ancient horses 2

germtales...

The Mystery of the Ancient Horses

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The viral modus operandi is genetic reassortment, shuffling bits of DNA into new combinations with new capabilities. Different strains of a flu virus, for example, can meet and mix in a host and result in a new, more lethal version. Viruses can also pick up genes from the cells they infect. Bacterial microbes routinely share up to quarter of their genomes with other microbes through lateral transfer. (This is how antibiotic resistance genes spread.) The realm of the invisible is actually a giant genetic swap meet. And newly acquired genes can be passed down to new generations, with natural selection weeding out what’s useful and what’s not. Mosquitoes and ticks often vector more than one germ at a time, making them perfect little cauldrons for cooking up new disease combos (the tick that carries Lyme Disease, for example, can carry a dozen different pathogens). Insects are also well positioned to then deliver these new plagues to an unsuspecting—and unprepared—world. Disease trouble starts whenever old meets new. A prehistoric epizootic (an animal pandemic) could have been caused by a first encounter with an Old World disease, or possibly by a hybrid New/Old World pathogen. Either way, a whole continent full of unprotected potential victims awaited. THE PAST AS PROLOGUE: A CAUTIONARY TALE Disease alone probably wasn’t enough to do in the horses. Given their numbers, distribution and sheer toughness, some surely would have survived. Even with West Nile, where the horse mortality rate is considered quite high, only a third of confirmed cases die. But could disease have been the tipping point event leading to extinction? What if…
….human hunters killed off the mammoths, enormous, slow-to-mature, slow-to-reproduce animals that literally shaped the Pleistocene environment. The mammoths themselves may have been weakened by diseases, making them easier to hunt (perhaps a new strain of mange from new-to-the-continent dog mites attacked their famously shaggy fur, driving them slightly mad…) Mammoths kept the landscape open, just the way horses preferred it, by eating and trampling young trees. The demise of the mammoths also meant that an important part of the prey base collapsed. Humans were now competing with accomplished predators such as short-faced bears, dire wolves and saber-toothed tigers for what was left. Meanwhile, habitat was being hammered by climate change. There were droughts and fires and floods. Forced by a scarcity, horses found themselves increasingly pushed into more dangerous predator-filled areas. Then a vector-borne West Nile-like virus hit hard, flaring up every summer as birds migrated North. Both birth rates and death rates were affected. Pregnant mares either died or aborted. The young were left to fend for themselves. As the horse population dwindled, so did genetic variability. The end was in sight. And the last ancient horse trotted off into the Pleistocene sunset… We may never know exactly what happened. But we do know the result: 135 megafauna species gone from the continent in comparatively short order. Is it happening again? The climate is changing. And species of every kind are moving into new areas, including humans by the millions via plane, train, boat and automobile. Habitats are shifting. The pressure on dwindling resources is intense. We are also in the midst of what has been described a wildlife disease crisis. There is “an explosion, a renaissance of disease that’s unprecedented in modern time,” according to wildlife biologist Milt Friend, founder of the USGS National Wildlife Health Center. ("Disease Emergence and Resurgence: The Wildlife-Human Connection") Outbreaks of duck plague and avian cholera have killed off tens of millions of birds, while TB and Chronic Wasting Disease threaten deer. From rabies in raccoons to plague in prairie dogs, the number, variety and severity of outbreaks is on the rise. This time it is not just North American phenomenon. Nor it is not just wild animals. As bird flu, monkey pox, SARS, West Nile and a long list of other headline disasters keep proving, many germs make no distinction between wildlife, livestock, pets and humans. A host is a host is a host. RETURN OF THE NATIVE? Whatever caused the Pleistocene horses to die out in North America thousands of years ago, they are back now, and when left alone, thriving. Horses that escaped from a conquistador’s corral or a settler’s stable kicked up their heels, took off for the hills and never looked back. Some American herds have been living wild now for hundreds of years. But today mustangs tread a fine line between Cultural Icon and Reviled Pest. To most ranchers, they’re thieves-on-the-hoof, stealing water and forage that rightfully belong to livestock. To environmentalists, mustangs are an alien species, destroying the landscape with their sharp hooves. But are they really thieves and aliens? Or are we witnessing one of Nature’s most dramatic and successful wildlife reintroductions? Ten thousand years is a blink in evolutionary time, and horses were an important part of the prehistoric landscape. Mammoths, woolly rhinos and short-faced bears are gone forever. But horses have returned, once again filling the North American air with the thunder of their hooves and joy of their whinnies. In their wildness, they provide us a glimpse a world that used to be.