Sex Determination 
Determining the sex of mountain beaver is not difficult. It is 
easy to distinguish the prepuce of the males from the vulva of the 
females. In addition, the large (about 1 inch or more in length) 
bifurcated penis of the male is readily extended. Males are also 
slightly larger than females. The conspicuous black hairs around the 
nipples of parous and lactating females, is a practical external 
character that distinguishes the sexes. Taylor (1918) stated that 
these black hairs are inconspicuous or lacking in the winter. 
IN CAPTIVITY 
Harry D. Hartwell (personal communication) wrote that mountain 
beaver can be held in captivity for extended periods of time in small 
enclosures of about 6 square feet. Pfeiffer (1953) discounted some of 
the statements made in the literature that the mountain beaver is diffi- 
cult to keep in captivity. He kept several of these rodents for long 
periods of time (6 months or more) in small laboratory cages designed 
for rabbits and guinea pigs. Also, mountain beaver have recovered com- 
pletely from such operations as ovariectomy and epididymal biopsy. 
Large amounts of water and an assortment of vegetation are neces-~- 
sary to keep the rodent in good health, 
PROSPECTS 
Much has been written on the toxonomy, paleontology, and life his- 
tory of the mountain beaver. Walter P. Taylor summarized the full taxo- 
nomig history of the genus, and McGrew reviewed the paleontology of the 
rodent. Monographic studies of the life history include those of 
H. E. Anthony, Charles L. Camp, Theo. H. Scheffer, and L. G. Ingles. 
E. W. Pfeiffer studied reproduction in the mountain beaver, and various 
parasitologists contributed information. But there is a dearth of 
knowledge regarding its ecology. We need to know more about its adapta- 
tion to new environments, direct effects on man's activities, depredation, 
food habit studies, predatory relationships, movements (immigration and 
emigration), population dynamics, methods of inventory, and carrying 
capacity to mention a few. For example, E. W. Pfeiffer, W. C. Mungesser, 
and D. A. Iverson (1960) found that the mountain beaver has a very limited 
ability to concentrate urine. Pfeiffer (personal communication) suspected 
that this inability limits the species' range to areas of a constant ade- 
quate water supply. Could this inability be a factor which limits the 
distribution of the mountain beaver in North America? 
One essential step in the process of research is the combing of the 
literature, The purpose of this review was to summarize the available 
references for use in future research on the mountain beaver, This is 
followed by intensive field investigations. 
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