242 



NORTH AMERICAN FAUNA 68 



percent. Since the young sea otter remains with its mother for 

 about a year, the annual rate of reproduction is 14 percent, or 

 about 16 young born per 100 independent animals per year. 



From Chapman's analysis (see later section) it is shown that 

 each 100 adult females in the breeding population produce 50 

 young per year. Thus, in the adult population (male and female) 

 there are theoretically 25 dependent young per 100 adults of both 

 sexes. It was calculated that 31 percent of the independent animals 

 observed during field counts are juveniles. Thus, 69 percent of 

 those observed are adults. It follows that, with a group of 69 

 adults there are 17 (i.e., 25 percent) dependent young, or — 



Number Percent 



Dependent young 17 14.5 



Independent animals: 



Juveniles 31 26.5 



Adults 69 59.0 



Total 117 100 



Information from reproductive tracts, from field counts of 

 dependent and independent animals, and the ratio of males to 

 females in the kill are used in two ways to demonstrate that the 

 annual rate of reproduction of the Amchitka population is about 

 14 to 15 percent. 



Twinning 



Barabash-Nikiforov (1947, p. 98) and Snow (1910) report 

 observations of mothers with two young, and reproductive tracts 

 containing two fetuses. Our observations indicate that twinning 

 is unusual. In 278 reproductive tracts taken in 1954-63, 178 of 

 which showed an indication of pregnancy, no twin fetuses were 

 found. One specimen (No. JEB 63-166) taken at Amchitka on 

 25 March 1963, had well-defined corpora albicantia in both ovaries 

 and darkened scar tissue (placental scars) in both uterine horns. 

 This female was not, however, accompanied by young. 



That activity in both ovaries is not necessarily an indication 

 of twinning was shown by one specimen ( ADFG 515) . Both ovaries 

 contained well-defined corpora lutea but only the left uterine horn 

 contained a conceptus. 



In early April 1955, a female and two large young (each about 

 18-20 lb.) with her were captured on shore at Amchitka. They 

 were held in captivity and liberated at the Pribilof Islands. During 

 the days that the three animals remained in captivity, the adult 

 allowed both young to nurse. She appeared equally permissive to 

 both. She groomed only one of the pups, however, and when the 



