Male and female chimpanzees receive social rank differently

A new Duke University research paper on chimpanzees suggests that members of both sexes achieve high-ranking social status in dramatically different ways, with the payoff for high rank being better food, higher reproductive success and increased offspring survivability.

Social ranking is present among a variety of species and is maintained differently depending on the species. Animals such as female elephants, female mountain goats and male rats maintain social rank by age. Meanwhile, male clownfish achieve rank based on body mass and female Japanese macaques, female yellow baboons and both sexes of spotted hyenas rank through social support.

Male chimpanzees, who tend to be more social than females, were found to fight for their social status, with aggression being the main indicator of dominance or submission. Females, who rarely interact with other females, were found to never challenge one another, but rather wait to move up in social rank when another member of their social group left, either by death or dispersal. Another large difference in the fight for social status was the stability of their rankings. While males fight for a higher social status that will peak in their early 20s and gradually decline, females’ social statuses will continuously rise throughout their lives.

Male chimpanzees begin their lives at the very bottom of the social ladder, and must fight their way through their group’s social ranks to garner any esteem. Female members of the group, however, have built-in boosts to their status based on parentage, with female children typically inheriting their mothers’ statuses. A female who joins a group of chimpanzees as a foreigner or a female whose mother is killed has a harder time moving up through a group’s social ranks. Unlike males who can actively fight for their rank, female rank is predominantly stagnant, only gradually rising in time. This means that entering the social ladder at a higher point for a female chimpanzee brings about many more benefits than gradually moving up from the bottom throughout their lifespan, which often exceeds 30 years.

The data that the researchers used for this study was drawn from almost 40 years of study at the Kasekela community of chimpanzees at Gombe Stream National Park in Tanzania. Almost 100 chimpanzees were studied at the national park over the 40-year period, during which researchers recorded chimpanzees’ pant-grunt interactions.

Pant-grunting interactions among chimpanzees are typically a way for subordinates to interact with a superior. These interactions were seen in females toward dominant males and in lower ranking males to appease an alpha male. This form of interaction was primarily how social ranking was categorized and understood.

The research originally posed the problem of how to measure social rank over time. To record the data on the chimpanzees in an accurate numerical fashion, the researchers of the university used a method of comparing competitor versus competitor skill, called ELO Ranking. The system was originally developed by Arpad Elo to rank players in chess but has since been used to rank skill levels in many different areas.

The ELO system starts all competitors at an arbitrary number like 1,500. With each win or loss the number goes up or down, respectively. If the win was an easily predicted one, such as a higher-ranked chimpanzee winning a fight with a younger one, the number changes only slightly. If the win was greatly unexpected, the ELO of both parties changes dramatically, such as a low-ranking chimpanzee winning against a much higher one. The researchers also compensated for the boost that mothers provided for their female young by increasing their base number accordingly.

Reproductive success is the most important benefit gained from rank for chimpanzees. Even if a male only holds high rank for a short period of time, if he manages to fertilize enough females, he has achieved high reproductive success, according to Anne Pusey, an evolutionary anthropology professor at Duke University and a senior author on the research paper. For females, however, high reproductive success is correlated to, rather than caused by, high rank and reproductive success, since a female who has lived longer has most likely reproduced more successfully.

These new developments shed light on why chimpanzees interact the way they do, and highlight the differences between how both sexes of the species approach ranking. Above all, it illustrates that chimpanzees do not differ greatly from many other species. Their ultimate goal, however, is still simply survival.