrease in Nev ) and 19.5% mated when female RS was accounted for (polygyny responsible for 67% decrease in Nev ). Heritability of female RS also affected Nev ; hf2=0.91 (heritability responsible for 41% decrease in Nev ). The low effective size is of concern, and we suggest that management actions focus on factors identified as strongly affecting Nev, namely, increasing the availability of artificial water sources to increase number of dominant males contributing to the gene pool. This approach, evaluating life-history hypotheses in light of their impact on effective population size, and contrasting predictions with genetic measurements, is a general, applicable strategy that can be used to inform conservation practice.

A model to simulate the spread and management cost of kudzu (Pueraria montana var. lobata) at landscape scale

Invasive plant species are a global problem causing damaging ecologic and economic impacts valued in billions annually and requiring agencies and local governments to contain, eradicate, or otherwise manage them. The spread of invasive species is complex and involves spatially and temporally variable processes. The life history characteristics of invasive species are frequently poorly known. This makes the development of effective strategies problematic. Spatially explicit population models, offering a virtual platform to test the implications of life history hypotheses or alternative management strategies can therefore provide valuable and inexpensive support on which to base management decisions. However, two problems limit the use of such models in the actual management of invasive species: (1) field practitioners and management groups often lack the expertise (or funds) to develop their own models and (2) existing invasive species models are often too cryptic to be readily understood and modified by novices.We present a model simulating the spread and population dynamics of kudzu and illustrate its potential through a case study application. The model, which could be adapted to fit the characteristics of other invasive plant species, couples species biology, dispersal and local management practices to generate local and regional scale outcomes that can be compared to identify those most effective given the level of available resources. This model could be further expanded, improved, and utilized as a medium through which the weed management, modeling, and decision-making communities could collaboratively increase the knowledge pool and effectiveness of their management decisions.

Life History theory hypotheses on child growth: Potential implications for short and long‐term child growth, development and health

Life history theory integrates ecological, physiological, and molecular layers within an evolutionary framework to understand organisms' strategies to optimize survival and reproduction. Two life history hypotheses and their implications for child growth, development, and health (illustrated in the South African context) are reviewed here. One hypothesis suggests that there is an energy trade-off between linear growth and brain growth. Undernutrition in infancy and childhood may trigger adaptive physiological mechanisms prioritizing the brain at the expense of body growth. Another hypothesis is that the period from conception to infancy is a critical window of developmental plasticity of linear growth, the duration of which may vary between and within populations. The transition from infancy to childhood may mark the end of a critical window of opportunity for improving child growth. Both hypotheses emphasize the developmental plasticity of linear growth and the potential determinants of growth variability (including the role of parent-offspring conflict in maternal resources allocation). Implications of these hypotheses in populations with high burdens of undernutrition and infections are discussed. In South Africa, HIV/AIDS during pregnancy (associated with adverse birth outcomes, short duration of breastfeeding, and social consequences) may lead to a shortened window of developmental plasticity of growth. Furthermore, undernutrition and infectious diseases in children living in South Africa, a country undergoing a rapid nutrition transition, may have adverse consequences on individuals' cognitive abilities and risks of cardio-metabolic diseases. Studies are needed to identify physiological mechanisms underlying energy allocation between biological functions and their potential impacts on health.

Inventory and Assessment of thePan troglodytes(Blumenbach, 1799) Skeletal Collection Housed at the Yale Peabody Museum

