Extinction and the Mammoth Steppe

During the Pleistocene Ice Age that ended 12,000 years ago, many large mammals coexisted in an ecosystem that paleontologist Dale Guthrie dubbed the “mammoth steppe,” an icy prairie too cold, windy, and dry to support more than a few trees. When this ecosystem faded away at the end of the lce Age, many of its characteristic animals, like the mammoth, the wolly rhino, and the short-faced bear, went extinct, while a new set of megafauna—moos,elk, and bison—invaded Alaska and the Yukon. To understand the varied fates of these big herbivores, Guthrie looks to their digestive systems.

Any creature that lives off leaves or stems must overcome an array of defenses to absorb the nutrients inside. Many plants produce toxins designed to discourage herbivores — and the longer the vegetation grows, the more poisonous it gets. Even the youngest, freshest buds and leaves are full of cellulose, the complex sugar that builds plants’ cell walls. Only bacteria are able to break down cllose, so every herbivore from rabbit to elephant has evolved ways to nurture celose-digesting bacteria in its gut. Wild ruminants like bison and moose ferment (breakdown) their food in a large forestomach called the rumen. Inside this chamber, microbes tear apart plant cells and reconstruct their contents into a complete nutrtional package that includes every B vitamin and every essential amino acid. Because they ferment their food before it reaches their intestines, ruminants can eat toxic plants that would sicken or kll a horse. However, their digestion is slow by comparison—and because of the way their guts are designed, it cannot be sped up.

Elephants and horses also fement their food, not in a rumen but in a large pouch called the cecum, a hindgut that lies between the small inestine and the colon, after the stomach. The fermentation process is the same in both groups of animals, yet their survival strategies are very different. Since a horse’s fermentation chamber lies farther along in its digestive tract, it can absorb proteins that would otherwise be broken down by microbes. Horses can make do on tough, old grass—a diet that could never sustain a ruminant. If the available food is poor, heavy on useless fiber and low on nutrients, horses respond by eating constantly, running more and more food through their systems. The dry, cold plains of the Pleistocene had a short growing season but lots of grass. An ability to make it through long winters on tough, withered stems gave hindgut fementers, like the horse and mammoth, a major advantage. As the lce Age faded, willow trees began to colonize the banks of newly forming streams. Other trees and shrubs followed, many of them heavily armed with toxins only a ruminant could digest. In Guthrie’s view, these habitat shifts drove the decline of the horse and mammoth.

Guthrie has gathered an impressive collection of old and new radiocarbon dates on fossil mammoth, horse, bison, elk, and moose from Alaska and the Yukon. The patterns of animal distribution in time and space show that the transition from dry steppe to wet tundra kicked in between 14,000 and 15,000 years ago. The last of the region’s horse fossils date from that window of time, a moment when bison, elk, and moose — all ruminants — exploded in numbers and moved far into the north. Mammoths seem to have become less common around the same time, and vanished by 13,000 years ago.

To further support his climate-related scenarlo. Guthrie has shown that Alaskan horses began shrinking thousands of years before the first people settled the region, continuing to grow steadily smaller until they disappeared. That kind of pattern implies a gradual response to changing climate rather than a sudden impact of human hunters. Guthrie bases his claim on measurements of hundreds of Pleistocene horse metacarpals, foot bones that are critical in carrying the animal’s weight and thus make a good indicator of its body mass.

The data show that woolly mammoths and humans coexisted in Alaska for at least a few hundred years,and Guthrie agrees that people may have helped drive the mammoth into oblivion. If this happened, however, he emphasizes that the giants’ condition had already been weakened, the result of a climate change that degraded the environment in which hindgut fermenters had thrived and roamed for so long.?

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1.During the Pleistocene Ice Age that ended 12,000 years ago, many large mammals coexisted in an ecosystem that paleontologist Dale Guthrie dubbed the “mammoth steppe,” an icy prairie too cold, windy, and dry to support more than a few trees.?When this ecosystem faded away at the end of the lce Age, many of its characteristic animals, like the mammoth, the wolly rhino, and the short-faced bear, went extinct, while a new set of megafauna—moos,elk, and bison—invaded Alaska and the Yukon. To understand the varied fates of these big herbivores, Guthrie looks to their digestive systems.?