236 



NORTH AMERICAN FAUNA 68 



(1947) did not observe copulation during October and December. 



Data from field observations of breeding behavior and from 

 reproductive tracts give conflicting indications of the period of 

 maximum mating activity. Field observations appear to indicate 

 that most breeding activity occurs in June. By months, observa- 

 tions of breeding behavior were as f ollov^s : 



January 1 May 9 September J2 



February 2 June 17 October 0 



March 3 July 1 November 1 



April 1 August 4 December 0 



Field observations of behavior are hampered in fall and winter 

 by stormy weather conditions. Because comparative field observa- 

 tion time was not available in all seasons, I consider that field 

 observations of mating activity are not quantitatively useful. 

 They are qualitatively useful, however, in demonstrating that 

 breeding activity does occur in all seasons. 



Because the frequency of pregnancy in our samples increased 

 from 27 percent in early fall (October) to 70 percent in the 

 January-February sample (table 47), the available data indicate 

 that maximum breeding activity occurs in the late fall to winter 

 period. Further studies, particularly of reproductive tracts taken 

 in fall and winter months, are needed to reveal with certainty 

 the period of maximum breeding activity. 



Gestation period 



Sinha et al. (1966) demonstrated that after fertilization in the 

 sea otter the blastocyst enters a period of rest, i.e., undergoes 

 ''delayed implantation." Thus, presence of a corpus luteum indi- 

 cates pregnancy whether or not a conceptus is visible on gross 

 examination of the tract. When the ''resting stage" is completed, 

 the blastocyst becomes implanted in the mucosa of the uterine 

 horn and proceeds through embryonic and fetal stages of growth. 



To gain some knowledge of the length of the gestation period 

 and the duration of its unimplanted and implanted stages, data 

 were obtained from the ovaries and conceptuses of 275 reproduc- 

 tive tracts (table 47 and fig. 91). 



One method of estimating the gestation period of the sea otter 

 is to assume that during the period of implanted pregnancy the 

 rate of fetal growth may be comparable to that of the European 

 river otter (Lutra Ultra) and American river otter (L, canadensis). 

 (A more sophisticated method, given below, is employed by Chap- 

 man.) The European animal differs from the American river otter 

 and sea otter in that the blastocyst does not undergo a delay in 



