Pfeiffer estimated the fertility and fecundity of females by 
ovarian and uterine examinations. He found little loss of ova or 
embryos between ovulations and implantation, suggesting a very short 
estrous period. He also found that all females in a given population 
ovulate about the same time each year so that there is a very sharply 
defined and extremely limited breeding season within a population, 
These criteria enabled him to determine that estrus occurs within a 
short period of from 5 to 7 weeks in mid- or late winter. 
Gestation Period 
From data which gave dates of ovulation and parturition, Pfeiffer 
estimated that the gestation period is 28 to 30 days. He found early 
embryos during the last half of February and large fetuses during the 
first half of March. Scheffer (1929) reported early stages of pregnancy 
in mid-March, and found that by April 25th, of 23 parous females 11 were 
nursing young, and 12 were pregnant. He examined 26 females on April 5, 
of which 1l were pregnant. Of the 15 nonpregnant females, Scheffer 
thought 6 to be matured but unmated, and 9 to be immature yearlings. 
He suggested that the gestation period was about 30 days. 
The difference in embryo stages between Pfeiffer and Scheffer's 
findings is due to different geographic ranges of individual populations. 
Pfeiffer's embryos were from a Point Reyes, Calif. populations; whereas 
Scheffer's were from the Puget Sound Country in Washington. 
Parturition 
Cramblet and Ridenhour (1956) live-trapped a pregnant female 
mountain beaver in Humboldt County, Calif., on March 15, 1954. Palpa- 
tion showed that it was about 1 week prepartum., They determined the 
prepaftum period by comparison with fetuses of rodents of comparable 
size, 
On March 19, the authors observed that the animal lactated slightly. 
On the following day they noted wavelike contraction of the female's 
abdominal muscles which appeared to be due to fetal movements. Similar 
contractions were observed on the following 2 days. On March 23, they 
observed that the female pressed her forefeet against the genital region 
and gnashed her teeth. This activity of 2 to 18 minutes duration was 
repeated periodically for the next 1-1/2 hours, Immediately before 
parturition, the female positioned herself on her haunches and lowered 
her head between her hind legs. When the tail of the first fetus ap- 
peared, she assisted its birth by pressing her forefeet around the vulva 
and licking the region. The birth of the first fetus was completed in 
4 minutes, The head of the second fetus appeared 21 minutes later, and 
the birth process was completed in 5 minutes, One minute later the tail 
of the third fetus appeared, and the birth was completed in 2 minutes. 
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