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Big Boys

The National Zoo’s Grevy’s zebras are growing into stallion studs.

by Jennifer Zoon

A rumble brews between two Grevy’s zebra (Equus grevyi) stallions at the National Zoo’s Cheetah Conservation Station. In one corner of the yard, weighing in at 750 pounds, is six-year-old Gumu (pronounced “GUH-moo”). He’s enjoying lunch—left out by African mammal specialist Kate Volz and other zookeepers—until four-year-old Dante moseys over and noses around his food. Instead of foraging in other areas of the yard, Dante commits a hierarchical faux pas and swipes some hay from Gumu’s pile.

Dante’s impertinence is met with a hearty bray—Gumu’s way of warning the 950-pound heavyweight not to take any more. It’s not as though either animal lacks food. Both zebras are well fed and fall within the appropriate weight range. Indeed, this dispute isn’t really about hay. It’s about who’s boss.

Gumu swings his head, forcing his rival’s mouth away from the food. The two spar and nip at each other. The clash is enough to convince Dante to back off a few paces. After several tense moments of silence and stillness, the younger zebra moves on to another haystack. Then, as though nothing had taken place, the two stallions stand with their backsides together and nonchalantly resume eating.

Grevy's Zebra stallions
Who's the boss? Gumu (at left) and Dante horse around in their bachelor pad. (Mehgan Murphy/NZP)

“Don’t let them fool you,” Volz said. “They look like they’re ignoring one another, but each is very aware of what the other is doing. Subtle movements, even as much as a twitch or a bray, are their attempts to jockey for dominant position.” Confrontations like this are typical male Grevy’s zebra behavior, especially now that both stallions are sexually mature. No serious injuries have occurred, but, Volz noted, “We’re keeping a close eye on them to make sure one doesn’t pick on the other more.”

Keepers are vigilant because Dante could react more aggressively to Gumu’s horseplay when he reaches his full weight in a few months. Like parents of bickering children, they face a tough question: How can they divert the boys’ pent-up energy in a positive direction?

The Mating Game

Grevy's zebras
Change is ahoof for the Zoo's Grevy's zebras. (Mehgan Murphy/NZP)

The National Zoo keeps only male Grevy’s zebras, which means no breeding—and no birth—takes place on site. This arrangement is part of the Species Survival Plan (SSP) for Grevy’s zebras. Eighteen years ago, a panel of SSP scientists determined that the Zoo’s Cheetah Conservation Station would serve as a holding facility for juvenile males and young stallions. Only a handful of North American zoos play this role. It’s an unusual situation for the National Zoo, which normally devotes much time and effort to breeding endangered species.

But it benefits our bachelors: By sharing an enclosure now, these highly territorial animals learn to accept another zebra’s presence and build invaluable social skills that will help them get along with future mates.

Now is the opportune time for Gumu and Dante to breed with mares from other zoos. Last year, National Zoo keepers sent both stallions’ records to the SSP panel, hoping to find them suitable mates. Only 83 individuals were selected for breeding and transfer in 2010, and Dante was one of them. Later this year, he will transfer to a South Dakota zoo, where he will have the opportunity to become a father. Why did the SSP scientists recommend Dante and not Gumu? The answer lies in their genes.

North American zoos are home to 163 Grevy’s zebras, all of which descend from an original population of 64 individuals. Each year, the SSP panel collects information about potential male and female breeders. They compile the data in one large matchmaking document, aptly known as the studbook. Before zebra studs can be matched up with a mate, they are put through a rigorous background check. They must be healthy, sexually mature, and capable of breeding. To ensure a good personality match, keepers detail their zebras’ social needs as well.

An animal’s genetic value, however, tops this list of criteria. The SSP panel reviews a zebra’s entire family history, including all known living and dead relatives. Zebras with small family trees (like Dante) have rarer genes than those with lots of relatives, so they are more likely to be paired with mates. That way, zoos minimize the likelihood of inbreeding.

The SSP scientists’ pickiness pays off. As of 2009, they determined that North America’s Grevy’s zebra population is 97 percent genetically diverse. This is an exceptionally good record, given that most species participating in an SSP aim for a 90 percent genetic diversity goal.

“It’s not surprising that the SSP panel recommended one of our boys and not the other,” Volz explained. “Only 38 zoos in the entire country house Grevy’s zebras, so we’re very fortunate and happy Dante made the cut.”

Soon after Dante moves to South Dakota, two yearling Grevy’s zebras from Tampa’s Busch Gardens will join Gumu on exhibit. It’s common for several young wild males to stick together, so keepers believe the trio will get along.

“Gumu will probably approach them the same way he approaches Dante—with a little skepticism,” Volz predicted. “As long as they recognize him as the dominant male, which they most likely will, this arrangement shouldn’t pose a problem.”

Grevy's Zebra stallions
How do zebras tell one another apart? By their stripe patterns, which are as unique as human fingerprints. (Jessie Cohen/NZP)

A Horse of a Different Color

Gumu and Dante are striped members of the equid family and belong to one of three zebra species. But a close inspection reveals the differences between the Grevy’s zebra and its cousins: This species sports a larger head; a coarser, bristly mane; and rounder, trumpet-shaped ears. In fact, it is more physically and socially similar to wild asses than to horses.

Beneath the zebras’ hair is dark skin that lays an old debate to rest: These animals are black—with black and white stripes. Those stripes shrink and tighten as they approach the hooves, stacked atop one another as neatly as bracelets. Zebras’ hides boast a plethora of different stripe widths, curves, brown-black color variations, and occasional scars inflicted by rivals and predators. The resulting pattern is as distinct as a human fingerprint. Zebras’ unique looks, along with vocalizations and mannerisms, help the animals identify one another.

