Pleistocene Mammals in the Midwest
Giant Beaver, Castoroides ohioensis
The giant beaver was the largest rodent in North America during the Pleistocene. Although it is similar in appearance to the modern beaver, it actually may have been more similar to the modern capybara (water hog) from South America.
Length: 1.9-2.2 m (6.2-7.2 ft)
Weight: 90-125 kg (200-275 lbs) (McDonald 1994)
The giant beaver was larger, with proportionally shorter limbs than its modern counterpart. The vertebrae that make up the tail are wide, with flaring processes, indicating that it was flat, although proporationally narrower than the modern beaver tail. Their front incisors were extremely large (up to 15 cm [6 in] long), had numerous thin grooves on their front surfaces, and were tapered to blunt, rounded points, rather than wide sharp edges. Their cheek teeth (molars) were also noticeably different from those of modern beavers, and were structured in a manner that is very similar to those of the modern capybara. Finally, the brain of the giant beaver was relatively small and smooth, unlike the large, wrinkly brain of the modern beaver, which may indicate that giant beavers were not capable of the complex behaviors exhibited by their modern counterparts (e.g., dam building).
Although most giant beavers inhabited lakes and ponds that were bordered by swamps, they are also present in spruce tundra habitats. The shape of their incisors are unlike modern beavers, and would not have been efficient at cutting trees, nor do scientists think they built dams or lodges (Swinehart and Richards 2001).
Paleontologists often compare the diet and behavior of the giant beaver to a muskrat. It did not eat woody vegetation but had a diet dominated by aquatic plants, including "coarse leaves, the roots of sedges , cattails, and other vegetation" (Swinehart and Richardson 2001).
It is thought that these animals were clumsy walkers but strong swimmers and probably spent most of their time in the water. Their ungainliness out of the water seems to have restricted their ability to disperse rapidly over land, which may have played a role in their extinction. There is no evidence that giant beavers built dams or cached food for the winter.
North American Ice Age Distribution:
During the last ice age, giant beavers were restricted primarily to the central and eastern U.S. (McDonald and Bryson 2010), and were most abundant south of the Great Lakes in Illinois and Indiana.
Status at the end of the Pleistocene:
Giant beavers went extinct towards the end of the Pleistocene. It is generally thought that these animals went extinct in large part due to the reduction and/or disappearance of their preferred habitat as the climate warmed and the glaciers retreated north, and to increased competition with modern beavers. Recent work at a site in Indiana (Swinehart and Richards 2001), though, suggests that their preferred habitat (ponds, lakes and marshes) may actually have expanded as the climate warmed during the terminal Pleistocene.
Results from a new study (McDonald and Bryson 2010) suggests that changes in temperature and precipitation patterns may have contributed significantly to the extinction of these animals, but not in the ways previously thought. These authors found that giant beavers preferred areas with cooler annual temperatures and a strong summer growing season, which would enable them to store sufficient fat stores to survive the winter. During the terminal Pleistocene, changing climate patterns lead to higher annual temperatures and a seasonal shift towards high springtime precipitation that may have affected the growth of plants available to giant beavers. The authors suggest that while a northern shift in distribution would have provided these animals with a more acceptable temperature range, it also would have shortened the growing season, and thus limited the amount of time available for accumulating fat stores. These are intriguing hypotheses that will need to be tested with additional data.
Midwestern Paleontological Finds:
Remains of giant beaver have been recovered at paleontological sites located throughout the Midwest, and especially from Illinois. This species has been documented from at least seven localities in central and northern Illinois, including Alton, Hopwood, Clear Lake Sand and Gravel, Polecat Creek, Bellflower, New Bedford and Phillips Park, and from at least three localities in central Indiana, including Prairie Creek, Shoals, and Christensen Bog. It has also been documented in Michigan at the I-96 site, Dowagiac River site and near the city of Ludington. In Ohio, this species has been documented from the Carter Site and from Sheriden Pit, and it has been recovered from the Witte Farm locality in southern Wisconsin. Finally, giant beaver remains have been documented at Boney Springs in central Missouri as well as from two localities near Minneapolis, Minnesota.
McDonald, H.G. 1994. The late Pleistocene vertebrate fauna in Ohio: Co-inhabitants with Ohio's Paleo-indians. Pp. 23-41, In The first discovery of America: Archaeological evidence of the Ohio area. (W.S. Dancey, ed.). The Ohio Archaeological Council, Inc. Columbus
McDonald, H. Gregory, Bryson, Reid A. 2010. Modeling Pleistocene local climatic parameters using macrophysical climate modeling and the paleoecology of Pleistocene megafauna. Quaternary International 217: 131–137
Swinehart, A., Richards, R. 2001. Palaeoecology of a northeast Indiana wetland harboring remains of the Pleistocene giant beaver (Castoroides ohioensis). Proceedings of the Indiana Academy of Science 110: 151–166.