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a big old Sequoia, Sierra Nevada's -- California

About two million years ago Earth entered the last Ice Age, known more correctly as the Pleistocene epoch. Paleo-humans coexisted with bizarre-looking Ice Age mammals like woolly mammoths, saber-toothed cats and massive ground sloths. Rather abruptly, 10,000 years ago, the mega Ice Age mammals disappeared en masse. Did the climate change too quickly for them to adapt or did some other event drive them to extinction?

At the peak of the Ice Age continental-sized ice sheets covered roughly 30 percent of the Earth’s land surface. In comparison, today ice only covers 10 percent of the world’s surface. Most of Canada was smothered under three kilometers of ice and the oceans worldwide were lowered by 279 feet (85 meters).  Conclusive evidence supports lowered sea levels because numerous teeth of mammoths and mastodons were dredged from present-day submerged continental ocean shelves.

Many animal immigrants were able to enter North America from Asia by way of Alaska across the present Bering Strait. This route was called Beringia and emerged because the sea levels dropped and exposed bare ground for travel.

During the Pleistocene there were tremendous shifts in climate and scientists have documented 17 glacial-interglacial cycles. Of these events, seven complete glacial-interglacial cycles have occurred over the past 620,000 years, lasting between 88,000 and 118,000 years.

Cores from Greenland and Antarctica ice caps reveal pulses of major glacial expansions and minor glacial retreats. These changes apparently happened suddenly within decades or less. Moreover, deep-sea sediments have different forms of shell life. They contain special oxygen with neutral particles called neutrons in their atoms, decay at different rates depending upon whether they grew in warmer or cooler seas and accurately reflect climate change.

So what causes an Ice Age? A combination of events occur to cause Earth’s global temperature to decrease 3 to 5 degrees Celsius enabling the accumulation of snow leading up to advancing glaciers. Some of the more common hypotheses include: sunspot cycles which cause short term variations of solar output but it is unclear whether they cycles can last for 10,000 years or more. The Earth’s orbit can change as much as 8,639,308 miles (16,000,000 kilometers) over a 100,000 year period which would induce the onset of glaciation. In addition, the tilt of the Earth has changed and is changing by .01 degrees per century, and in conjunction with a wobble brought on by the gravitational perturbations by the sun and moon would also account for conditions favorable for glaciation. And interplanetary dust could block solar radiation, thus lowering the Earth’s temperature.

Ice Age mammals were covered in fur because it was so frigid, but why were they so large? Large animals have less surface area relative to their body mass compared to smaller animals. Larger animals lose proportionally less heat and can withstand cold temperatures more successfully than smaller animals. Bigger animals are better insulated with thick skin with or without more body fat. When size and insulation are increased the metabolic rate goes down and animals breath slower and have fewer heart beats per minute.

However, bigger animals need more food and therefore larger foraging areas. But larger animals have larger strides compared to smaller ones and therefore use proportionally less fuel to obtain food. Larger animals may have to search more widely for a mate. And there would be no surplus of species to survive adverse conditions or disease. This is applicable to both vegetarians and meat-eaters alike.

Bigger animals also have lower birth rates, longer gestation times and smaller-sized offspring compared to final adult size and are vulnerable to predation. Bigger bodies are structurally complex and with complexity comes specialization. Specialization makes a species less adaptable to changes in habitat such as climate swings of the Ice Age.

Some of these mega Ice Age mammals were really weird looking. For example, huge Rusconi ground sloth herbivores that were six meters tall and weighed an astounding 5,952 pounds (2,700 kilograms)! Short necks, powerful chests and long, sharp claws made them formidable. Its only real enemy were paleo-humans.

There were giant two-meter tall short-faced bears weighing in excess of 2,204 pounds (1000 kilograms). They were carnivorous and scavengers. Enormous beaver weighing a whopping 441 pounds (200 kilograms) with 6 inch (15 centimeter) long incisors must have built remarkable dams.

Fearsome saber-toothed cats with 7-inch (17 centimeter) front teeth were a feared predator as were the 860 pounds (390 kilograms) American lions which were believed to be the most widespread land mammals in North America. They hunted the gigantic mastodons and gargantuan woolly mammoths. These long-lived beasts had 9-foot (2.7 meter) tusks and could live for about 80 years.

Many wooly mammoths and other Ice Age mammals have been found in the far north where they have been entombed in perennially frozen grounds called permafrost. An excellent site to witness a diverse collection of late Pleistocene mammal fossils is Rancho La Brea tar pits in the heart of Los Angeles. Ten thousand animals were trapped over a 40,000-year period in asphaltic seeps representing 565 species.

Between 12,700 and 9,400 years ago North America lost 31 genera of animals each weighing 100 pounds (45 kilograms) or more. Scientists believe that these beasts perished because habitat changed dramatically due to climatic shifts at the end of the last Ice Age. And as human populations began to grow, and they doubled every 20 years, so too did the demand for meat. With Clovis stone projectile points, humans began to decimate North American mega fauna. The bigger animals did not have a chance to learn evasive behavior and survive.

Unless we significantly reduce polluting the stratosphere with fossil fuels (currently we spewing about 83 million tons a day of greenhouse gases) there’s a high probability humans will follow, shortly, the fate of the mega Ice Age mammals.

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Earth Dr Reese Halter is an award-winning broadcaster and distinguished biologist. His latest books are The Insatiable Bark Beetle and The Incomparable Honeybee

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Text © by Dr Reese Halter 2012. All rights reserved.


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