Dropping by. During rainy months in African savannas, zebra may benefit cattle by unearthing nutrient-rich grasses.

Rob Pringle

Want Fatter Cows? Bring In a Zebra

Climb to the top of a hill along one of the few remaining undisturbed grasslands in East Africa, pull out the binoculars, and you may spy a black-and-white zebra herd. You may also see a few gazelles, buffaloes, and elephants. "Natural selection has favored that mix," says Johan du Toit, an ecologist at Utah State University in Logan. Natural selection, maybe, but not people. Convinced that other grass-chomping animals will drive their herds to starvation, ranchers in Kenya and elsewhere tend to keep their cattle separate from wildlife. But a new study suggests that thinking may be wrong. Wildlife, particularly zebras, can actually help a ranch thrive.

To test the common wisdom, Wilfred Odadi, a rangeland ecologist at the Mpala Research Centre in Nanyuki, Kenya, and colleagues visited a local 20,000-hectare savanna where livestock and protected wildlife, including close to 2000 zebras, still mingle. The team fenced off pastures within the grazing lands, opening some to cows only and the others to cows and whatever wildlife happened to pop by. And many animals did. Throughout the study, the unrestricted cow herds brushed shoulders with a veritable Disney movie cast of creatures, including zebras, oryx, hartebeest, elephants, and even giraffes.

During the dry season, when much of the grass turns wispy, the constant rotation of wild ungulates did seem to take a toll on the cattle, and many lost significant weight. But during the wet season, the livestock rebounded. In rainy months, cows with wild companions beefed up much faster than their solo counterparts, Odadi and colleagues report today in Science.

Zebras seem to be responsible for the effect. Giraffes only nibble trees, and elephants don't gorge on grass in dry months. But zebras, easily the most abundant wild grazers in the region, can swallow grasses that many other herbivores avoid, thanks to their specialized digestive tracts. Throughout the study, herds of the animals tramped through the pastures, chewing the taller, less nutritious leaves, possibly exposing the richer vegetation below. That hoof print was obvious even to the naked eye, Odadi says: "You would see that [the grassland] is greener and leafier, especially after it had started raining."

The team's findings are "of wide, practical importance," although not necessarily surprising, says Norman Owen-Smith, an ecologist at the University of the Witwatersrand in Johannesburg, South Africa, who was not involved in this study. Wild ungulates such as zebras are, by nature, wanderers. In the dry season, they cluster around water holes, then cut a wide path in the rainy months to track down quality feed. But in recent decades, most have been confined to relatively small national parks that can't sustain large herds. If ranchers learned to tolerate a few zebra visitors, he says, some fences could come down, and the animals could return to their routine.

Zebras are few and far between in the western U.S., adds du Toit, who was not involved in the study, but the same rules may apply. He points to old prairies in the Great Plains, where plentiful bison would march along, followed closely by pronghorn and then prairie dogs. Changing ranchers' minds in the United States and elsewhere on wildlife would be tough, he admits, but might be possible if scientists and policymakers can communicate the potential economic benefits. "The traditional inertia is huge," he says. "But, you know, money talks."