Maturation is prolonged and variable in female chimpanzees

Abstract

Chimpanzees are important referential models for the study of life history in hominin evolution. Age at sexual maturity and first reproduction are key life history milestones that mark the diversion of energy from growth to reproduction and are essential in comparing life history trajectories between chimpanzees and humans. Yet, accurate information on ages at these milestones in wild chimpanzees is difficult to obtain because most females transfer before breeding. Precise age at first birth is only known from a relatively small number of non-dispersing individuals. Moreover, due to small sample sizes, the degree to which age at maturation milestones varies is unknown. Here we report maturation milestones and explore sources of variance for 36 wild female chimpanzees of known age, including eight dispersing females born in Gombe National Park, Tanzania. Using Kaplan-Meier survival analysis, including censored intervals, we find an average age of 11.5 years (range 8.5-13.9) at sexual maturity and 14.9 years (range 11.1-22.1) at first birth. These values exceed previously published averages for wild chimpanzees by one or more years. Even in this larger sample, age at first birth is likely underestimated due to the disproportionate number of non-dispersing females, which, on average, give birth two years earlier than dispersing females. Model selection using Cox Proportional Hazards models shows that age at sexual maturity is delayed in females orphaned before age eight years and those born to low-ranking mothers. Age at first birth is most delayed in dispersing females and those orphaned before age eight years. These data provide improved estimates of maturation milestones in a population of wild female chimpanzees and indicate the importance of maternal factors in development.

Sources of variation in weaned age among wild chimpanzees in Gombe National Park, Tanzania

“…We used long‐term demographic records to determine mother's age at offspring birth, parity (primiparous or multiparous), offspring birthdate, and offspring sex. Maternal dominance rank at birth was determined by the direction of pant grunts (a submissive vocalization) and decided aggressive interactions, using a modification of the standard Elo ranking to assign normalized cardinal Elo scores to each mother (Foerster et al, ; Walker, Walker, Goodall, & Pusey, ). For visualization purposes, we placed females in three rank categories (high, middle, and low) using Jenks natural breaks.…”

Heterochrony in chimpanzee and bonobo spatial memory development

“…For example, chimpanzees exhibit slower rates of white matter maturation (Sakai et al, ) and longer periods of synaptogenesis in childhood (Bianchi et al, ) compared to Old World monkeys. While there are not comparable studies of bonobo brain development, studies of physical maturation indicate that chimpanzees and bonobos exhibit largely similar life history milestones, such as age of weaning, sexual maturity, or first birth (Robson & Wood, ; Walker, Walker, Goodall, & Pusey, ). Thus, the life history hypothesis predicts that chimpanzees and bonobos will both exhibit similar patterns of cognitive development, given their similar life history patterns.…”

Intra-community infanticide in wild, eastern chimpanzees: a 24-year review

“…These fitness costs are quantifiable. Female chimpanzees give birth on average every 5-7 years (Emery Thompson et al 2007b), and average expected reproductive lifespan for Sonso females is roughly 26 years, given an average age at first conception in East African chimpanzees of 14.2 years (Walker et al 2018b) and an oldest age for a live birth by a Sonso female of 40.6 years (Emery Thompson et al 2007a, b). The most fecund Sonso female had eight recorded births, with four surviving infants, losing infants at 1 week, 1 week, 1 day, and 65 days post-partum, a total loss of 1944 days to gestation, lactation, and delay to conception.…”