Speed Goats


These are pronghorn antelope:


But they aren't actually antelope like the ones in Africa (Antilopinae). Pronghorn are all by themselves in their own taxonomic family of American antelope (Antilocapridae), roaming the sage flats around the Wild West, including here in Grand Teton National Park. They used to have about 12 other brother and sister species (some the size of small rabbits!), which, for whatever reason, didn’t have the evolutionary edge to survive the last two ice ages. The lone surviving species, however, is probably the park’s most spectacular example of adaptive success.

Most animals exist in constant stress with not enough food. Imagine being a ground squirrel in this ecosystem. Your preferred plant food is only available for a few short months, and everything is trying to eat you: wolves, bears, cougars, bobcats, hawks, falcons, badgers, coyotes, fox, weasels, etc. Your best bet to avoid becoming dinner is to spend a few short weeks aboveground to frantically gather and store food, then return to your relatively safe burrow for the next 8 months. If you are an elk and don’t even have the luxury of hiding underground, you now have to deal with 6 feet of snow and -40 degree nights.

Pronghorn seem to be the impressive exception to this stressful reality, so if I had to be any animal in the park, it would be a pronghorn. For starters, they almost always have more than enough food. Despite growing everywhere here, sagebrush is only eaten regularly by one mammal: pronghorn. Sagebrush leaves are full of noxious alkaloids that mildly poison most herbivores, but pronghorn have the right complement of intestinal bacteria to break down and digest this otherwise unpalatable forage. And if I were a pronghorn in a vast expanse of sage, I would almost certainly be able to see a predator coming. My eyes would be so big and set so far to the side, I could see more than 270° without even turning my head.

Now suppose a pronghorn was so focused on its sagebrush lunch that a wolf managed to wind up within 50 yards of it. The wolf starts the chase, running flat-out at 42 mph towards the surprisingly unconcerned antelope. At this point, if I were a pronghorn, I would take a few more bites of sagebrush, chew for a while, scratch an itch, stretch my legs, look at some clouds, and maybe decide what direction I should move. No need to panic, because when I do decide to distance myself from the wolf, I could do so at 60 mph, and I could keep up this speed for a very long time. Within seconds I would leave the wolf in hundreds of yards of dust, and merrily return to eating sagebrush.

Why evolve the ability to run so fast? It reminds me of the wily coyote vs. the roadrunner. Natural selection in a predator/prey relationship results almost every time in a prey species that is just barely faster than its predator. If you could run, say, 45 mph, you could easily outrun the fastest thing chasing you in the western hemisphere. Nothing requires a pronghorn to run 60 mph, so, ostensibly, there is no evolutionary advantage to do so. Well, not anymore at least.

This is (a rendering of) a North American Cheetah:
File:Miracinonyx trumani.jpg
Once the fastest thing in the world, it went extinct during the last ice age.
 The pronghorn has outlasted its only historical predator, and now enjoys a life of bountiful food and virtually no threat of predation.

The pronghorn in Grand Teton National Park have one more trick up their sleeve. In the winter, the snow gets too deep for the antelope to move around in. So they migrate. They head east over the Gros Ventre mountain range, 150 miles southeast into central Wyoming, where the snow is thinner and the temperature ever-so-slightly warmer. It’s the second longest land migration in the western hemisphere (after caribou in Canada). The “Path of the Pronghorn” is known only by the 400 antelope that summer here in the park. They learned this ancestral trail from their parents, generation after generation, and walk such a reliable route every year that they leave, in some places, a trail as wide as a road.


Isn’t nature awesome?

Comments

  1. I believe the Arctic Tern and the Humpback whale have longer migrations. However I'm assuming you're just talking about land mammals.

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