Hark, the Mountain Beaver

Hark to the mountain beaver, one of California's most mysterious and best kept secrets. It's not a beaver, and it doesn't live exclusively in the mountains. So . . . you may want to call it by its other name --boomer. Not that that is any better, because they don't "go boom" either.



We should take pride in this lovely, furry muskrat-sized rodent with a stub-tail. It's a living fossil and a unique American mammal. Aplodontia rufa is the only member of its family, the Aplodontiidae. Its ancestral relatives were digging up the Great Plains back in the Late Oligocene and Miocene, and were related to the Mylagaulids, which were more diverse and included the horned gopher -- Ceratogaulus of the Great Plains. That makes the mountain beaver a sole survivor of a once diverse assemblage of rodents.

Until a few weeks ago I knew the species only from the plush furs of museum specimens and the literature. But I always wanted to know the animals in the flesh, so to speak. My chance came this spring when the National Park Service approved my application to do a camera trap survey of wildlife in Point Reyes National Seashore.



The coastal scrub of California seems an unlikely place for mountain beavers. They live in lush conifer-shaded creekside habitats in most of their range, and usually on steep slopes. In addition to the main populations in the Sierra Nevadas and Cascades, California has two small isolated populations at Point Reyes (285 sq. km.) and Point Arena (62 sq. km.). These are in the fog belt of the north coast.

These island populations were obviously connected at some point in the past. As recently as about 3,000 years ago, mountain beavers lived betweeen these two sites. Gnawed bones from Duncan's Point Cave tell us that carnivores were catching them there near the mouth of the Russian River.

During a state-wide distributional survey of mountain beavers, biologist Dale Steele also discovered an even more unlikely habitat in the alkali scrub of Mono Lake. This truly looks like a no-mans-land for mountain beavers, because water limits its distribution. Unlike other rodents, mountain beavers are unable to produce a concentrated urine by resorbing water, because their kidneys lack the loops of Henle which perform this function. They need plenty of free water and succulent vegetation to survive.

Nonetheless, there they are at Mono Lake, pocketed in an oasis with a questionable future. I don't think it is known whether these particular mountain beavers are brave recent colonizers, or old stragglers from earlier and wetter times.

The coastal scrub at Point Reyes on the other hand is sodden during the winter and spring, and fog-bathed in summer and fall. The coastal scrub forms impenetrable thickets of coyote bush, blackberry, Euonymous, poison oak, cow parsnip, and thistle (among others). Here and there deer and elk have blazed trails through the stuff, which brings coyotes and bobcats a little closer to the mountain beavers' inner sanctum. But I got a clear impression that only smaller predators like weasels, skunks, and perhaps gray foxes can penetrate the tangles to the heart of boomerland.

When wildfire burned 5000 acres of Point Reyes's habitat in 1995 wildlife biologist Gary Fellers and co-workers discovered that mountain beavers were more abundant than previously estimated. With home ranges no larger than a half acre and a fairly tolerant disposition, quite a few can pack into an area, but for the most part they are hidden in the thickets.

During 27 camera trap days, the cameras made 858 exposures of which 31% contained animal images. Of the 8 species photographed, mountain beavers were in only 7% of the photos, and most of them looked like this.



I am not satisfied. I want a full-body shot worthy of a centerfold. Clearly I need more time to catch the boomer disporting itself out of its burrow.



Acknowledgement:
Many thanks for the cooperation of Point Reyes National Seashore, National Park Service. I am grateful for the assistance of Drs Ben Becker (NPS) and Gary Fellers (USGS).

References:
Beier, P. 1989. Use of habitat by mountain beaver in the Sierra Nevada. Journal of Wildlife Management, 53(3):649-54.

Carraway, L.N. and B.J. Verts. 1993. Aplodontia rufa. Mammalian Species, No. 421, 1-10.

Fellers, G.M, D. Pratt, and J.L. Griffin. 2004. Fire effects on the Point Reyes Mountain Beaver at Point Reyes National Seashore, California. Journal of Wildlife Management, 68:503-508.

Martin, P. 1971. Movements and activities of the mountain beaver (Aplodontia rufa). Journal of Mammalogy 52(

Steele, D. T. 1974. An ecological survey of endemic mountain beavers (Aplodontia rufa) in California (1973-83). State of California, The Resources Agency, Department of Fish and Game. 51 pp. (pdf)

Steele, Dale. Mountain Beaver (Aplodontia rufa) Journal

Wake, T.A. 2006. Archaeological sewellel (Aplodontia rufa) remains from Duncan's Point Cave, Sonoma County, California. Journal of Mammalogy 87(1):139-147.