Raising Wild

by Michael P. Branch

  Several other pronghorn adaptations for speed are specific to does and fawns. The first has to do with the pronghorn’s distinctive reproductive strategy of birthing twins. While a doe breeding for the first time usually calves a single fawn, nearly all her subsequent births—and every healthy doe will be pregnant every year for the rest of her life—will result in twin fawns weighing about eight pounds each. This twinning is ensured not only by a complex process of intrauterine competition among embryos, but also by the fact that the pronghorn uterus, unlike that of a human, is twin horned, thus virtually ensuring that an embryo will develop in each uterine horn. The pronghorn’s primary defense against predators is speed, so the development of twins ensures that the doe’s body will remain balanced and fast, even as she becomes heavier. Because Eryn is an identical twin, as were my uncles (who were named Robert and Bruce after Robert the Bruce, the warrior who led Scotland to independence during the early fourteenth century), I’m enamored of the idea that for pronghorn twinning is not an anomaly but rather an essential means of bringing a new generation of children into this world.

  A related adaptation is the pronghorn’s exceptionally long gestation period, which, at about 252 days, is much longer than that of larger ungulates, such as bighorn sheep (about 180 days) or mule deer (about 200 days). This protracted gestation is an evolutionary adaptation that reduces the window of vulnerability for fawns. Because fawns stay in the oven so long, they remain protected from predation longer and are more fully developed at birth. Pronghorn fawns can stand up just thirty minutes after they are born, and four days later they can outrun you or me. Although many fawns fall prey to coyotes, the fact that pronghorn gain size and speed so quickly allows many more to survive than would otherwise be possible. Because I am now a father, it is impossible for me not to see compelling correspondences between pronghorn fawns and human kids. The pronghorn’s extended gestation of about thirty-six weeks is remarkably close to the human gestation period of forty weeks, and I too have a fawn who weighed not much less than eight pounds when she entered the world. Whenever I find fawn bones in the desert hills I struggle to negotiate my own allegiances, for both the pronghorn and coyote, like me, have children to feed.

  The primary fuel of a pronghorn’s speed is oxygen, and it is here that the animal’s evolutionary adaptations are most impressive. Where you and I do a respectable job metabolizing oxygen so we can have muscular strength sufficient to thumb text messages and hoist pints, the pronghorn consumes three times the oxygen we do. Its heart is enormous, and its lungs are triple the size of those found in comparably sized mammals. In fact, all the organs and processes associated with burning oxygen are radically developed in pronghorn. Compared with other mammals their size they have a third more blood cells, half again as much blood, and two-thirds more hemoglobin; their muscle cells are loaded with the mitochondria necessary to process all this oxygen. Unlike the three-quarter-inch tube through which we pathetically slow bipeds suck wind when we’re out for a jog, the pronghorn has a two-inch windpipe that allows it to shotgun air while running. In fact, the minute it takes off sprinting it lolls its tongue out and opens its mouth to clear the tracheal opening for a full blast. If you don’t think this form of air gathering works, try sticking your open-mouthed noggin out the window of your car while going sixty miles per hour. One unfortunate by-product of all this oxygen-stoked muscular performance is tremendous heat, which is especially threatening to the animal’s brain. But pronghorn have developed a number of novel thermoregulation strategies, among which is a fascinating circulatory system that shunts overheated blood through a network of superficial arteries and veins that cool the blood before it is redirected to the brain.

  Although these and many other adaptations for speed have been sculpted by the pressures of natural selection, the creation of the amazing feat of form and function that we call the pronghorn has required eons. When I despair at how shortsighted we humans often are, I find it comforting to remember that Homo not-so-sapiens has had only a quarter of a million years or so of practice. Even the aches in our backs and knees remind us that our ancestors haven’t been standing upright for very long—and, in fact, that the evolutionary jury is still out on whether this bold uprising to verticality really pays off for anybody other than NBA players and chiropractors. Pronghorn, by contrast, have been trying to get it right for at least eighty times longer than we have, which provides some hope that in another nineteen and a half million years we might gain wisdom enough to be worthy of our name, sapiens. Then again, since we were the ones foolish enough to name ourselves in celebration of our wisdom, it is a good bet that we may lack the humility necessary to survive that long.

