With an alleged top speed of 60 miles an hour, the pronghorn antelope is second only to the cheetah. But whereas the cheetah is a mere sprinter, the pronghorn is an endurance runner par excellence. Herds of pronghorn galloping across the high prairies of Wyoming can average 40 miles an hour for half an hour or more. If pronghorn ran marathons, they would complete the course in 40 minutes. Human marathoners manage two hours and eight minutes at best. How does the pronghorn do it?
Stan Lindstedt, a comparative physiologist at Northern Arizona University, put two pronghorn antelope on a treadmill and had them wear face masks connected to a gas analyzer. To measure the animals’ peak oxygen consumption, Lindstedt then made them gallop up an 11 percent incline at 22 miles an hour--about the speed Carl Lewis achieves on flat ground. We found that pronghorn have an extraordinary capacity to process oxygen, says Lindstedt. Each antelope consumed between six and ten liters of oxygen a minute, which is five times as much as a typical mammal of similar size would burn--a 70-pound goat, say--and more than four times as much as Carl Lewis would consume if he were shrunk to the size of a pronghorn antelope. (A pronghorn stands about three feet at the shoulder.)
The next question, then, is how pronghorn manage to burn vast quantities of oxygen. Birds do it, too, thanks to a neat anatomical trick: they have one-way lungs, with a separate entrance and exit, which allow them to inhale continuously without stopping to exhale. Lindstedt teamed up with anatomist Hans Hoppeler of the University of Bern in Switzerland to determine whether the pronghorn also relies on some structural novelty for its high performance.
It doesn’t. Instead, the researchers discovered that the pronghorn is just a little bit better at everything. Compared with the goat, it has bigger lungs with which to absorb oxygen, slightly more blood hemoglobin with which to transport the oxygen from the lungs to the muscles, and slightly bigger and leaner muscles containing a higher concentration of mitochondria--the cellular organelles that burn oxygen to provide power for muscle contraction. In other words, there are no tricks to the pronghorn antelope. It has simply perfected the same equipment that all mammals have.
But if that’s all it takes to be a champ, why haven’t other animals evolved in a similar manner? The answer is that there must be a catch--a cost that comes inevitably with the benefit the pronghorn derives from its remarkable endurance.
The benefit is clear enough. Until the wolf population in the Rockies began to dwindle, wolves were the pronghorn’s principal predator. Unlike big cats (such as the cheetah), which lie in wait for their prey and attempt to grab it with a short, sharp dash, wolves are pursuit predators. Once a pack has started to chase something, it will chase on and on. To avoid annihilation, the pronghorn was forced to reach the heights of endurance running, surpassing even the dogged wolf.
The cost of the pronghorn’s supreme endurance, however, is less clear. Initially, Lindstedt and his colleagues thought that pronghorn, like gas-guzzling race cars, must burn lots of energy even when they’re idle, which would force them to eat massive amounts of food. If so, then for any animal that is not terrorized by wolves, the burden of finding food might outweigh the benefits of being able to run like a pronghorn.
But in fact, says Lindstedt, when we calculated the amount of forage consumed by our two antelope, we found that they actually ate less than similar-size goats. There is nothing remarkable, he says, about the resting metabolic rate of the pronghorn.
A better theory, perhaps, is that the downside to being an endurance runner is extreme vulnerability to food shortages. Like human marathoners, pronghorn have limited stores of fat. And they do appear to suffer miserably when food is short, says Lindstedt. In the harsh winter of 1984, thousands of pronghorn carcasses piled up outside the fences of Wyoming cattle ranches. The animals had died while trying to reach the grass on the other side.