Variability of weaning age in mountain gorillas (Gorilla beringei beringei).

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. RESULTS: 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). DISCUSSION: 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.

Wild bonobo and chimpanzee females exhibit broadly similar patterns of behavioral maturation but some evidence for divergence.

OBJECTIVES: Primates exhibit variation in rates of growth and development. Variation in female growth and development across ape species appears to be explained by the Ecological Risk Aversion Hypothesis (ERAH). Indeed, existing data on variation in somatic growth and reproductive maturation between humans' closest living ape relatives, bonobos and chimpanzees, appear to be consistent with this hypothesis. However, existing data on behavioral maturation between the two species appear to contradict this hypothesis. We present novel behavioral data on infant and juvenile females from wild populations of both species in order to further evaluate predictions of the ERAH as it relates to the speed of behavioral maturation. MATERIALS AND METHODS: We analyzed 3 years of behavioral data on 17 female bonobos (<8 years of age) from LuiKotale, Democratic Republic of the Congo and 40 years of behavioral data on 30 age-matched female chimpanzees from Gombe, Tanzania. We compared the timing of (a) the attainment of independence from mothers and (b) the development of social skills using the following proxies: proximity between females and their mothers and the time that females spent engaged in eating, suckling, social play, social grooming, and riding on their mothers. RESULTS: We did not find species differences in the proportion of time that females spent in contact with their mothers or engaged in eating, suckling, social play, or social grooming. Female bonobos spent more time riding on their mothers than did female chimpanzees. Female bonobos spent more time at distances greater than 5 m from their mothers during the ages of 3-8 years, but females did not differ during the ages of 0-3 years. DISCUSSION: Behavioral maturation is largely similar between females of the two species based on the ages and proxies considered herein. We propose alternative explanations for the differences that we found in proximity and riding that do not invoke differences in underlying rates of maturation.