  To understand something of how pronghorn run so fast remains a far cry from understanding why they do so, especially in the context of such core evolutionary concepts as selective pressure and speciation. The problem with pronghorn speed is that it is anomalous. No predator in North America can catch an adult pronghorn, and none even comes close. While coyotes exact a heavy toll on each year’s crop of pronghorn fawns—and golden eagles and bobcats pick off a few as well—Old Man Coyote can’t begin to catch an adult pronghorn. The sophisticated pack-hunting strategies used by wolves gives them a better shot at relay racing a pronghorn to death, but even here predation is minimal. In fact, some studies suggest that the presence of wolves on the range increases pronghorn populations because wolves regularly haze the coyotes that are the major predator of pronghorn fawns. Like every other major predator in North America, coyotes, wolves, and mountain lions are so incredibly slow compared with pronghorn that predation by these animals cannot have provided the selection pressure necessary to create the pronghorn’s speed. Where, then, did this amazing speed come from? Evolutionary theory allows only one explanation: pronghorn run so fast because they are being chased by ghosts.

  Imagine a lovely, sunny spring day out on the North American prairie, a million years ago. You are one of tens of millions of pronghorn that live on the savannah, which you share with a great many other mammals, most of which will be extinct a million years from now. As you stand eyeing the prairie horizon—or perhaps you are reclined, chewing your cud—you are worried, and it isn’t because you have bills to pay or your car is making a funny sound. You have real, urgent, and incessant reasons to be terrified. Of course Old Man Coyote wants to snap your fawns’ necks today—just as he will on this day a million years from now—but at least he isn’t fast enough to catch you. Wolves are a more serious concern, as they’ll separate you from your herd and pack hunt you if they can. Thank goodness the gluttonous plundering dog Borophagus finally went extinct after seven million years of crunching your kids’ spines in its powerful jaws. Although evolution is busy replacing Borophagus with the huge, bone-crushing dire wolf (Canis dirus), it will be a few hundred thousand years before you really have to worry about that. In the meantime, you can handle most of these Canis types. After all, how scared can you be of a genus that will eventually produce animals that prance along on a leash and wear little sweaters? The hyenas are a bigger problem. Ever since they crossed the Bering land bridge back in the Pliocene, they’ve been getting better and better at chasing you. In fact, when he’s hot on your white rump, the running hyena Chasmaporthetes looks more like a jaguar than it does like the slouchy little hyena that will be around a million years from now.

  That’s just for starters. The short ambush predators can really ruin your day. Of these, the gnarliest is Arctodus simus, the giant short-faced bear. Don’t try any of your excellent short-face jokes on him, because he weighs eighteen hundred pounds, is seven feet tall at the shoulder and ten feet tall on hind legs, and has a reach of about fourteen feet. He needs to eat thirty-five pounds of meat each day, which is about what you weigh once he picks his teeth with your ribs and gets to the juicy part. What’s worse, unlike bears a million years from now—the ones that lumber over to the park dumpster to scavenge from discarded buckets of KFC—his feet are oriented w
ith toes straight forward, which means he can really run. He can’t outrun you in a prairie race, of course, but if you spend one second too long daydreaming about the upcoming rut while drinking from the spring, you’re a goner.

  The stalk-and-ambush crew also includes the scimitar cats (Homotherium) and the saber-toothed cats—which aren’t named Smilodon fatalis for nothing. There are a bunch of species of each, all of them weighing five or six hundred pounds, with six- or seven-inch-long teeth hanging out of their mouths, quiet as a mouse when stalking, and not too shabby running short distances. The “smaller” cats also cause problems. The American lion (Panthera leo atrox) is five hundred pounds of bad-tempered nuisance, and the cougar (Felis concolor) and jaguar (Panthera onca) are always staking out your migration routes. A good rule of thumb with all these cats is that you should just stick with your herd, eat a nutritious diet of succulent forbs, get lots of exercise, and stay out of situations where you can get bushwhacked. Of course they’re going to eat most of your children, but as long as you stay out in the open and remain vigilant, you can outrun any of these carnivores in a pinch. Still, I’ll bet you can hardly wait for the Pleistocene extinctions to get some of these giant monsters off your back.

