Having a higher rank in the social hierarchy means you get nicer things: designer clothes, a bigger house, a better iPhone. If you’re a hyena, it also means you get a better immune system. In female-dominated spotted hyena clans, females at the top of the pyramid take fewer sick days than their lowly peers, biologists report in a study published in the September issue of Functional Ecology.
The finding could offer key insights into the human immune system—and even how contagious diseases like Ebola spread among human communities. "By studying these long-lived animals, it can provide a fresh perspective on how the immune system works," says Andrew Flies, a research fellow in immunology at the University of Tasmania and the University of South Australia and author of the new study. Flies and his colleagues examined a 30-year dataset on spotted hyenas in the Maasai Mara National Reserve in southern Kenya.
Both male and female spotted hyenas are revered by ecologists for their remarkable disease-fighting powers. Hyenas often survive rabies in the wild, despite being regularly exposed to the virus, as evolutionary ecologist Marion East at the Leibniz Institute for Zoo and Wildlife Research in Germany and others found in a 2001 study. Other studies have shown that hyenas seem to be resistant to anthrax as well as the canine distemper virus, a close cousin of measles that has killed massive numbers of lions in the Serengeti.
Spotted hyenas “really seem to be resilient animals both from wounds like broken bones and injuries from getting attacked by lions,” says Flies, who conducted the research as part of his Ph.D. work at Michigan State University. In the field, the team observed a hyena running around with a snare left by a hunter constantly tightening around its neck for a year. Once researchers removed it, it survived for at least another three years. “This hyena had a seeping, oozing, bloody wound—the perfect spot for an infection," Flies says. "But it never seemed to get sick."
Part of hyenas' remarkable disease resistance has to do with their high levels of disease-fighting antibodies. But not all members of the pack have the same antibody levels. Flies and his coauthors examined blood samples from a long-term dataset compiled by previous researchers, who had anesthetized hyenas with tranquilizer darts. They found that females had higher levels of certain antibodies on average than males did—as well as a better capacity to kill bacterial infections. "Those are baseline resources that are good at preventing infection," Flies says. After all, the best way to stay healthy is not to get sick in the first place.
Why were the alpha females’ antibody levels so much higher than the rest? Researchers hypothesize it’s because alpha females live generally more pampered lives: they’re better fed, and spend less energy on hunting and nursing injuries. "They have better access to resources, which allows them to maintain better defenses," says Flies.
Female spotted hyenas are born leaders. They enjoy absolute mate choice in their clan, in part due to their larger power and size and bizarrely penis-like, challenging-to-mate-with clitorises. They also follow a strict, if unconventional, hierarchy: When an alpha female dies, it is her youngest daughter, rather than the oldest, that assumes power. If there is more than one female in the same litter, dominance is decided through a sisterly spat.
The social rank determined early in life will be respected, regardless of whether the female turns out to be smaller or weaker than others later on. “It’s rare for a low-ranking male to attack a high ranking female. They will usually turn tail and run if the female is aggressive to them,” says Flies. Moreover, every hyena seems to remember exactly where it sits in relation to its peers—even when the clan numbers 120 animals, like the clan that Flies observed.
Males, on the other hand, always rank lower in the pecking order than their sisters in the same litter. As long as they stay within their clan, these males can still enjoy a relatively high standing if they are born of high blood. But most—about 90 to 95 percent, according to Flies—instead wander out in search of other mating opportunities. While this may be better for the overall hyena gene pool, it means those males fall from social grace, ending up near the bottom of the hierarchy new clan.
The strict rules that govern hyena communities have serious consequences for all its members. When the clan takes down an animal or scavenges a carcass, for instance, they will always defer to the alpha female. “We think that the higher-ranking hyenas have much better access to a kill,” Flies says. “Sometimes there will be 10 hyenas sitting around waiting while she eats her fill.” As a result, females work less for their meals, receive less kicks from giant wildebeests or injuries from lions—and thereby conserve more of their energy for fighting off infection and disease.
These trends have parallels in human societies, where higher-ranking males and females tend to enjoy better health than those of lower social standing. "There are several medical studies on humans that clearly show that individuals holding high social status in human societies are healthier in general and have better values in several standard tests of immune function," East wrote in an email. "Furthermore, several studies show that the outcome of infectious diseases is more severe in humans of low status than those of high status."
Besides their complex hierarchies, hyenas are relatively long-lived, surviving for two decades or more in the wild. That means that studying their immune systems could help us glean new insights into human immunity, especially with regards to diseases that occur later in life like dementia or arthritis, which are difficult to study in lab mice that only live for a couple years. Says Flies: “The immune system in an animal that lives for 20 years is a lot different than an animal that lives for two years."