by Lorena Lyon

figures by Lorena Lyon

If you’ve ever backed down from an argument with a popular kid, you might relate to a male spotted hyena.

Spotted hyenas are highly social animals, forming large groups called clans, which range from 6 to over 100 members. Hyenas in clans, like eighth graders in middle school, are sorted into complex social hierarchies. At the top of the pecking order are female hyenas. Though female hyenas are not significantly larger than males, their slightly larger size and increased aggression has previously been theorized to explain why they dominate the social order.

Also, female spotted hyenas have a pseudopenis.

It’s not a Freudian complex, but rather a nickname for a body part. Female hyenas have a clitoris that is so large and elongated that it looks like a penis. Instead of a separate vagina and urethra, the elongated clitoris contains a single urogenital canal used to pee, have sex, and bear offspring.

Given their pseudopenis, more aggressive behavior, and larger size, it was no surprise to researchers that female hyenas dominate clans. However, a new study of hyena social behaviors suggests that these characteristics, termed “intrinsic factors,” are not responsible for female hyena’s social dominance. Instead, an “extrinsic factor,” social support, was found to be more predictive of a hyena’s dominance in a one-on-one interaction. If a hyena has more social support, it is more likely to win an antagonistic social challenge.

The lunch line: hyena social organization

Why do spotted hyenas have highly organized social groups? Think of any group project you’ve worked on. In groups where everyone wants to lead, did it go well? Probably not. Animals that live in groups also need to organize themselves to avoid in-fighting, collaborate, and gain more resources as a group. Within spotted hyena clans, social dominance hierarchies are basically a lunch line, with higher ranking females and their cubs getting first dibs on meals.

As mentioned above, spotted hyena clans are matriarchies, led by females. This is partly due to a phenomenon called male dispersal. After puberty, males leave the clan into which they were born. When they join a new clan, these “immigrant” male hyenas become the lowest-ranking members of their new clan. They are subordinate to any offspring of that clan’s females, any males native to that clan, and immigrant males who joined the clan before them. However, immigrant males do get a perk that native males do not: females are more likely to mate with low-ranking immigrant males, which helps diversify the gene pool. Also, females completely control mating. Remember the pseudopenis? Because of its unusual shape, it is physiologically impossible for males to mate with females who aren’t into them. Since females have absolute say in mating and tend to mate with lower-ranking males, there is no pressure for males to fight for mates, which is a hypothesis that might explain males’ lower aggression levels.

In addition to these male-female dynamics, relationships between female hyenas in a clan are also hierarchical and are typically stable for many years. Offspring inherit a social rank below their mothers. Thus, clans are made up of multiple female-led lineages, or multiple “matrilineal kin groups” (Figure 1). These kin groups are organized by how the oldest females relate to each other. Though rare, a high-ranking matrilineal kin group may be toppled if a coalition of lower-ranking groups challenges the smaller, higher-ranking group.

Though spotted hyenas develop long-lasting relationships with their clan mates, clan members spend most of their time alone or in smaller groups. However, they come together to help each other hunt large prey and defend their kills from other hyenas and predators. Contrary to popular depictions of spotted hyenas as scavengers (for example, in Disney’s Lion King), hyenas get at least half of their meals through hunting fresh prey. After working together to bring down a large prey, spotted hyenas then enter what looks like a competitive eating contest. Hyenas eat rapidly, taking in as many calories as possible. A scientist observed a clan of 35 spotted hyenas turn a 1,500 pound African cape buffalo into skull and bones within a few hours.

During feeding, higher ranking females, their cubs, and adult offspring have first dibs, showing where social dominance hierarchies are most important: access to food.

It’s hypothesized that female dominance was evolutionarily selected for because females and cubs get more access to food. How females were able to establish and maintain this dominance, though, is only being revealed now, through new research.

Sex versus Support

In a recent study, researchers observed more than 4000 one-on-one aggressive interactions between over 700 hyenas from eight different clans. In each of these interactions, both hyenas were characterized by two intrinsic factors, sex and body mass, and the extrinsic factor of social support. The interaction was further categorized by whether it was between two hyenas of the same clan or not. When hyenas came from the same clan, the researchers also noted whether the hyena was born in that clan, or an immigrant.

To evaluate social support, the authors examined whether a hyena would be able to recruit backup from fellow clan mates. When faced with a challenge, a hyena can emit a long-distance call, called a whoop, which brings other hyenas to its rescue. In this study, the measure of a hyena’s social support depended on the geographic location of the interaction, how long the hyena had been part of the clan, family connections, and prior engagements.

After crunching the data, the numbers showed that better social support predicted a winning outcome for a hyena over intrinsic factors, such as larger body size. This finding overturns the hypothesis that female hyenas gained dominance through increased aggression and body size. It is also consistent with observations of female hyena health and reproductive success. Higher-ranking female hyenas are observed to start breeding earlier, live longer, and produce more surviving cubs. Their longer lifespan and larger kin group makes for a larger social web. They have more kin members to rely on, and thus, have better social support.

By quantifying the factors that lead to spotted hyena social dominance, this study offers a more complete picture of hyena social systems and the circular logic that puts female hyenas on top. Female hyenas stay in their birth clan and produce offspring there, while males must break their social network to reproduce. Social rank determines food access, giving higher ranking hyenas more calories and increasing their fitness. Increased fitness allows higher-ranking females to have longer lives and produce more surviving cubs, surrounding them with a larger kin group. A larger kin group provides those hyenas more social support. Social support helps hyenas win challenges with other hyenas, solidifying social hierarchies (Figure 2).

Looking beyond physical traits was key to this research, showing that sometimes what scientists observe (female social dominance) is not necessarily tied to obvious traits associated with the phenomenon (larger size and aggressive behavior).

Lorena Lyon is a recent graduate of Harvard College and currently is a research assistant in the department of Systems Biology at Harvard Medical School. Tweet @renatyger.

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