Masters of Landscape Engineering

Man nearly extirpated beavers from Georgia by the end of the 19th century, much as he did other integral members of the state’s fauna such as the white tail deer.  But just like the deer, beavers staged a remarkable comeback and are now of a pestiliferous population.  During the Pleistocene before any humans were here to hunt this interesting animal, there must have been sizeable numbers of them in the environment, and they shared this ancient land with another larger species, the giant beaver (Casteroides ohioensis).

The habits of modern beavers (Castor canadensis) are well known to science, but knowledge of the extinct giant beaver is limited.  Here’s what we do know about Casteroides:  It weighed up to 400 pounds, averaging about 8 times larger than the still extant species.  It sported a longer, narrower tail than modern beavers.  And the teeth were adapted to a diet of aquatic vegetation vs. the mostly woody plants the smaller species prefers to eat. 

Giant beavers were widespread–their fossil remains found throughout the midwest and southeast, including a few specimens found in Georgia from Chatham and Glynn counties near the coast.

We do not know for sure whether giant beavers built dams like their smaller cousins.  The available evidence seems unclear.  Here, I divide the evidence in favor and against the possibility the giant beavers built dams.

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Evidence favoring the hypothesis that giant beavers built dams.

Fossil evidence at a Pliocene-age site on Ellsemere Island in the Canadian Arctic conclusively proves that Dipoides, an extinct species of beaver closely related to Casteroides, did build dams because the dam itself was actually found with specimens of Dipoides, and the chewed wood matched the teeth marks of this species.  Dipoides had teeth even less suitable to chewing wood than Casteroides, yet it did build dams from felled trees.  If a species like Dipoides, which was closely related to the ancestor of all aquatic beavers, built dams, than dam-building is an ingrained behavior for all beavers, and Casteroides must have built dams to.

Evidence against the hypothesis that giant beavers built dams.

Its teeth were not at all adapted for chewing wood.  Isotopic analysis of Casteroides’ fossil bones from one site suggests they lived on a diet consisting exclusively of sedges, and nodding water nymph, not wood.  Giant beaver fossil specimens recovered from the Erb site in Wisconsin date from a time when the local environment was an open marsh with few trees; fossil speciemens of the smaller beavers at this site date only from a time when the local environment became a closed canopy woodland.  This data indicates habitat differentiation and suggests that giant beavers preferred sparsely wooded habitats where few trees existed for dam-building as opposed to the closed canopy forests preferred by the smaller species.

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I theorize giant beavers did build dams in some circumstances but were not as dependent on them as Castor canadensis is and could occupy habitats not suitable for the smaller species.

Some scientists suggest giant beavers became extinct because our extant beavers outcompeted them.  This makes little sense.  The two species co-existed for at least 2 million years.  Moreover, beavers and giant beavers apparently preferred different habitats, the former preferring closed canopy woodlands, the latter open canopy marshes.  The smaller species actually created habitat favorable to Casteroides when they gnawed down trees and dug canals, thus expanding the size of the open wet meadows the giant species liked.  If any species competed with Casteroides, it would have been the muskrat, because they eat many of the same plant foods.  Muskrats today thrive in many aquatic habitats.  To me, this shows that habitats favorable for giant beavers are still available.  The main survival differences between muskrats and giant beavers are the larger size and slower reproducing capabilities of the latter species.  Casteroides couldn’t survive human hunting pressure, while smaller, faster reproducing species such as the muskrats and extant beavers could.

Beaver meadows and ponds must have abundantly dotted Georgia’s Pleistocene landscapes, especially during wetter interstadials (warm moist climate cycles during Ice Ages) and interglacials.  Beavers not only created ponds for aquatic life, but they also create meadows.  The longer a beaver dam is maintained, the more the number of surrounding trees get gnawed down until after many years the area is converted to a meadow.  In this self created landscape when  trees become scarce because of their own tree-destroying habits, beavers avoid traveling great distances overland where they’d be vulnerable to predators, by digging canals to wooded patches that are farther away from their home lodges.  In this way they expand meadows even more.  In Pleistocene times this meant they increased habitat for mastodons, mammoths, horses, bison, deer, and elk, as well as the aquatic life already flourishing in the ponds and canals.

I’m sure if we were able to take a time trip back to Pleistocene Georgia, we’d find both species of beavers, perhaps even living side-by-side.

Sources:

Kurten, Bjorn; and Elaine Anderson

Pleistocene Mammals of North America

Columbia University Press 1980

Rybczyinski, Natalie

“Casteroid philogenetics: Implications for the Evolution of Survival and tree exploitation in beavers”

Journal of Mammalian Evolution 207 (14) 2007 pp. 1-35

Yansa, Catherine; and P.M. Jacobs

“Pleistocene age Giant Beaver (Casteroides ohioensis) and extant beaver (Castor canadensis) environments of southern Wisconsin”

Geological Society of America abstract

October 15-21 2009

See also google images for pictures and illustrations of beaver dams, beaver lodges, beaver meadows, and fossil specimens of giant beavers.

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