Museum collections are critical resources for examining comparative anatomy, developmental biology and life history hypotheses. Evaluation of skeletal collections provides insight into spatiotemporal, species, population and individual variation associated with environmental, social and epidemiological history. For endangered species such as primates, these collections provide data that are nearly impossible to replicate today.In this first in a series of papers reviewing the great ape holdings of the Peabody Museum of Natural History at Yale University, we describe the Pan troglodytes skeletal collection. Although most of the collection is from Central Africa's Atlantic coast, it includes several Pan troglodytes subspecies. Multiple age and sex classes are present, with craniodental and postcranial elements available for each age class. All material was assessed for developmental, disease, trauma and socioecological indicators. Multiple indicators of metabolic stress are present and likely associated with nutritional and epidemiological factors. Instances of trauma and injury, ranging from antemortem to perimortem events, are described. For some individuals, these injuries are likely associated with intraspecific and intrasexual competition and violence, whereas others are suggestive of infanticide attempts. Other injuries associated with interspecific violence are of value for forensic examination.Our evaluation of the Yale Peabody Museum collection provides a baseline for future research and testable hypotheses for alternate techniques, such as isotopic analyses of calculus and noninvasive genetic testing. Museum collections continue to provide new insights into taxonomic and individual variation and environmental cues, and ultimately allow for comparisons between modern and historical environmental and behavioral variables.

Genetic confounding of the relationship between father absence and age at menarche

Abstract Research in evolutionary psychology, and life history theory in particular, has yielded important insights into the developmental processes that underpin variation in growth, psychological functioning, and behavioral outcomes across individuals. Yet, there are methodological concerns that limit the ability to draw causal inferences about human development and psychological functioning within a life history framework. The current study used a simulation-based modeling approach to estimate the degree of genetic confounding in tests of a well-researched life history hypothesis: that father absence ( X ) is associated with earlier age at menarche ( Y ). The results demonstrate that the genetic correlation between X and Y can confound the phenotypic association between the two variables, even if the genetic correlation is small—suggesting that failure to control for the genetic correlation between X and Y could produce a spurious phenotypic correlation. We discuss the implications of these results for research on human life history, and highlight the utility of incorporating genetically sensitive tests into future life history research.

Environmental Unpredictability in Childhood Is Associated With Anxious Romantic Attachment and Intimate Partner Violence Perpetration.

Human life history theory describes how resources are allocated among conflicting life tasks, including trade-offs concerning reproduction. The current research investigates the unique importance of environmental unpredictability in childhood in association with romantic attachment, and explores whether objective or subjective measures of environmental risk are more informative for testing life history hypotheses. We hypothesize that (1) unpredictability in childhood will be associated with greater anxious attachment, (2) anxious attachment will be associated with intimate partner violence (IPV) perpetration, and (3) anxious attachment will mediate the relationship between unpredictability in childhood and IPV perpetration. In two studies (total n = 391), participants in a heterosexual, romantic relationship completed self-report measures of childhood experiences, romantic attachment, and IPV perpetration. Study 1 provides support for Hypothesis 1. Hypothesis 1 is replicated only for men, but not women, in Study 2. Results of Study 2 provide support for Hypothesis 2 for men and women, and Hypothesis 3 was supported for men but not women. The findings contribute to the literature addressing the association of environmental risk in childhood on adult romantic relationship outcomes.

Life history data of lizards of the world

Life is defined by a capacity for reproduction, yet the ways in which animals reproduce vary tremendously among species. Reproductive life histories are complex phenomena influenced by a variety of factors, such as physical condition of individuals, food supply, bauplan, and phylogenetic history, which are often correlated. Understanding life histories is crucial in evolutionary ecology because they represent different strategies that evolved to maximize individual fitness. Variation in life history can be attributed to both historical (=phylogenetic) and non‐historical (=environmental) causes. We compiled a large data set on lizard life history variables. Our data set consists of life history data for 737 lizard populations, representing 337 species in 33 families from 280 study sites globally. About 64% of these data were collected directly by the authors. Regarding data collected by authors, we sexed lizards by dissection and direct examination of gonads. Females were considered reproductive if vitellogenic follicles or oviductal eggs were present. We regarded the simultaneous presence of enlarged vitellogenic follicles and either oviductal eggs or corpora lutea as evidence for the sequential production of more than one clutch of eggs per year. We considered clutch size as the number of vitellogenic follicles or oviductal eggs in mature females. For each population, we recorded the following variables, if available: adult female mass (g), adult female SVL (mm), female SVL at maturity (based on SVL of smallest reproductive female), offspring SVL (based on hatchling size or smallest individual in population), clutch or litter size (number of offspring per clutch or litter for all reproductive females in the population), total number of clutches or litters per year, clutch frequency (single‐ or multiple‐brooded), relative clutch or litter mass [total volume of eggs or embryos (cc) divided by adult female mass (g)], reproductive mode (oviparous or viviparous), foraging mode (sit‐and‐wait or active), distribution (tropical or temperate), and preferred habitat type (aquatic, arboreal, bromelicolous, fossorial, psammophilous, saxicolous, semi‐arboreal, or terrestrial). These data are valuable to test various life history hypotheses, including phylogenetic design constraints and effects of ecology and climate on reproductive tactics.

