266 DISCOVERY REPORTS 



Table 20 compares these results from the North Atlantic with findings on the sexual maturity of 

 sperm whales from the North Pacific and from the southern hemisphere. Because whaling in the 

 Azores is not restricted by the size regulations enforced elsewhere, the North Atlantic data have the 

 advantage of including more immature whales of both sexes than have been available in any area, 

 except the pelagic Antarctic where Nishiwaki (1955) considered sixteen of his large sample of males 

 to be immature. For male whales captured off Japan workers have failed to detect any change in the 

 rate of increase of testis size when plotted against body length, and this is probably due to the scarcity 

 of the smaller (immature) whales in their samples. Nishiwaki & Hibiya (1951, 1952) and Nishiwaki 

 (1955) have based their findings on the presence or absence of spermatozoa in histological preparations 

 of the testis. Since there is evidence that male sperm whales have a sexual cycle (p. 273), it is doubtful 

 whether the presence or absence of sperms is a very reliable means of discriminating between mature 

 and immature animals. Nishiwaki & Hibiya estimated that males from the North Pacific matured at 

 a length of 35 to 37 ft. Because of the enforced size regulation they examined no material from whales 

 shorter than 35 ft., so it may well be that the four whales they considered to be immature corresponded 

 to large immature animals of the overlap in Table 18. Unless small whales become available to workers 

 in the North Pacific, it is not possible to say whether or not male whales from the North Atlantic and 

 North Pacific mature at different mean lengths. Probably there is no real difference, because, turning 

 to females, it is seen that those data from the North Pacific which were mostly collected in 1948, when 

 the length restriction had not been raised above 30 ft. and which therefore include most immature 

 females (three), suggest that female whales mature at 30 ft. or a little less (Omura, 1950). This is 

 virtually identical with the present estimate for Azores females. Recent work, reviewed by Pike (1953), 

 has shown that whalebone whales mature at a greater length in the southern than in the northern 

 hemisphere. Consequently it is interesting to note that, although data on southern female sperm 

 whales are few and include no immature animals, there have been two estimates (based on testis size, 

 and on the presence or absence of sperms) which suggest the considerable mean lengths at sexual 

 maturity of 39 ft. and 41 ft. respectively for male sperm whales in the southern hemisphere 

 (Matthews, 1938, p. 129; Nishiwaki, 1955, p. 148). Further work is required, but there may be a 

 real difference here between northern and southern sperm whales. 



Breeding 



Table 21 gives details by months of forty-two sperm whale foetuses examined at Horta between 

 1949 and 1954. The foetal lengths fall clearly into a group representing early pregnancies, ranging 

 from 0-15 to 1-25 m., and into a second group of late pregnancies with lengths between 3-10 and 

 4-10 m. Foetuses from the Bonin Islands, North Pacific, show a similar grouping (Mizue, 1950, p. 118). 

 That there should be two such distinct groups, without intermediate lengths being represented, points 

 at once to the presence of a breeding season, and to a period of gestation which extends over more 

 than one year. 



In Fig. 6 the foetal measurements from Horta are twice plotted against months in successive years. 

 The bottom left-hand and the top right-hand groups of points are now seen to represent on this two- 

 year time scale the early and late stages of foetal growth. The average length of the foetus in each 

 month, at the mean time in the month, has been calculated and marked in the figure, and a freehand 

 curve drawn giving the best fit to these average values. The number of foetal measurements is not 

 large, but there are fairly adequate data for the average size of the early pregnancies in July and Sep- 

 tember and for the late pregnancies in July; the curve joins these three average points, and the sparser 

 points for other months are seen to be fairly distributed about the curve. The curve is pecked for the 

 first two months, April and May, when early foetuses are minute and when few data can in any case 



