Image of two hyena cubs. Photo courtesy of Jennifer Smith.

Establishing dominance

 

Photo by Kay HolekampSocial dominance plays a primary role throughout a hyena's entire life and this is a major research focus of the Holekamp Lab.

Hyenas usually give birth to one or two cubs that enter the world with sharp teeth and pitch black fur. The mother and cubs live in an isolated natal den for up to four weeks after birth. The newborn cubs begin to fight immediately after birth as a way to sort out which cub will be socially dominant. The dominant cub wins priority in everything, whether it is the best nursing position or ordered feeding priority at ungulate kills. The early fighting for dominance between siblings can occasionally turn deadly as newborns sometimes kill or wound each other. 

After three or four weeks, the mother moves her newborn cubs to a communal den, where all cubs less than about nine months of age reside. A communal den may provide shelter for over twenty cubs at once. The den is usually a complex of holes in the ground, leading to several underground chambers too small for any hyenas but cubs to enter. To nurse her cubs, the mother usually lies at the entrance to the den which often appears as a shallow depression on the plains.

Cubs graduate from the communal den when they are 9 to 12  months old. At this point they start traveling around the clan's territory with their mother. When they are over two years old, males usually move to a new clan to mate, whereas female tend to remain in the natal clan throughout their lives.

Among hyenas, many traditional gender roles are reversed.

Unlike most animals in which dominance is determined by size, strength or the ability to gather food, hyenas "inherit" their social ranks from their mothers. That is, mothers intervene to protect their cubs in fights with other hyenas, and other clan members quickly come to associate a particular cub with a particular mother, such that the cub comes to assume a rank immediately below hers.

Females control the power in hyena society, and the female wielding the most power is the alpha female in the clan. The alpha female always has first access to food and other resources.  All other hyenas fall into place beneath this top-ranking female. The alpha female's cubs attain social ranks immediately hers, so they outrank all other clan members except her, regardless of their opponent's relative age or size.

The dominance relationships in a hyena clan are complex.

Holekamp and her students have been tracking these complexities through years of research. They identify all clan members by their unique spots, and they  have built a database to track all hyenas from several different clans in Kenya. As lab members conduct field research, they sometimes use tranquilizers on a hyena to gather measurements and other data.

Among the data being collected are blood samples which can reveal the health of the animals as well as its genetic characteristics and other traits. How hyenas understand and process their positions in the clan's hierarchy is one of the puzzles Holekamp and her lab are studying. The cognitive skills involved in understanding and processing these roles may be unique to hyenas and other long-lived mammals that live in very complex societies.

A hyena's social rank affects virtually every aspect of its daily life. When a hyena makes a kill for food, dominant hyenas have the option of eating first and displacing any lower-ranking hyenas from the food.

This dominance relationships apparent in hyena societies illustrate very clearly the process of natural selection. Because higher ranking hyenas receive more food than their lower-ranking counterparts, they are larger, healthier and better able to survive and reproduce.

Holekamp's studies have revealed that high-ranking female hyenas reproduce more often than low-ranking females, and their cubs grow faster and survive better. Natural selection continues to favor higher ranking hyenas, as lower ranking bloodlines can die out altogether over periods of several years.

 

Female spotted hyenas are surprisingly "masculine" in their behavior and appearance. Unlike most other mammals, female hyenas are substantially more aggressive than males and they also are about ten percent larger than males.

Androgens are hormones that normally abound in the blood of male mammals after puberty.

Holekamp's research team has shown that dominant females in the final stages of pregnancy have higher concentrations of androgen than do subordinate females. This suggests that cubs born to dominant mothers are exposed to higher concentrations of androgens while still in the womb, and this may lead these cubs to exhibit higher rates of aggression after birth.

 

Prenatal androgen exposure appears to have adaptive consequences for young hyenas of both sexes.  Enhanced aggressiveness may help females compete for food at kills whereas males exposed to higher concentrations of androgens in the womb tend to practice mating more as cubs than their peers exposed to lower androgen concentrations. This practice  may provide them with enhanced mating ability as adults and they should therefore be more successful in mating than sons of less dominant females. Regardless of their gender, cubs born to dominant females have a greater chance at survival and reproduction.

Whereas spotted hyenas exhibit many interesting traits that set them apart from other mammals, one of the most unusual is the external genitalia of the female spotted hyena.

The female hyena's clitoris is elongated and virtually indistinguishable from the male's penis. Female hyenas urinate, copulate and give birth through this unique anatomical feature.

There is no definitive explanation for the evolution of the unique female anatomy in the spotted hyena. While many different hypotheses have been offered through the years, most of these can be ruled out, and scientists believe the explanation is most likely one of only two remaining possibilities.

Graphic by Christine Drea.First, if the female hyena's "masculinized" genitalia have an adaptive function, they might play an important role in post-copulatory choice by female hyenas regarding which sperm will fertilize their eggs. Females in the wild often mate with multiple males when they are in estrus, and the sperm from these competing males must therefore often occur together in the female's reproductive tract. The ovaries of the female spotted hyena contain mainly hormone-producing tissue, and they contain very little egg-producing tissue, so the female hyena may have very few eggs relative to those produced, stored and released by other mammalian carnivores.

A second hypothesis suggests that, instead of having an adaptive function, the female's odd genitalia merely represent a side-effect of selection for other male-like traits in females such as large body size or enhanced aggressiveness. However, when pregnant female spotted hyenas are treated during pregnancy with drugs that block the action of androgens, female offspring still develop the unique anatomy.

Furthermore, giving birth through a penis-like clitoris has high costs in that some cubs inevitably suffocate during the birth process. These costs suggest that the odd female genitalia must have been favored by natural selection because their benefits more than outweighed these costs. In any case,  the explanation for the unique anatomy of female spotted hyenas continues to elude and fascinate scientists.