Scientists have discovered that wild female chimpanzees don’t seek out status the same way male chimps do. Rather than challenge their superiors, female chimpanzees wait until the hierarchy shifts as older chimps pass away. The details are in a paper that was just published in the journal Scientific Reports.
Researchers from Duke University combed through observational records on eastern chimpanzees (Pan troglodytes schweinfurthii) at the Gombe National Park in Tanzania. The team used 40 years of collected data to investigate how the chimps change dominance ranks in the social hierarchy. The researchers also noted information such as which chimpanzees were related. In total, the team studied about 100 individuals.
According to their data, male chimpanzees compete for status while females follow a queue system based on age. Males actively fight and challenge each other, resulting in constantly changing rankings. Females are patient, however, and rarely fight. Instead, female social rankings often stay stable until an older female dies and the next one takes her place. Ranks are mostly determined at birth and after that, the female just waits for her “turn”. The team isn’t sure what decides their birth rank but preliminary data suggests that having a high-ranking mother helps her daughter maintain a higher status.
The team’s findings show that females don’t use challenges to climb social rankings. The researchers believe this is because it would be too costly. They could get injured and their offspring would be at risk. It makes more sense for females to cooperate and let the hierarchy change with time. Males, on the other hand, have more to gain by fighting. High-ranking males gain access to more females, a benefit that doesn’t require them to stay in charge long. This makes even potentially risky challenges valuable to male chimpanzees.
This was the first time that sex differences were recorded in chimpanzee rank relationships. The study highlights the importance of birth rank for females since it’s difficult for them to climb the hierarchy otherwise. The team intends to conduct further research on the factors that determine female birth rank.
Foerster et al. Chimpanzee females queue but males compete for social status. Scientific Reports (2016).