Social bonds motivate hyenas to take big risks together
It’s not unusual to see hyenas and lions fighting on Kenya’s Maasai Mara. As part of these duels, hyenas sometimes bunch into small groups and rush a lion, a behavior called mobbing. Successful mobs can drive a lion out of hyena territory or off a tasty antelope carcass. But mobbing is also risky, says behavioral ecologist Tracy Montgomery, a postdoc at the Max Planck Institute of Animal Behavior in Konstanz, Germany. Lions are twice the size of hyenas and can swipe hyenas out of the mob and kill them.
To understand why hyenas engage in such risky behavior, Montgomery and Kenna Lehmann, an evolutionary biologist at Michigan State University in East Lansing, co-led a recent study in Proceedings of the Royal Society B, modeling the situational factors that predict whether a hyena mob will form, who participates, and who benefits. The researchers found that certain hyenas—in particular, middle-aged females who know each other well and are high-ranking in the clan—are generally the ones to band together against a lion. Tight social relationships, it seems, motivate hyenas to cooperate, even when risks are high.
For 35 years, the coauthors and other biologists have observed six hyena clans at the Maasai Mara National Reserve, almost 600 square miles of savanna habitat. The researchers drive Land Cruisers around the reserve twice a day, tracking some 400 to 500 hyenas using radio collars and GPS trackers. When they find a clan, the biologists record which hyenas are interacting with which. In cases of mobbing, they also jot down who participated and whether the mob was successful, as well as which hyenas got to eat after the mob and in what order (from any carcass won in the fight).
Sometimes, out in the field, the biologists find hyenas sleeping or hanging out around their den. “And sometimes, they’re in this epic battle with lions,” Lehmann says. “There are lots of times we’re watching the hyenas and thinking, ‘why in the heck are you doing this? This is pretty dangerous.’”
To begin to understand hyena motivation, Montgomery and Lehmann combed through the entire dataset for every instance of hyena mobbing. They used generalized linear mixed models to ask which factors might predict when mobs formed, which hyenas participated, and who benefitted. Past studies suggest that the number of lions present relative to the number of hyenas, as well as the strength of the social bonds between the hyenas at a given site, influence when mobs occur.
Montgomery and Lehmann found, based on their data, that mobbing was most likely in situations where only lionesses were present, while stronger male lions were absent. They also found consistent shared traits among individual hyenas who participated in mobs. Adult females, between 6 and 7 years old, who are high-ranking in the clan and had engaged in consistent greeting behavior, even 5 minutes before the mob formed, were most likely to band together against lions.
Lastly, the researchers asked how hyenas benefit from mobs, based on access to food. They found that hyena clans consistently allow mob participants to eat. In fact, the strongest predictors that any individual would feed on a carcass were mob participation, age, and social rank. “Perhaps hyenas are keeping track of who is cooperating,” Montgomery says, and trying to ensure that they get a share of the benefits.
Daniel Blumstein, a behavioral ecologist at the University of California Los Angeles, who did not participate in this work, says it’s “extraordinary” to be able to go back through decades of field notes, to extract and then analyze a set of relatively rare observations.
This study suggests that hyenas are somehow weighing costs and benefits, including peer relationships, to decide whether to mob or not. That degree of risk assessment is “rather sophisticated,” says Terry McGlynn, a professor of biology at California State University, Dominguez Hills. “We have seen this type of behavioral sophistication in groups of social insects, but it’s harder to document in vertebrates.”
Looking ahead, Lehmann says she’d like to do more fieldwork studying how hyenas signal their decisions to one another. Hyenas are loud. They giggle, whoop, and growl, especially during mobs. However, it’s unclear exactly what these vocal cues mean. Lehmann says she would also like to understand the benefits of mobbing for social rewards—for instance, whether males who mob are allowed to mate more often. For now, these latest analyses help begin to explain the appeal of joining in, rather than sitting back while the other members of the clan risk life and limb.
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