Using 27 years of detailed data on hyena social interactions, a team led by Penn biologists nailed down a pattern of social network inheritance and its implications for social structure, rank, and survival.
Hyenas are a highly social species, living in groups that can number more than 100. But within their clans, there is order: A specific matrilineal hierarchy governs societies in this species where females are dominant to males.
While researchers have intensively studied the social structure of hyenas and other animals, it’s only recently that scientists have begun to investigate how this structure arises. A new study led by Penn biologists, which relies upon 27 years of detailed observations of hyena social behavior collected by researchers at Michigan State University, pulls back the curtain on how social order comes to be.
Their findings show that hyenas inherit their mother’s social networks, so their social connections resemble their mother’s. However, offspring of higher-ranking individuals more faithfully replicate their mother’s interactions, winding up with social networks that more closely resemble their mother’s than do offspring of females that rank lower on the clan’s social ladder. The team reported their findings in the journal Science.
“We knew that the social structure of hyenas is based in part on one’s rank in the agonistic hierarchy, which we know is inherited from mothers,” says Erol Akçay, a study coauthor and associate professor in Penn’s School of Arts & Sciences. “But what we found, that affiliative, or friendly interactions, are also inherited, hadn’t been shown.”
“This is a very simple process of social inheritance that we show works very, very well,” says Amiyaal Ilany, a senior lecturer at Israel’s Bar-Ilan University. “Individuals that were born to higher rank are more accurate in their inheritance, and they have good reason to do so. It fits well with what is already known about inheritance of rank. There are very strict rules about what place you sit in the hierarchy if you are a hyena.”
The work builds on a theoretical model of social network inheritance Akçay and Ilany developed in 2016. According to that simple framework, animals establish their networks by “social inheritance,” or copying their mother’s behaviors. The model fits well with snapshots of real-world social networks from not only hyenas but also three other social species: bottle-nosed dolphins, rock hyrax, and sleepy lizards.
In the new work, the team aimed to refine their model to better understand the intricacies of social inheritance in hyenas. They were fortunate to have a robust dataset collected by Akçay and Ilany’s coauthor, zoologist Kay Holekamp of Michigan State University, consisting of 27 years of detailed accounting of a clan’s social interactions.
“We realized we could use that dataset to directly test our model, to see if social ties are inherited or not,” Akçay says.
Field biologists from Holekamp’s research group had meticulously tracked how hyenas in a clan interacted, including who spent time with whom as well as the social rank of each member. To do so, researchers spent months getting to know each member of the clan by sight.
“They are there year-round, every day, identifying individuals by their specific spot patterns and other characteristics,” Ilany says.
These observations allowed Akçay, Ilany, and Holekamp to map out hyenas’ social networks based on which individuals spent time close together.
“This use of proximity to track social networks isn’t possible with humans, as two strangers might randomly get into an elevator together,” Ilany says. “But with hyenas, if one individual gets within a few meters of another, that suggests that they have a social connection.”
With this picture of each individual’s social affiliations in hand, the researchers compared the social networks of mothers to their offspring. “We developed a new metric to measure social inheritance, to track how faithfully an offspring’s network reproduces its mother’s network,” Akçay…