By Kathleen Holder
Why do people ride horses but not their striped African cousins?
A few zebras have accepted a rider or pulled a cart, but zebras have never been truly domesticated — and for good reason: They can be aggressive, panicky and unpredictable, making them difficult to halter and saddle train. While smaller than horses, they have powerful legs that can carry them at speeds up to 35 mph, and with a kick, can break the jaw of a predator. Those Chuck Norris-like skills are useful when you have lions, cheetahs and hyenas chasing you down for lunch.
But a new study by UC Davis researchers suggests that fear of four-legged carnivores may not be the sole explanation of why zebras are hard for humans to tame. Their enduring wildness may be the evolutionary legacy of a long relationship with predators on two legs —humans themselves.
Alexali Brubaker, who earned her Ph.D. in psychology from UC Davis in 2103 and is now research coordinator for the Third Millennium Alliance, and psychology professor emeritus Richard Coss compared the flight behavior of plains zebras in Africa with that of feral horses in Nevada and California when a human approached on foot.
In areas frequented by people, feral horses allowed a researcher to approach much closer than did zebras — waiting until they got about 54 yards away before going into alert mode and an average 18 yards before running away, compared with the zebras’ 68 yards alert distance and 40 yards before fleeing.
Brubaker and Coss say zebras’ wariness may be an evolutionary adaptation that allowed the species to survive hundreds of thousands of years of hunting by humans in Africa. Their 40-yard no-human zone is just outside the effective range of poisoned arrows used by African hunters for at least 24,000 years.
In Central Asia, early horses were hunted initially by archaic humans. Even then, Ice Age weather conditions provided long periods where horses, better adapted to cold climates, saw few human hunters. However, modern humans who replaced them after migrating to Asia from Africa 40,000 to 50,000 years ago were capable hunters of horses. Coss said that timeframe was not long enough to evolve an instinctual fear of humans.
The researchers were surprised to find, on the other hand, that in remote areas where people are rarely seen, modern feral horses exhibited as much or more wariness as zebras. Horses showed alert behavior (raising their heads, stopping grazing), on average, when a person got within 218 yards and then moved away when the human was 160 yards away. For the zebras in unpopulated areas, the average distances were 167 yards for an alert response and 115 yards for flight.
“This finding indicates,” Coss said, “that despite domestication, horses have not lost their keen awareness that an upright, approaching shape viewed from a distance could constitute a predatory threat.”
Their study, reported online Sept. 7 in the Journal of Comparative Psychology, also sheds new light on the question about where horses were first domesticated through selective breeding.
Coss said the findings point to Central Asia where, “their initial wariness of humans was likely assuaged by frequent exposure to humans as it is today when wild horses are rounded up and find homes under private care.”
Kathleen Holder writes about social sciences for the UC Davis College of Letters and Science. Follow Kathleen at @kmholder.