Embargoed by Science until 3 p.m. CDT Thursday, Aug. 7.
To interview Pusey, call Deane Morrison, University News Service, (612) 624-2346, [email protected]
U of Minnesota, Jane Goodall Study Shows That for Female Chimps, Even Weak Rank Has Its Privilege
Drawing on 25 years of data from Gombe National Park in Tanzania, University of Minnesota researchers Anne Pusey and Jennifer Williams, along with Jane Goodall, have shown that even a weak social hierarchy can have a profound impact on individual chimpanzees' reproductive success. The researchers studied female chimpanzees, whose social rankings are subtle and hard for humans to figure out. But the researchers were able to assign loose rankings based on behavior and found that each female's reproductive success was closely correlated with her social station. The study will be published in the Aug. 8 issue of Science.
"We're the first to show an effect of rank on anything in the lives of female chimps," said Pusey. "I was surprised that we found such strong effects of rank, given how hard it is to determine rank. These effects are at least as clear as they are in more obviously hierarchical societies such as baboons and macaques. It appears that dominance [of lower-ranking individuals by their social superiors] could still be important in species where hierarchies are less obvious."
The higher-ranking chimps reaped reproductive benefits in the form of higher infant survival, faster-maturing daughters and more rapid production of young. All that points to better nutrition, said Pusey, and so high rank probably confers its rewards by helping females gain and keep control of good foraging areas rather than by sparing them stress from aggression.
Because female chimps living in stable groups or pairs display little overt aggression, it's difficult to detect linear hierachies that dictate who dominates whom. That has led some researchers to doubt that dominance behavior is important for the reproductive success of female chimps. But Pusey, Williams and Goodall analyzed all pant-grunts--a chimp sound that signals submission--recorded between females from 1970 to 1992. Using that data, they were able to assign females to either high, low or middle rank in their group.
The researchers found, first, that infants of low-ranking females suffered much higher mortality than those of high-ranking females during the first seven years of life. Second, the age at which a young female reached sexual maturity was related to her mother's rank. Daughters of low-ranking females reached sexual maturity as long as four years later than daughters of high-ranking females. Third, high-ranking females tended to live longer. Finally, the annual production of offspring surviving to weaning age (5 years) was correlated with rank.
Since food is unevenly distributed at Gombe, high-ranking females may well occupy or have first access to better feeding areas, the researchers said. This would help their daughters grow faster and reach sexual maturity earlier, as well as benefit other aspects of reproduction. Williams is now investigating the differences in food availability among the females' ranges. It isn't clear, said Pusey, whether high-ranking females get the best ranges or the first pick of shared ranges. Also, more research is needed to find out how females achieve their rank, especially when they leave their mothers' group to join a new one, as half the Gombe females do.
Data for this study came from the records of Goodall's 35 years of work at Gombe, which are stored in the Jane Goodall Institute's Center for Primate Studies in the department of ecology, evolution and behavior at the University of Minnesota.
###
RSS