Do the stripes serve a purpose beyond identification? Scientists are not sure. Folklore says that stripes provide camouflage for zebras and dissipate the sun’s brutal heat, but those ideas are scientifically unproven. For now, the purpose of those eye-catching coats remains a zebra’s secret.

Good Neighbors

Visitors sometimes voice surprise that the Zoo’s zebras and cheetah (Acinonyx jubatus) live side by side. Yet the two species are good neighbors. Cheetah biologist Craig Saffoe clarified the seemingly macabre aspect of this placement: “Our cheetahs rarely trouble the zebras, or vice versa. The truth is, cheetahs prefer to hunt prey that weighs close to 100 pounds, such as antelope and gazelles.”

Gumu tries to use his color and weight advantages to one-up his cheetah neighbor. He often trots along the fences that separate the two species’ enclosures and watches as the cheetah’s head darts back and forth, following his every strut. “These interactions,” Saffoe said, “are part of both species’ enrichment and encourage natural behaviors—at a safe distance, of course.”

Zebras can live up to 30 years in captivity, and such social interaction is key to their well-being and keeps their minds active. Since zebras are neophobic—afraid of new things—they tend to turn to familiar faces for entertainment: one another and their cheetah and scimitar-horned oryx neighbors. When keepers introduce a horse toy, such as a large ball or traffic cone, the stallions generally ignore it.

There is one object, though, that Gumu and Dante toss around without fear: their food bowls. The boys often engage in tug-of-war battles over the rights to these objects. These keep-away games are subtle displays of dominance. Zoo staff have also watched the twosome throw smaller toys over the fence with such frequency that it’s almost as if they’re playing fetch with their keepers.

“It’s fascinating to watch their social interactions change as they get older,” Volz said. “Both have such unique and charismatic personalities. That’s why our mission is to show Zoo visitors how charming these endangered creatures are and inspire them to care about Grevy’s conservation.”

Grevy's Zebra stallions
Dante (at left) and Gumu graze in the Zoo's Cheetah Conservation Station. (Mehgan Murphy/NZP)

On the Edge of Extinction

Another thing Gumu and Dante definitely don’t fear is food. They spend about 80 percent of their day chomping on compressed grass pellets and hay. Even mealtimes are a part of the zebras’ enrichment program: Keepers strategically place food in the crevices of trees or on the ground. This encourages the boys to forage just as their wild cousins do.

For wild Grevy’s zebras, finding food is not so easy. They are native to the semiarid grasslands and acacia savannas of East Africa’s “horn”—an area comprising Kenya, Ethiopia, and Somalia. In these dry, dusty landscapes, food is scarce, and water is even rarer. So when a zebra finds a watering hole, it will stay in that territory until a more dominant rival forces it to move on to new pastures.

The search for water can take Grevy’s zebras out of protected areas. That makes them vulnerable to poachers, who sell the animals’ striking hides to collectors. The meat is sold for food, and other bits of their bodies are carefully cut out for use in traditional medicine.

In some areas, farmers present a larger problem than poachers. “Pastoralists want every drop of water and each blade of grass for their own stock,” Volz explained. “When Grevy’s zebras are forced to share water resources with the livestock, they hang back and wait until nighttime. This increases their exposure to predation when they finally have the chance to drink.” Death from dehydration is one of the largest threats to foals and their mothers, who stop producing milk if they do not drink at least every other day.

A zebra that does not want to wait to drink faces an angry farmer. It is a challenge the animal rarely walks away from. “People should understand that these farmers aren’t villains. They kill wild animals for the sake of their livelihoods,” Saffoe added. “Just one livestock can feed an entire family for a week. The loss of any domestic animal, from either natural or environmental causes, has devastating effects on these villagers’ lives.”

After wild Grevy’s zebra populations fell by 50 percent, the IUCN classified them as endangered. Kenya boasts the largest population, some 2,300 individuals. Another 150 or so live in Ethiopia, and even fewer roam Somalia’s border, their numbers uncharted.

“Most conservation efforts are geared toward providing locals with jobs that help scientists monitor and scout for wildlife,” Volz explained. “Educational programs and Grevy’s zebra scholarships have also helped communities see the value of the unique species that lives only in their homeland.” Local programs such as the Grevy’s Zebra Trust work to provide better and more abundant resources for both livestock and zebras. These efforts have saved lives on both sides.

Still, wild Grevy’s zebras live on the cusp of extinction. They are threatened daily by disease, poaching, lack of resources, and habitat destruction, which is why there are currently no plans to release zoo-bred populations into the wild. This makes the Zoo’s participation in the SSP all the more important: Dante’s genes will one day help ensure his species’ future. At present, the fight to keep captive Grevy’s zebras thriving and reproducing—the work of Volz, her fellow keepers, and the SSP panel—is an ongoing battle, but Gumu and Dante don’t appear intimidated. Instead, they blithely graze, ever ready to defend their bachelor pad.

Did You Know?

Grevy’s zebras owe their name to Jules Grévy, who served as President of France in the late 1800s. He received a zebra as a gift from Emperor Menelik II of Abyssinia. The unfortunate animal died soon after its arrival in France, so Grévy had it stuffed and displayed in a Paris museum. A scientist there named the zebra after the president.

Freelance writer and editor JENNIFER ZOON is an assistant on the Zoo’s communications team and a former Smithsonian Zoogoer intern.

 

More: "Social Life, Grevy's Style"

 

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Smithsonian Zoogoer 39(1) 2010. Copyright 2010 Friends of the National Zoo. All rights reserved.