Giant Ground Sloth

Giant ground sloths (Megatherium and Ere-motherium spp.) roamed the Americas at the end of the last ice age. Shortly after humans entered the scene, the giant sloths disappeared from the New World's cast of characters, as did mammoths (Mammuthus spp.), the big-horned Bison antiquus, and many other large mammals. The exact cause of the Late Pleistocene megaherbivore extinctions remains a matter of debate, but the timing of these extinctions, following so quickly upon the heels of man's arrival, suggests more than mere coincidence.

Several species of the genus Megatherium inhabited North and South America. These creatures, weighing several tons, were some of the largest mammals ever to walk the earth. Next to a giant ground sloth, a 500-pound grizzly bear would seem puny in comparison. Unlike their much smaller arboreal relatives, the modern two- and three-toed sloths (Choloepus and Bradypus spp.), the giant ground sloths were terrestrial, and skeletal evidence indicates that despite their massive size they were able to stand up on their hind legs using their stout tails for balance (Casinos, 1996). The smaller forelimbs armed with huge claws were most likely used to strip leaves and bark from trees. One author (Farina, 1996) has recently theorized that some of the giant sloths may in fact have been carnivores, using their dexterous forelimbs and sharp claws for hunting, but the small, blunt teeth would appear to have been better adapted for a herbivorous diet.

A few early paleontologists were tantalized by the discovery of bits of sloth skin, which seemed so fresh that they believed the animals had been recently alive. However, further examination of these specimens using radiocarbon dating has revealed that they are in fact much older and simply very well preserved because of environmental factors. It is generally agreed that all of the giant ground sloths became extinct about 10,000

Print of the fossil skeleton of a sloth discovered in South America, from an 1825 book by Georges Cuvier (Library of Congress)

years ago—shortly after the arrival of humans in the New World.

Scientists who theorize about the cause of the great wave of extinctions that swept through the large and medium-size herbivores in the Late Pleistocene are generally divided into two camps. Some believe that climate change and the retreat of the glaciers led to changes in vegetation, which caused the herbivores' demise. Others believe that the primary force was predation by human hunters. Large herbivores have relatively low reproductive rates, and their populations decline rapidly if predation surpasses the low population growth threshold (Owen-Smith, 1989). Most scientists believe that a combination of factors was involved.

The "keystone herbivore" hypothesis proposed by Norman Owen-Smith suggests that the megaherbivores were not only among the victims of the great wave of extinctions that occurred in the Late Pleistocene, but they may also have played a key role in the genesis of this extinction event. According to the hypothesis, large herbivores are instrumental in modifying the landscape by clearing old growth and making way for savanna and forest regeneration. These changes may make the landscape more favorable for smaller, more selective herbivores. A modern example of this has been observed in areas where elephant populations have been eliminated. In the absence of the megaherbi-vores, thickets and woody vegetation overgrow the savannas, making them less hospitable to other grazing herbivores. Elimination of the Pleistocene megaherbivores by human predation or climate change would have also resulted in secondary changes in vegetation that might have had a cascade effect, resulting in the loss of other species dependent upon the megaherbivore-modified landscape (ibid.).

—Julie Pomerantz

See also: Ecosystems; Extinction, Direct Causes of; Herbivory; Mass Extinction; Order Uranotheria; Paleontology; Pleistocene Epoch

Bibliography

Casinos, Adria. 1996. "Bipedalism and Quadrupedal-ism in Megatherium: An Attempt at Biomechanical Reconstruction." Lethaia 29, no. 1: 87-96; Farina, Richard A., and R. Ernesto Blanco. 1996. "Megatherium, the Stabber." Proceedings of the Royal Society of London. Series B: Biological Sciences 263, no. 1377: 1725-1729; Owen-Smith, Norman. 1987. "Pleistocene Extinctions: The Pivotal Role of Megaherbi-vores." Paleobiology 13, no. 3: 351-362; Owen-Smith, Norman. 1989. "Megafaunal Extinctions: The Conservation Message from 11,000 years B.P." Conservation Biology 3, no. 4: 405-412.

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