skip to main content

Title: Sources of variation in weaned age among wild chimpanzees in Gombe National Park, Tanzania
Abstract Objectives

A key feature of human life history evolution is that modern humans wean their infants 2–4 years earlier on average than African apes. However, our understanding of weaning variation in apes remains limited. Here we provide the first such report in chimpanzees by examining weaned age variation using long‐term data from Gombe National Park, Tanzania.

Material and Methods

We analyzed 41 years of observational behavioral data from 65 offspring of 29 mothers to examine the relationships between weaned age (defined as cessation of suckling) in wild chimpanzees and maternal age, dominance rank and parity, and offspring sex. We used Cox proportional hazards regression with mixed effects to model time to weaning and to examine potential sources of variation in offspring weaned age.


We found that male offspring were less likely than female offspring to wean by a given age and that weaned age of males varied more than weaned age of females. In addition, maternal dominance rank interacted with offspring age, such that low‐ranking mothers were less likely to wean offspring early, but this effect decreased with offspring age.


We found that male offspring and offspring of low‐ranking females were less likely to wean early, but did not find evidence for variable weaning according to maternal age or parity. As more data accumulate, we will be better able to disentangle the effects of maternal dominance rank, age and parity. Such studies will not only provide a richer understanding of living ape life history characteristics, but will also provide an important framework for understanding the evolution of early weaning in humans.

more » « less
Award ID(s):
Author(s) / Creator(s):
 ;  ;  ;  
Publisher / Repository:
Wiley Blackwell (John Wiley & Sons)
Date Published:
Journal Name:
American Journal of Physical Anthropology
Page Range / eLocation ID:
p. 419-429
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. Abstract Objectives

    Weaning is a key life history milestone for mammals that represents both the end of nutritional investment from the perspective of mothers and the start of complete nutritional independence for the infants. The age at weaning may vary depending on ecological, social, and demographic factors experienced by the mother and infant. Bwindi mountain gorillas live in different environmental conditions and have longer interbirth intervals than their counterparts in the Virunga Volcanoes, yet other life history characteristics of this population remain less well known. We use long‐term data from Bwindi Impenetrable National Park, Uganda to examine factors related to weaning age.

    Materials and methods

    We analyzed data on infants born in four mountain gorilla groups in Bwindi to quantify their age of weaning (defined as last nipple contact) and to test if the sex of offspring, parity, and dominance rank of mother influences age of weaning. We also compared the age at weaning and time to conception after resumption of mating in Bwindi and Virunga gorillas.


    Bwindi gorillas were weaned at an average age of 57.5 months. No difference was found between age of weaning for primiparous and multiparous mothers, nor did maternal dominance rank influence age of weaning, but sons were weaned at a later age than daughters. The majority of Bwindi mothers were still suckling when they resumed mating and mothers generally conceived before they weaned their previous offspring. The age of weaning was significantly later in Bwindi than in Virunga gorillas. After mothers resumed mating, the time to conceiving the next offspring was not significantly longer for Bwindi females than Virungas females (6 vs. 4 months).


    Later weaning age for sons than daughters is similar to findings of other studies of great apes. Bwindi mountain gorillas are weaned at approximately the same age as western gorillas and chimpanzees, which is more than a year later than Virunga mountain gorillas. The results of this study suggest that variation in ecological conditions of populations living in close geographic proximity can result in variation in life history patterns, which has implications for understanding the evolution of the unique life history patterns of humans.

    more » « less
  2. Abstract Objectives

    In humans and other mammals, maternal hormones are transferred to offspring during lactation via milk and may regulate postnatal development, including the pace of early growth. Here, we used a nonhuman primate model to test the hypotheses that milk cortisol and dehydroepiandrosterone‐sulfate (DHEAS) concentrations reflect maternal characteristics, and that changes in these hormones across lactation are associated with early postnatal growth rates.


    Demographic information, morphometrics, and milk samples were collected from rhesus macaque mothers and their infants at the California National Primate Research Center in Davis, California. Using linear models, we examined the relationship between maternal traits and milk hormone concentrations (N = 104 females) and explored the effect of milk hormones on the rate of offspring growth (N = 72 mother‐infant dyads), controlling for available milk energy.


    Contrary to previous studies, we found that milk cortisol concentrations were categorically higher in multiparous females than in primiparous females. However, milk DHEAS concentrations decreased with maternal parity. Neither milk cortisol nor DHEAS were related to maternal rank. Finally, changes in milk hormones predicted offspring growth in a sex‐specific and temporal manner: increases in cortisol from peak to late lactation predicted faster female growth, and increases in DHEAS concentrations from early to peak and peak to late lactation predicted faster male growth.


    Our findings shed light on how hormonal components of milk have sex‐specific effects on offspring growth during early postnatal life with varying temporal windows of sensitivity.

    more » « less
  3. Abstract

    An individual's size in early stages of life may be an important source of individual variation in lifetime reproductive performance, as size effects on ontogenetic development can have cascading physiological and behavioral consequences throughout life. Here, we explored how size‐at‐young influences subsequent reproductive performance in gray seals (Halichoerus grypus) using repeated encounter and reproductive data on a marked sample of 363 females that were measured for length after weaning, at ~4 weeks of age, and eventually recruited to the Sable Island breeding colony. Two reproductive traits were considered: provisioning performance (mass of weaned offspring), modeled using linear mixed effects models; and reproductive frequency (rate at which a female returns to breed), modeled using mixed effects multistate mark–recapture models. Mothers with the longest weaning lengths produced pups 8 kg heavier and were 20% more likely to breed in a given year than mothers with the shortest lengths. Correlation in body lengths between weaning and adult life stages, however, is weak: Longer pups do not grow to be longer than average adults. Thus, covariation between weaning length and future reproductive performance appears to be a carry‐over effect, where the size advantages afforded in early juvenile stages may allow enhanced long‐term performance in adulthood.

    more » « less
    more » « less
  5. Abstract Objectives

    Lactational programming, through which milk‐borne bioactives influence both neonatal and long‐term biological development, is well established. However, almost no research has investigated how developmental stimuli during a mother's early life may influence her milk bioactives in adulthood. Here, we investigated the association between maternal birth weight and milk epidermal growth factor (EGF) and epidermal growth factor receptor (EGF‐R) in later life. We predicted there would be a decrease in both milk EGF and EGF‐R in the milk produced by mothers who were themselves born low birth weight.


    Study participants are from the Cebu Longitudinal Health and Nutrition Survey. Mothers (n= 69) were followed longitudinally since birth with prospective data collection. Anthropometrics, health, and dietary recalls were collected with early morning milk samples when mothers were 24 to 25 years of age. Milk samples were analyzed for EGF and its receptor (EGF‐R). Analysis of variance was used to test for differences in milk EGF and EGF‐R between low and average birthweight mothers after adjustment for parity, age, and maternal dietary energy intake.


    Mothers who were low birth weight produced milk with significantly less EGF and more EGF‐R which resulted in a lower ratio of EGF to EGF‐R. These associations persisted after adjustment for infant age, maternal adiposity, and dietary energy.


    While this is a small sample size, these preliminary findings suggest that maternal early life characteristics, such as birth weight, may be important contributors to variation in milk bioactives. Future work is necessary to understand how variation in maternal early life may influence milk composition in adulthood.

    more » « less