  But here’s the real problem, the one you’d really prefer not to discuss. There’s one thing out there that is faster than you are, at least in a short sprint—and if you get taken down in the first hundred yards, then your ability to run fast mile after mile won’t help you, will it? What’s worse, you made that thing fast. You made it fast by being so fast yourself. It is thanks to you that the American cheetah can sprint seventy miles per hour and is thus among the swiftest of terrestrial mammals. His adaptations for speed are numerous, fabulously inventive, and all your fault. Many’s the day you pranced around racking your brain for a new way to pick up just a few MPHs. Put your ears back to reduce wind resistance? Stick out your tongue to shotgun air? Find some way to stretch your stride length from twenty-eight to twenty-nine feet? Seems like every time you came up with something, though, he did too; no matter how fast you became, the American cheetah was a little faster, at least in the short run. But if it bothers you that he’s faster than you are, remember that you also have him to thank for making you so fast. If you weren’t running from him all the time, you’d never have become fast enough to outrace all those other voracious predators, and then where would you be? Your speed and his have coevolved, each causing the other. I know you hate it when he eats you, but he really has made you the best pronghorn you can be.

  Nobody knows for certain why most large mammals went extinct around eleven thousand years ago. In North America, roughly thirty-three of forty-five genera of mammals larger than about ninety pounds disappeared in a wave that erased the mastodons, mammoths, dire wolves, giant short-faced bears, saber-toothed cats, and many other mammals—including the American cheetah and all species of antilocaprids except the pronghorn. Some believe that overhunting by the humans who were then on the scene and wielding the new Clovis spearpoint technology was a major factor. Others find it more likely that a changing climate disturbed the ecosystems upon which large carnivores depended for survival. Yet others subscribe to the theory that the late Pleistocene extinctions are attributable to a wave of highly infectious diseases against which few large mammals had adequate defenses.

  We do know that the lightning-fast American cheetah (Miracinonyx trumani) went extinct about ten thousand years ago, while its prey, the pronghorn, did not. As a result, Antilocapra americana has been alone out there for the past ten thousand years, running much faster than anything in North America, though the pronghorn doesn’t seem to know that yet. And this is the most fascinating part of the pronghorn’s story. Evolutionary theory provides no way to account for Antilocapra’s excessive speed except to say that the pronghorn still sees the sleek shadow of the American cheetah over its shoulder. The pronghorn has experienced twenty million years of directional selection for speed: it just kept getting faster and faster in an attempt to outrun its ever speedier predators. For the last ten thousand years, though, that selection pressure has been relaxed. Now you might think that if you had ten thousand years to change your ways you could do so (though my experience with humans suggests this isn’t a sure thing), but the pronghorn’s speed—and a number of its other adaptations to avoid Pleistocene predators that no longer exist—is inscribed in its body. It can’t simply decide that it really doesn’t need so large a windpipe or even that it can abandon its vigilance while browsing. If we assume a twenty-million-year evolutionary history for the pronghorn, then the ten-thousand-year period during which directional selection for speed has been relaxed amounts to just 0.0005 percent of that history. The pronghorn is a product of millions of years, not thousands. Pronghorn run fast because evolution hasn’t yet had time to slow them down.

