FIN AND BLUE WHALES 295 



Standard errors is about 0-26 m. Even in January and February, in which the sample sizes are large, 

 the variation is of this order. The explanation of this relatively large variation is undoubtedly that the 

 pairing season extends over several months, thus increasing the length variation within the monthly 

 samples. Comparison of Tables 4 and 5 shows that a similar variation occurs in the blue whale. 



Symons (1955) compared foetal length data for fin whales taken in area IV in 1954 and in areas 

 III and IV in 1955s and suggested that those in the second year w^ere 'some i to 2 weeks late in their 

 peak period of conception '. He then put forward a hypothesis based on this supposed difference. 



MONTHS 



Text-fig. 8. Foetal growth in length of southern hemisphere blue whale, Balaenoptera mtisculus, and fin whale, B. pliysalus- 

 Rectangles represent monthly means, plus or minus two standard errors. (Black — blue whale ; white — fin whale). 



In fact the greatest differences between his mean lengths are almost within the range of four standard 

 errors (about 0-26 m. or 10 in.) calculated for the present material and certainly well within plus or 

 minus two standard errors of the means for his very much smaller samples. The supposed difference 

 between these two years is therefore unlikely to be significant. 



The average growth curve for the fin whale has been fitted as follows. The mean foetal lengths of 

 the samples for October, November and December are thought to be very close to the true mean 

 lengths for these months; they lie on a straight line which cuts the abscissa, when extrapolated, in 

 mid-July. The slope of this line is very similar to that calculated for the sperm whale (Text-fig. 12) 

 and the period from conception to Lt^ is estimated to be 33 days in the fin whale (Table i). This gives 

 the second week in June as a mean conception date for the southern hemisphere fin whale, which 

 agrees very well with independent conclusions from other data. (These include the follicular cycle 

 of females at puberty, and the histological cycle of testis activity and of testis tubule diameters, 



' There are six Antarctic whaling 'areas'. Areas III and IV are south of the Indian Ocean. 



