BREEDING AND LIFE CYCLE 273 



Dates of capture of the whales and the mean diameter of their testis tubules are plotted in Fig. 9, 

 which also shows the average monthly tubule diameter plotted at the average date in the month. The 

 diagram shows that there is a steady decline in the diameters from month to month, from a high range 

 of values in June to a low range in September. There are not sufficient data for October to say whether 

 or not the apparent rise in this month is real. There is one anomalous plot, for whale F246, 9-8 m. 

 long, captured on 2 October 1953 : the mean tubule diameter was 182 ft, but in this whale the testis 

 sampled was very much larger (1-35 1.) than its pair (0-57 1.), and a few tubules in the section seemed still 

 to be maturing; so it would appear to be abnormal, presumably an individual which has matured out 

 of season: it has been neglected when calculating the average tubule diameter for October (Table 23). 

 My colleague, Mr D. E. Cartwright, working from the correlation coefficient between date and 

 tubule diameter (^0-58 for the sample from eighty whales), has calculated that the probability is 

 much less than 1 % that the decline over the months is merely due to chance. The tubule diameters 



200- 



cr 

 O 



<j 



E 



£ ISO- 



a) 



E 



D 



C 



o 



IOO- 



O Monthly average tubule diameter 



• •F246 



• t /. . 



• t 



•• •• 

 I. • 



o 



Oct 



Nov 



May Jun July Auq Sep 



Fig. 9. Males. Diameters of testis tubules in sexually mature whales and the dates these whales were examined at 



Horta in 1949, 1953 and 1954. 



have presumably declined from greater diameters during a period when the tubules, characterized in 

 the present collection by degenerating spermatids or spermatozoa, have been active with spermato- 

 genesis outside the months shown. 



Thus the observed decline, combined with the other histological evidence, shows that the male 

 sperm whale has a sexual cycle, and that the sexual season occurs later than October and earlier than 

 June. 



This sexual season is a protracted one, for Fig. 9 indicates that tubule diameters in any one month 

 vary widely, but steadily, at least for the three months (June, July and August) where data are most 

 plentiful and where the monthly differences between the smallest and largest diameters are almost 

 identical, being 50, 48 and 48^ respectively (Table 23). 



Except for rare instances where a mechanism exists separating the period of germ cell production 

 from the period of pairing, sexual seasons coincide when they occur in both sexes of an animal. The 

 season of the female sperm whale is mostly contained between March and May (p. 270), so it is most 

 probable that the sexual season of the male is in spring also. 



Matthews (1938, p. 132) considered that there was probably no sexual cycle in the southern male 

 sperm whale. However, Nishiwaki & Hibiya (1951 ; 1952, p. 123), reporting on two collections from 



