238 



NORTH AMERICAN FAUNA 68 



less than a week old were 285 and 290 g. (Jensen, 1964). If the 

 smaller of the two is presumed to approximate the birth weight 

 and its cube root is plotted according to the method of Huggett 

 and Widdas (1951) for a gestation period of about 62 days, a fetal 

 growth rate line is established (fig. 94). If the full-term fetus 

 weighing 132 g., that was recovered by Hamilton and Eadie 

 (1964), is plotted on the same growth scale as the European otter, 

 it is indicated that the period of implanted pregnancy in the 

 American river otter may be less than 2 months. Liers (1951) 

 states that "The eyes open when the young are about thirty-five 

 days old." This indicates that at birth the river otter is in an 

 early stage of development compared to the sea otter which is 

 born with the eyes open. 



To obtain an approximation of the period of implanted preg- 

 nancy in the sea otter, it appears reasonable to compare its fetal 

 growth rate with that of its closest relatives. If the cube root of 

 the sea otter weight at birth (generally at least 1,900 g., see Body 

 Measurements) is plotted on the same line of growth, then the 

 indicated period of implanted gestation is at least 4 months. Data 

 from reproductive tracts indicate that maximum breeding activity 

 probably occurs in the fall to winter period and that the maximum 

 number of births occur in the summer to early fall period. The 

 samples indicate that the period of unimplanted gestation in the 

 majority of animals (table 47) extends into the January to April 

 period, i.e., to last for about 6 to 8 months. This would indicate 

 that the total gestation period may be about 10 to 12 months. 

 Because mating and parturition occur during all months, this 

 method of estimating the gestation period from the available data 

 do not permit a more precise definition. 



Pupping season 



Birth may take place at any season. This was stated by many 

 early sea otter hunters, and confirmed by modern observers 

 (Barabash-Nikiforov, 1947; Fisher, 1940a; Murie, 1940; etc.). 

 I have handled and examined newly born young in all months. 

 Many of these newborn were captured with their mothers during 

 tagging operations and were returned to the mother when she 

 was released after tagging. 



Scheffer (1951) raised the question whether there was a season 

 of birth : "The evidence of five specimens . . .is that the young 

 are born in April and May, rather than the year around." Data 

 from our samples indicate that there is a season of maximum 

 birth frequency. The data in table 52 demonstrate that the per- 