Do avian cooperative breeders live longer?

Cooperative breeding is not common in birds but intriguingly over-represented in several families, suggesting that predisposing factors, similar ecological constraints or a combination of the two facilitate the evolution of this breeding strategy. The life-history hypothesis proposes that cooperative breeding is facilitated by high annual survival, which increases the local population and leads to a shortage of breeding opportunities. Clutch size in cooperative breeders is also expected to be smaller. An earlier comparative analysis in a small sample of birds supported the hypothesis but this conclusion has been controversial. Here, I extend the analysis to a larger, worldwide sample and take into account potential confounding factors that may affect estimates of a slow pace of life and clutch size. In a sample of 81 species pairs consisting of closely related cooperative and non-cooperative breeders, I did not find an association between maximum longevity and cooperative breeding, controlling for diet, body mass and sampling effort. However, in a smaller sample of 37 pairs, adult annual survival was indeed higher in the cooperative breeders, controlling for body mass. There was no association between clutch size and cooperative breeding in a sample of 93 pairs. The results support the facilitating effect of high annual survival on the evolution of cooperative breeding in birds but the effect on clutch size remains elusive.

Insights into life history theory: a brood size manipulation on a southern hemisphere species, Tachycineta leucorrhoa, reveals a fast pace of life

Life history traits exhibit substantial geographical variation associated with the pace of life. Species with a slow pace are expected to invest more in their future/residual reproductive value and are more common at tropical latitudes, whereas species from high latitudes, with a faster pace, are expected to prioritize the current reproductive effort. Most evidence supporting this pattern comes from studies conducted in tropical and north temperate species; very little is known about patterns in southern South American species. Here, we describe the life history of a southern swallow Tachycineta leucorrhoa and use an experimental approach to test their breeding strategy over four breeding seasons. We manipulated brood size for 105 nests of white‐rumped swallows to measure whether costs of reproduction were borne by adults or nestlings as alternative selection strategies towards maintaining residual or current reproductive value. Adults increased their feeding effort in enlarged broods, at least enough to maintain nestlings’ development/growth. In addition, adults decreased the number of visits to the nest (without having a negative effect on nestlings) in reduced broods. We did not detect differences in fledging success among treatments, suggesting there were no differences in nestlings’ survival. However, enlarged broods more frequently incurred in complete nest failure, suggesting only some adults were able to cope with increased costs of reproduction. We conclude this species is characterized by a fast pace of life similar to their northern congeners and less like its tropical ones. This is one of the first studies to use an experimental approach to test a life history hypothesis of pace of life using data from a southern South American species. We encourage researches to include southern species when evaluating latitudinal variations as we still do not have enough evidence to assume all southern subtropical species are indeed similar to tropical ones.

