High-ranking hyena mothers pass their social networks to their cubs

Hyenas are a very social species, living in groups that can number over 100. But within their clans, there is an order: a specific maternal hierarchy governs societies in this species where females are dominant over males.

While researchers have extensively studied the social structure of hyenas and other animals, scientists have only recently begun to investigate how this structure came to be. A new study led by biologists at the University of Pennsylvania, which draws on 27 years of detailed observations of the social behavior of hyenas collected by researchers at Michigan State University, is pulling the curtain on how social order emerged.

Their findings show that hyenas inherit their mother’s social networks, so their social connections are similar to those of their mother. However, the offspring of higher-ranking individuals faithfully replicate their mothers’ interactions, and end up with social networks that are more similar to their mothers than the offspring of females ranked lower in the social ladder of the clan. The team reported their findings in the journal Science.

“We’ve learned that hyenas’ social structure is based in part on an individual’s rank in the antagonistic hierarchy, which we know is inherited from mothers,” says Erol Akshay, a co-author and associate professor in the University of Pennsylvania’s College of Arts and Sciences. “But what we found, that sympathetic or sympathetic interactions are also inherited, was not shown.”

“This is a very simple process of social genetics that we show works very well,” says Amil Ilani, a senior lecturer at Israel’s Bar Ilan University. “Individuals born in a higher rank are more meticulous in their inheritance, and they have good reason to do so. It fits well with what is already known about rank inheritance. There are very strict rules about where you sit in the hierarchy if you are a hyena.”

The work is based on a theoretical model of social network inheritance developed by Akçay and Ilany in 2016. According to this simple framework, animals create their networks through “social inheritance,” or copying the behavior of their mothers. The model fits well with real-world social networking shots of not only hyenas but also three other social species: glass-nosed dolphins, hyraxes, and sleeping lizards.

In the new work, the team sought to improve their model to better understand the complexities of social inheritance in hyenas. They were fortunate to have a robust data set compiled by Akshay and Ilani’s co-author, zoologist Kay Holkamp of Michigan State University, consisting of 27 years of detailed accounting of the social interactions of the clan.

“We realized we could use this data set to directly test our model, to see if social connections are inherited,” Akshay says.

Field biologists from the Holekamp research group meticulously tracked how hyenas in a clan interacted, including who spent time with whom as well as each member’s social rank. To do this, researchers spent months identifying each member of the clan by sight.

“They are present throughout the year, every day, identifying individuals by their specific positional patterns and other characteristics,” Ilani says.

These observations allowed Akçay, Ilany, and Holekamp to plot the social networks of the hyenas based on individuals who spend time close to each other.

“Using this proximity to track social networks is not feasible with humans, as two random strangers might go up the elevator,” Ilani says. “But with hyenas, if one of the individuals is a few meters away from the other, it indicates that they have a social connection.”

Young hyenas essentially duplicated their mothers’ social group, even as the cubs got older and stopped spending as much time in close proximity to their mothers. (Photo: Kate Shu Yoshida)

With this picture of everyone’s social affiliations at hand, the researchers compared mothers’ social networks to their offspring. “We developed a new social inheritance scale, to track how an offspring faithfully reproduces its mother’s network,” Akshay says.

Hyena cubs attach to their mothers for the first two years of their lives, so the mother and son networks were quite similar. However, the researchers note that even as the young stop spending as much time in close proximity to their mothers, they still have quite similar networks, especially for female offspring, who generally remain lifelong members of the clan. “We have data in some cases showing that the network similarity between mothers and boys, especially female offspring, was still very high after six years or so,” Ilani says. “You may not see your mother often, or she may have died, but you still have similar friends.”

This pattern was particularly strong for mothers of higher rank, for whom social inheritance was the strongest in the group.

“This is kind of an intuition because such things happen in human society as well,” Akshay says. “It happens a lot and we take it for granted. We inherit social bonds, and there is a lot of social science research showing that this has a huge impact on people’s life trajectory.”

Offspring of lower-ranking mothers were less likely to reproduce their mothers’ social networks, and might try to compensate for their lower parentage by associating with a larger group of individuals.

A mother hyena with her cub

The mother’s social standing was deeply felt by her cubs. “Order is very important,” Akshay says. “If you are born to a lower-ranking mother, you are less likely to survive and reproduce.” (Photo: Kate Shu Yoshida)

There is no genetic inheritance of rank or close associates in this species, so in Holekamp’s opinion one of the most fascinating things about the phenomenon documented here is that youngsters’ relationships with their mother’s close partners are all learned very early in life. One explanation why social network genetics works better with lower-ranking hyenas may be that lower-ranking females tend to set out on their own more often to avoid competition with higher-ranking hyenas, and thus their cubs have less learning than cubs from Females of high rank.

The team found that the mothers’ pairs of offspring with more similar social networks also lived longer. This effect on survival may owe to the fact that sons who spend more time with their mothers and thus replicate their social networks benefit from increased care.

Social rank also had an effect on survival and reproductive success.

“Order is very important,” Akshay says. “If you are born to a lower-ranking mother, you are less likely to survive and reproduce.”

The researchers note that the inheritance of social networks likely contributes to group stability and also has implications for how behaviors are learned and spread through groups.

The study also emphasizes how factors other than genes influence key developmental outcomes, including reproductive success and overall survival. “A lot of things that are hypothetically determined genetically may depend on environmental and social processes,” Ilani says.

Erol Akshay is an associate professor of biology in the University of Pennsylvania’s College of Arts and Sciences.

Miles Ilani is a Senior Lecturer at the MENA and Everard Goodman College of Life Sciences at Bar Ilan University and has completed a postdoctoral fellowship working with Akshay in Pennsylvania.

Kay Hollikamp is Professor of Zoology at Michigan State University.

The research was supported by the Israel Science Foundation (grants 244/19 and 245/19), US Army Research Office (Grant W911NF-17-1-0017), Israel-US Bilateral Science Foundation (grants 2015088 and 2019156), and the National Science Foundation (grants 1853934 and 1755089).

Leave a reply:

Your email address will not be published.