The authors assumed that the long period of time between the birth 
of the first and second fetus and the short time between the second and 
third fetus could possibly be attributed to the location of the fetuses 
in the uterine tubes. They stated that there might possibly be a lag 
between the descent and appearance of the single fetus from one uterine 
tube and the descent and appearance of the two fetuses from the opposite 
tube. 
Number of Young at Birth 
Lord (1866) asserted that the mountain beaver has from 4 to 6 young 
at a birth and 2 litters a year. Lum (1878) reported from 3 to 5 young 
per litter and that the weaned young make their appearance during June. 
Scheffer (1929) examined data from 16 litters, and found 7 litters 
of twins (44 percent) and 9 litters of triplets (56 percent). Seton (1928) 
said that the litter size is usually 2, but occasionally may number 3 
or 4, 
Young at Birth 
In their observation on parturition in the mountain beaver, Cramblet 
and Ridenhour (1956) reported that the young at birth are naked, blind, 
and helpless, The vibrissae are apparent. The auricular openings are 
sealed, and the external ear formation is observable but not extended. 
The front toes are well developed, but the hind toes are mere stubs. The 
tail is very noticeable. The eyes are closed during the first 10 days. 
For the three young examined at birth by Cramblet and Ridenhour, the 
average body Length was 87.7 millimeters and the average body weight 27.0 
grams. 
Lactation 
Pfeiffer (1958) thought that lactation did not continue beyond 2 
months after parturition since he found juveniles of that age with 
vegetable matter in the stomach; at this time they have been reported to 
leave their mother. He stated that in nursing females the pectoral mammae 
are often the most used, and the thoracic pair are commonly used, while 
the abdominal pair are sometimes not used. 
Physiology of the Mammae 
Pfeiffer (1955) wrote that pregnant, lactating, and nonpregnant 
postovulatory mountain beaver females exhibit dense patches of black hairs 
around the nipples. He found that estrogen directly or indirectly applied 
either by inunction or injection thickened the epidermis of the nipple 
and surrounding area. The estrogen also stimulated the hair follicles in 
this area to rapid growth and hyperpigmentation resulting in hairs larger 
and darker than before estrogen stimulation, 
14 
