Mountain Beaver (Aplodontia rufa) by Dale Steele, Conservation Biologist I want to open this article with a personal experience that helped lead me to a long-standing interest in mountain beaver. Years ago I was participating in ecological studies at Mono Lake (see Mono Lake Natural History for more information). In the heat of summer in this harsh climate, I noticed a strange animal scurrying among the tufa (calcium carbonate) towers. I was sure that the animal was a mountain beaver but when I researched the subject I found that all available information indicated the species couldn't possibly exist in this hot, dry climate. Nonetheless, some years later several roadkilled mountain beaver were in fact documented in the Mono Basin. It seems to me that if we can learn more about this unique species we can better understand how is has survived for so long and perhaps find ways to coexist with it. I've long been interested in this most unusual and interesting animal, the mountain beaver or Aplodontia. I want to share some my interest and hopefully to raise your curiosity in the species. If you end up agreeing that this is an amazing animal and want to know more, I will be happy to share stories and references that should hold you over until you can go looking for them yourself! The mountain beaver is not a real beaver. It has been compared to a muskrat without a tail. It's a little known but fascinating rodent, which occupies a unique taxonomic and ecological niche. Aplodontia are considered to be the oldest group of living rodents, being the sole extant member of the superfamily Aplodontoidea, which has been found almost morphologically unchanged in the fossil record since the Miocene. It is thought to be ancestral to the squirrel family. The first published account of mountain beaver, Aplodontia rufa (Rafinesque), comes from the journals of Lewis & Clark in 1805. Aplodontia are not considered game or fur animals today although, in the past, Native American Indians wore robes made of mountain beaver and valued their meat. The Indian robes, called "she-wal-lal," were the origin of the mountain beaver nickname, sewellel, which Lewis and Clark misunderstood to be the name of the animal. What makes mountain beaver so interesting? The mountain beaver is sort of a living fossil. While their range has decreased from that in the fossil record, probably because of geological and climactic changes, very little appears to have changed morphologically. The species seems to have a number of physiological limitations that should decrease its ability to adapt to new environments. Mountain beavers were used in early studies of kidney function because their primitive kidneys lack the structure to concentrate urine effectively. The animals need large amounts of water as a result. Other organ systems have not been well studied. There is a unique structure in their brains whose function is unknown. One theory is that it provides high sensitivity to changes in air pressure. These are mysterious animals. They spend most of their time in underground burrows and come out only to eat or during the short juvenile dispersal period when young animals leave the nest to establish their own burrow sites. They are difficult to study and so have not been given adequate attention by biologists. At one time it was thought that mountain beaver were colonial animals because several animals may inhabit a series of interconnected burrows. However, it is now agreed that there is little interaction between animals except for territorial behavior within burrows. Each animal has its own burrow nest area that is connected with tunnels opening to the outside. It is difficult to determine how many animals inhabit a burrow system since an area riddled with holes may contain only a few animals. Mountain beaver are not pet material. They are crotchety, vicious animals that canÕt be domesticated. In captivity, many have lived for a period of time and then died suddenly from undetermined causes. When disturbed, they secrete a thick material from their eyes which has been misinterpreted as tears. These behaviors combined with their poor eyesight sometimes lead to some unusual interactions between researcher and animal! Mountain beaver burrow systems support a community of vertebrates and other animals. Skunks, salamanders, moles, voles, shrews, chipmunks, ground squirrels, mice, woodrats, gophers, weasels, mink, hares and brush rabbits have all been trapped in mountain beaver burrows. These animals may have been present as commensals, predators, or by accident. There is also a unique invertebrate fauna associated with mountain beaver. Perhaps the most striking example is Hystricopsylla schefferi, the largest flea in the world, which grows to 9 mm (almost 1/2 inch) in length. The coevolution of the two organisms has never been investigated but, given the ancient lineage of mountain beaver, may provide some interesting insights. Mountain beaver are strict herbivores. They are known to eat a wide range of plant species, which often includes just about all species within reach of the burrows. Herbaceous plants are eaten whole while woody plants are discarded after the bark has been peeled off for food. Clipped vegetation can often be observed near burrow systems. Mountain beaver are voracious eaters. Studies have shown that the majority of their active time is spent gathering, handling and eating food. They seldom venture far from their burrows, which may open directly into suitable vegetative stands. The animals forage for short distances above ground, then carry or drag the cut vegetation, which may vary in length from a few inches to several feet, to the burrow. There the material is cut into short sections at the burrow entrance and carried into the burrow to be eaten or stored. Animals may eat vegetation outside of the burrow, but most often consume it in feeding chambers which are adjacent to the nest. While mountain beaver gather much of the vegetation in their vicinity, there appears to be a decided preference for certain types of plants including shrubs and smaller trees. Some of their preferred foods include species that are unpalatable or toxic to other mammals such as bracken fern, sword fern, nettles, thistles, corn lily, salal, foxglove, larkspur, and skunk cabbage. This gives the mountain beaver a largely uncontested food niche. The ability to consume plants with such a variety of toxic secondary compounds is unusual and may involve a metabolic "cost" to the animal. Mountain beaver require large amounts of succulent vegetation for survival. Distribution limits are associated with rainfall and edaphic conditions that promote succulent vegetation and high humidity within burrows. Studies suggest that the most important factors in habitat use are a cool thermal regime, adequate soil drainage, abundant food supply, and a high percent cover of small diameter woody material and soft soil. Mountain beaver have a limited distribution along the west coast of the United States, from Point Reyes, California north and slightly into British Columbia. Of the seven subspecies of mountain beaver, four are effectively endemic to California. Two of these, the Point Reyes mountain beaver (Aplodontia rufa phaea) and the Point Arena mountain beaver (Aplodontia rufa nigra), are restricted to very small ranges. The Point Arena mountain beaver is a federally-listed endangered species. I recently completed a recovery plan for this subspecies for the U. S. Fish and Wildlife Service. In the Pacific Northwest, mountain beaver are considered a pest species by many foresters because they are abundant and eat seedlings and young trees. However, in California, they are limited and considered a species of concern. Habitat destruction due to human interaction is a problem. Natural disasters have, in many cases, exacerbated the problem. A recent fire in Point Reyes destroyed about 50 percent of the known habitat of the entire subspecies. Studies are currently underway to determine the extent of damage to the population. Mountain beaver are known to be prey of bobcats, fishers, coyotes, great horned owls, skunks, eagles, minks, and other predators. Little is known of other mortality factors such as disease. The sensitivity of mountain beaver to disturbance is also not well known. For more information, contact Dale Steele who? by workphone/voicemail (916-653-3444) or email (dsteele@igc.org) I am maintaining this website as a communication and information sharing tool while I work with this species. If you are interested in seeing what else I am up to you can get a look here. |Return Home|