292 DISCOVERY REPORTS 



found that for the four species considered so far (in which Lt^ has been estimated from the length of 

 gestation (toothed whales), or by reference to an accurately fixed conception date (humpback whale)), 

 the slope of linear growth is closely correlated with Lt^ (Table i). As the correlation is so close the 

 values of Lt^ for other species in which a is known have been obtained by extrapolation (blue and 

 fin whale) or interpolation (northern sei whale, minke whale). These values are bracketed in Table i. 

 For the northern sei whale {Balaenoptera borealis Lesson), the mean monthly foetal length values 

 given by Mizue and Jimbo (1950) fit almost exactly a linear/log growth curve. The three points for 

 March, April and May fall on a straight line intersecting the abscissa at the end of December, and the 

 five means for the succeeding months, June to October, fall on a logarithmic curve (Text-fig. 7). 

 Moreover, the neonatal length given by Mizue and Jimbo (1950) for this species (14-15 ft., or about 

 4-4 m.) is very close to the length of the humpback whale at birth and the fitted curves of these two 

 species are almost identical (Text-fig. 12). If Lt^ can be taken to be 39 days (Table i) then the average 

 date of conception is in the third week in November. By extrapolation of the exponential growth 



Table 



curve in the second half of pregnancy the birth length is reached in mid-November, so the gestation 

 period estimated in this way is almost 12 months. Mizue and Jimbo (1950) also assume that the end 

 of November is the average time of birth, but that the time of pairing is in January, giving a gestation 

 period of less than 11 months. It should, however, be noted that these authors fix the time of 

 conception by freehand extrapolation. 



Since the work of Mizue and Jimbo (1950), Omura and Fujino (1954) have shown that the northern 

 and southern types of sei whale in the western Pacific Ocean are the species B. borealis Lesson, and 

 B. brydei Olsen, respectively. These two species are similar in size, the former being apparently 

 slightly larger. The main diflferences are in the quality and shape of the baleen plates, the shape of 

 the palate, and the extent of the ventral grooves. The catch at the Bonin Islands is now composed of 

 Bryde's whales, but formerly, when the material in Mizue and Jimbo's (1950) paper was collected, 

 the whaling season was earlier and the catch was almost entirely composed of sei whales. According 

 to Omura and Fujino (1954) sei whales are taken at the Bonin Islands from November to April and 

 in May and June the catch is composed of Bryde's whales; at Sanriku and Hokkaido the catch is 

 almost entirely sei whales. 



It seems reasonable, therefore, to conclude that the data presented in Text-figure 7 apply to the 

 sei whale, B. borealis. In view of the similarity in the size of B. borealis and B. brydei it is probable 

 that the average neonatal lengths and foetal growth curves are similar, though the breeding season 

 may be different. 



The mean monthly foetal lengths for July to October of the northern fin whale given by Mizue and 

 Jimbo (1950) also lie on an exponential curve. 



The foetal growth of the minke whale has been discussed by Jonsgard (1951) and Omura and Sakiura 

 (1956). There is no reliable information on the length of gestation but Jonsgard found that only 