  Unlike many people, I have a neighbor who has cheetahs. My friend Aaron, who lives just four miles from our place as the kestrel flies, has for more than twenty-five years managed an animal rehabilitation center that cares for injured wild animals and for exotic animals confiscated from traders trafficking illegally in endangered species. Many years ago a pair of orphaned African cheetah cubs found their way to Aaron, who immediately began working to figure out how best to keep them healthy. Veterinarians at zoos had long observed that captive cheetahs developed a wide variety of health problems and that gastrointestinal diseases often claimed their lives. Using the kind of common sense that is as valuable to a human as speed is to a cheetah, Aaron reckoned that animals evolved to run seventy miles per hour would be healthiest if they could run seventy miles per hour. In his early work with the cheetahs, Aaron took them out to a desert playa not far from here, attached a flagged lure to a rope behind his pickup truck, and literally ran the cats across the desert as fast as he could zigzag without rolling his F-150. Aaron tells a wonderful story of once seeing a group of pronghorn lining a ridge above him, watching intently as the cheetahs sprinted across the playa at top speed. I love to imagine what kind of evolutionary ticking must have been going on behind those huge antilocaprid eyes. Were they curious, surprised, terrified? Or had evolution hardwired them so effectively that they simply thought the pronghorn version of “You again, huh?” Indeed, if advocates of the radical idea of “Pleistocene rewilding” ever get their way, African cheetahs (Acinonyx jubatus) will be introduced to the pronghorn range as a substitute for the extinct American cheetah. And if that wild day ever comes, pronghorn will be mighty glad they didn’t become slackers just because the Pleistocene extinctions wiped out their fastest predator.

  It isn’t only pronghorn physiology and speed that are vestigial but many pronghorn behaviors as well. These “relict behaviors” are another kind of evolutionary shadow—another indication that it takes tens of thousands of years to unlearn the lessons that millions of years have taught. Among relict behaviors in pronghorn, the most dramatic is the animals’ congregation into large herds during winter. Typically, herding is a behavior that increases energy-draining competition for food, but it is worthwhile because it is an effective way to guard against predation. After all, your chances of seeing spring will be much better if a few dozen or a few hundred of your buddies are scanning the ridges while you’re pawing away snow to get at forage. And the pronghorn winter herd displays the additional skill of being synchronized to a degree of precision that is incomprehensible to a bumbling biped like me. When the herd moves across the land, it exhibits a 97 percent synchrony of gait, which means that whether there are twenty pronghorn or two hundred, 97 percent of them will put their four hooves on the ground in exactly the same sequence and at exactly the same time. Pronghorn have at least four distinct gaits, or running patterns, but even when they shift from one to another at high speed (which is analogous to your shifting the gears of your bike or car as you increase speed), there remains better than 90 percent synchrony in the herd. Even in the exact instant they change their hoof sequence and timing while running at forty or
fifty miles per hour, 93 percent of them will do so at precisely the same time—and the rest will correct within a fraction of a second.

  Although I have witnessed the full winter herd only once, there is no possibility that I will ever forget it. The flawless synchronization of the animals’ gait makes the herd’s movement indescribably graceful. These animals don’t appear to run at all but instead seem to flow across the land like a single organism. At first glance I assumed that I was seeing the shadow of a cloud rippling across the desert hills; when I looked again, my home desert had been fantastically transformed into the Serengeti, with graceful herds pouring out beneath a towering sky. As a result of the pronghorn’s incredible synchronization, predators find it difficult to single out an individual to attack, and some scientists have even suggested that the weird effect of the perfectly coordinated herd—which looks more like a school of fish than it does a bunch of long-legged mammals—may actually have a hypnotic effect on predators.

  Of course the fleet predators this herding instinct evolved to protect against no longer exist. So how long will it take before pronghorn will slow down, before evolutionary pressures will select for something different, like the ability to jump a fence or to recognize the glint of sunlight off gunmetal? Or will they remain incredibly fast, while their slower predators evolve to become as fast as cheetahs? Or will the ongoing biodiversity and climate change crises, which threaten the extinction of numberless species, wipe pronghorn out before evolution can correct for their anomalous speed? Nobody knows. In the meantime, the pronghorn is still out there, calving twins, hanging on, running as fast as it can. It is still out there, a ghost being chased by the ghosts of predators past.