THE HUMPBACK WHALE 

 Table XXI. Humpback luhale. Females examined by the Discovery Staff 



67 



difficult to reconcile with whalers' statements, reported by Risting, to the effect that the 

 female Humpbacks caught off South Georgia and the South Shetlands in the summer 

 are nearly all pregnant, or with that obtained from Captains C. A. Larsen and Thoralf 

 Sorlle {Report Interdepartmental Committee ofi Research, etc., 1920, p. 91) that about 75 per 

 cent of female Humpbacks taken at South Georgia were pregnant. But as Risting shows, 

 these statements of whalers as to yearly breeding have not much value, because the 

 majority of the Humpbacks referred to were caught when the whole carcase was not 

 utilised and was generally discarded after the blubber had been removed: even after 

 more modern methods were introduced nothing resembling a careful examination was 

 undertaken. Hinton (1925) points out that Risting evidently thinks, and has good grounds 

 for so thinking, that Humpbacks bear a young one in alternate years only. 



The oestrous cycle of the female Humpback whale may therefore be summarized 

 diagrammatically as in Fig. 58, in which the main cycle is shown as occurring once every 

 two years, and the broken line represents a possible post-lactation oestrous which occurs 

 occasionally, and is usually found in those individuals which have given birth early in 

 the season of parturition. 



GROWTH 



Examination of the genitalia showed that female Humpbacks become sexually mature 

 at a length of about 12-5 m. and males at a length of about 12 m. The shortest mature 

 female recorded was 12-35 "^^ lo^^g ^"^^ the shortest mature male 11-9 m. 



If all records of Humpback whales less than 12-5 m. in length are plotted by months, 

 a scatter diagram as shown in Fig. 59 is produced. In this figure there are included 

 nineteen immature whales recorded by Hinton (1925) from South Georgia. There 

 appear to be in this diagram two fairly well-defined age groups, those 10-12 m. long and 

 those 6-9 m. long. If now the average length by months is calculated, those under 9 m. 

 in length being separated from those over 9 m. for the months of November to March 

 inclusive, and the averages are plotted on an extended time scale with the low November 

 to March values on the left, the curve in Fig. 60 is produced. 



