Early one morning I caught sight of Morpheus, silhouetted against a pink African dawn. Her long, sloping neck was stretched out as she loped away from me, disappearing over a hill. I followed her to a nearby plain and was met with the unmistakable sound of a group of hyenas squabbling over a carcass. Morpheus entered the fray, first lunging at a smaller male on her right. A moment later, she looked up briefly, her nose and mouth covered in blood, then turned and snapped at a hyena feeding nearby.
I'm intimately acquainted with Morpheus and these other hyenas because they have been studied for more than twenty years by various members of the lab where I did my Ph.D. research; I've staked these hyenas out at dens for hours on end and followed them as they raced across open plains. From watching these animals, we've learned about hyenas' social system, their physiology, and the conservation challenges they face.
But to me, it's the aggression that is the most fascinating thing about hyenas. It's rule-based and constrained by specific social norms, but at the same time, it's incredibly primal and ruthless. Studying aggression has helped us understand what makes hyenas tick, offering us a glimpse into the evolutionary pressures that have made them one of the most unusual and misunderstood species in the animal kingdom.
For more than 1000 years, people believed that hyenas were hermaphrodites, since female hyenas have long, fully-erectile pseudopenises that mimic male genitalia. Seeing a hyena play the role of mom while sporting what looks like a penis would bewilder even an astute naturalist. Not only do female hyenas look like males, they are also the more aggressive and socially dominant sex, exhibiting aggression more than three times more often than male hyenas do.
For many animals, too much aggression is detrimental, at least in terms of reproductive success; in baboons, aggressive females have reduced fertility and increased rates of miscarriage , and in western bluebirds, overly-aggressive males tend to fledge fewer offspring than other males. But in these species, males are generally more aggressive than females; how is aggressiveness related to fitness in a species where females are the more aggressive sex?
Life in the clan
Hyenas live in huge social groups called clans that are structured by a "linear dominance hierarchy." That's the scientific way of saying that in these societies, a high-ranking individual is dominant to every lower-ranking animal in the clan: Morpheus is dominant to Scrabble, who is dominant to Hendrix, and so on. For hyenas, social rank isn't just a title or a badge of honor. Rank determines access to food, so a high-ranking hyena like Morpheus can drive a lower-ranking hyena off a kill at any time, no matter who hunted or scavenged the meat.
Social rank also plays an important role in aggressive behavior among hyenas, since dominance determines who can exhibit aggression toward whom. Aggression is nearly always directed down the hierarchy, toward lower-ranking hyenas (and if a hyena disregards this rule, it's not taken lightly by other clan members). This means that the highest-ranking hyenas have a lot of opportunities for aggression - they can attack nearly any other hyena in the clan - whereas lower-ranking hyenas have far fewer possible targets. Aggression can occur over food, in defense of cubs, or to reprimand a pesky suitor.
But unlike many species, aggression doesn't dictate social rank among hyenas; instead, social rank is inherited. Hyenas are stuck with their lot in life, unable to move up the hierarchy. So does all this aggression actually benefit hyenas, and if so, how?
The implications of aggression
Aggressiveness, it turns out, varies drastically among hyenas; some hyenas tend to threaten - or outright attack - group members more frequently than others do. There is more than a five-fold difference in the aggression rates of the least aggressive and the most aggressive females, even after controlling for social rank and the number of opportunities for aggression.
This type of consistent variation in behavior, called "animal personality," is being found in several traits, such as sociability, boldness, and docility, across many species. And aggressiveness, like other personality traits, can have major implications for fitness. However, for hyenas, aggression doesn't affect fitness by improving a hyena's own survival; aggressive females don't live longer or survive at higher rates than others that attack less often.
Instead, the benefits of aggressiveness are seen later down the line, in the survival of offspring. Female hyenas that are particularly aggressive over food successfully rear a larger proportion of their cubs to adulthood than do females that aggress less often over food. But interestingly, the benefits of aggressiveness depend on social rank. For high-ranking hyenas, aggressiveness doesn't matter much in terms of reproductive success; the offspring of dominant females do well no matter how aggressive their mom is. However, for hyenas low on the totem pole, aggression plays an important role in reproductive success, greatly improving their offspring's odds of surviving until adulthood. But how?
Competition and reputations
It all comes down to acquiring resources for your offspring. High-ranking hyenas already have prime access to food, so being super-aggressive at a kill or carcass isn't a huge advantage. However, for hyenas low on the totem pole, being able to secure a little extra food for a cub could mean the difference between its survival and starvation.
When cubs begin eating meat at around 4 months of age, they start visiting kills with their moms. But as these cubs attempt to eat, they are often harassed by older hyenas and chased off the carcass. Additionally, these young hyenas have another disadvantage when it comes to feeding: their skulls haven't finished developing yet. Although being able to crush bone is a big benefit for hyenas evolutionarily, it's a huge morphological handicap for cubs. It takes up to 35 months for a hyena's skull to develop the integrity and strength to crack bone, so until about three years of age, young hyenas feed more slowly and less efficiently than adults. Combine this physical disadvantage with the incredible feeding competition seen at kills, and cubs - especially low-ranking ones - often don't get much to eat during these communal feeding situations.
Here's where a mom's aggressiveness comes in: we found that the cubs of aggressive females are tolerated better, and are able to feed longer, at these kills than the cubs of less aggressive females are. By being super-aggressive, moms secure extra feeding time and valuable calories for their cubs during this particularly handicapped period in their lives. Although we don't completely understand the process yet, aggressive females appear to develop a type of "mean girl" reputation within the clan that gives their offspring a boost early in life. This effect is incredibly strong and persists even when the mom isn't present at the kill, allowing cubs to benefit from their mom's aggressiveness even in her absence. This increased access to resources benefits low-ranking hyenas disproportionately, since they generally have very limited access to food.
A combination of behavioral, morphological, and ecological research has helped us begin to understand why these highly aggressive and masculinized females have been favored evolutionarily. But even after 20 years of intensive research, there's so much more to learn; we still aren't sure what the functions and implications of male aggression are, and it's possible that there are consequences of aggression in females that we haven't yet discovered.
Morpheus and her clanmates are still being observed, and you can follow the trials, tribulations, and musings of the researchers studying these hyenas out in the field at the Mara Hyena Project blog.