The Ecology and Behavior of New Chimpanzee Mothers at Ngogo, Kibale National Park, Uganda.

Abstract

Female mammals invest heavily in reproduction. Females require food to reproduce and thus mothers compete for food. In group-living species, females who form strong intrasexual social bonds reproduce more than females who do not. To reproduce successfully, females must meet both nutritional and social needs. Although mothers face certain challenges involving competition and affiliation, not all mothers are the same. One factor, parity, affects females across taxa. Due to inexperience, primiparas, mothers raising their first offspring, face challenges not experienced by multiparas, mothers with multiple offspring. Therefore, primiparas behave differently than multiparas. Chimpanzees are an excellent species to investigate whether behavior varies with parity. Chimpanzees are long-lived and primiparas live with multiparas in multi-female, multi-male, fission-fusion communities. I investigated the relationship between parity and behavior by conducting a 15-month study of female chimpanzees living in the community at Ngogo, Kibale National Park, Uganda. During the study, several females gave birth to their first offspring. This produced an ideal situation to compare the behavior of mothers who differed in parity, testing the prediction that primiparas and multiparas behave differently. To determine how females compete with one another, I examined ranging and intrasexual aggression, which influence food access. To examine female social bonds, I analyzed observations of association and grooming. Females at Ngogo utilized small, overlapping ranges within the community territory. Range sizes differed, but parity did not predict this variation. As reported elsewhere, mothers were often aggressive toward adolescent nulliparas. Female aggression varied with parity; primiparas displayed more aggression toward adolescents than did multiparas. Regarding affiliation, mothers mainly associated and groomed with mothers, rather than with adolescent nulliparas. Examining mothers by parity class showed that primiparas groomed with adolescent nulliparas more than did multiparas. These results indicate that behavioral differences existed between primiparous and multiparous female chimpanzees. These differences involved mothers’ social interactions with adolescent nulliparas in competitive and affiliative contexts. These results emphasize the importance of examining parity because considering mothers as a single category can mask behavioral variation. This indicates the importance of examining the lives of primiparas in order to understand how evolution has influenced the mothers’ behavior.