Evidence for bluegill spawning plasticity obtained by disentangling complex factors related to recruitment

Fishes can exhibit many forms of plasticity to maximize fitness. However, limited information exists on the ability of freshwater fish to adjust spawning behavior and characteristics (e.g., timing, duration, magnitude of spawning events) to minimize mortality of recruits and ultimately maximize fitness. We wanted to test the life history hypothesis for bluegill (Lepomis macrochirus) (i.e., opportunistic strategy) utilizing existing literature and results from our study to further evaluate the potential for spawning plasticity in this species. Our objective was to identify bluegill recruitment bottlenecks (i.e., periods of high mortality) and factors associated with these events in a single lake during 7 consecutive years. Bluegills exhibited shorter spawning durations and fewer spawning pulses (i.e., peaks in larval production) compared with bluegill in previous studies. Late-hatched (compared with early-hatched) bluegills consistently contributed the most to the fall juvenile population; these recruitment patterns were primarily attributed to biotic drivers. Our study suggests that bluegill could exhibit spawning plasticity and extends our current understanding of adaptations that are potentially capable of increasing fitness for a freshwater fish species under a wide range of environmental conditions and uncertainty.

Patterns of physical and psychological development in future teenage mothers

Teenage childbearing may have childhood origins and can be viewed as the outcome of a coherent reproductive strategy associated with early environmental conditions. Life-history theory would predict that where futures are uncertain fitness can be maximized through diverting effort from somatic development into reproduction. Even before the childbearing years, future teenage mothers differ from their peers both physically and psychologically, indicating early calibration to key ecological factors. Cohort data have not been deliberately collected to test life-history hypotheses within Western populations. Nonetheless, existing data sets can be used to pursue relevant patterns using socioeconomic variables as indices of relevant ecologies. We examined the physical and psychological development of 599 young women from the National Child Development Study who became mothers before age 20, compared to 599 socioeconomically matched controls. Future young mothers were lighter than controls at birth and shorter at age 7. They had earlier menarche and accelerated breast development, earlier cessation of growth and shorter adult stature. Future young mothers had poorer emotional and behavioural adjustment than controls at age 7 and especially 11, and by age 16, idealized younger ages for marriage and parenthood than did the controls. The developmental patterns we observed are consistent with the idea that early childbearing is a component of an accelerated reproductive strategy that is induced by early-life conditions. We discuss the implications for the kinds of interventions likely to affect the rate of teenage childbearing.

Mount Pinatubo, Inflammatory Cytokines, and the Immunological Ecology of Aeta Hunter-Gatherers

Early growth cessation and reproduction are predicted to maximize fitness under conditions of high adult mortality, factors that could explain the pygmy phenotype of many rainforest hunter-gatherers. This life-history hypothesis is elegant but contentious in part because it lacks a clear biological mechanism. One mechanism stems from the field of human immunological ecology and the concept of inflammation "memory" across the life cycle and into subsequent generations. Maternal exposures to disease can influence immunological cues present in breast milk; because maternal provisioning via lactation occurs during critical periods of development, it is plausible that these cues can also mediate early growth cessation and small body size. Such epigenetic hypotheses are difficult to test, but the concept of developmental programming is attractive because it could explain how the stature of a population can change over time, in terms of both secular increases and rapid intergenerational decreases. Here we explore this concept by focusing on the Aeta, a population of former hunter-gatherers, and the Ilocano, a population of rice farmers. We predicted that Aeta mothers would produce breast milk with higher concentrations of four bioactive factors due to high infectious burdens. Further, we predicted that the concentrations of these factors would be highest in the cohort of women born in the early 1990s, when exposure to infectious disease was acute following the eruption of Mount Pinatubo in June 1991. We analyzed levels of adiponectin, C-reactive protein, and epidermal growth factor in the milk of 24 Aeta and 31 Ilocano women and found no detectable differences, whereas levels of transforming growth factor-?2 were elevated among the Aeta, particularly as a function of maternal age. We found no difference between cohorts divided by the volcanic eruption (n = 43 born before, n = 12 born after). We discuss the implications of our findings for the terminal investment hypothesis and we suggest that the historical ecology of the Aeta is a promising model system for testing epigenetic hypotheses focused on the evolution of small body size.