THE REPRODUCTIVE CYCLE 395 



probably two to three ovulations per 2-year cycle, which means, if correct, that there must usually 

 be more than one ovulatory period in the course of the 2-year cycle. Chittleborough is the only 

 worker who has been able to carrv out extensive studies on a large whalebone whale in low latitudes 

 during the winter, that is near the breeding grounds during the breeding season. His direct observa- 

 tions confirm the inferences about the breeding season and gestation period of humpback whales 

 made from records of foetal lengths (e.g. Matthews, 1937). This is encouraging and suggests that 

 such an approach should give fairly accurate results for other species (Laws, 1959a). 



It has been necessary in parts of the following account to draw analogies with other mammals, but 

 owing to the practical difficulties of verifying such points as, for example, the duration of the corpus 

 luteum of the cycle in the whale, this approach is unavoidable. 



The sex ratio 



Mackintosh (1942, tables 21 and 22) gives records of the sex of 13,379 fin-whale foetuses and 

 119,385 post-natal fin whales. The foetal records show a significant difference in the proportions of 

 the sexes (52-0% male, s.e. of the percentage 0-19), and the other records show an even greater 

 preponderance of males (54-5%, S.E. of the percentage 0-02). 



. The interpretation of the second figure is difficult. Lactating females are under-represented on the 

 antarctic whaling grounds and, as their taking has been prohibited for many years, even less well 

 represented in the catch. Conversely, there has probably been some selection by the whalers of 

 females because of their larger size, and the minimum size limits give greater protection to the males 

 (see Laws, i960). Mackintosh (1942, p. 267) concludes that there are slightly more males than 

 females. 



The breeding season 



The breeding season is here defined as the period of pairing and parturition. Since pregnancy extends 

 over almost a year these two activities take place at the same season of the year. It is clear from the 

 wide variation in foetal lengths in any one month that it is a protracted period. In fact, conceptions 

 occur and calves may be born in almost every month of the year, but most of this activity is confined 

 to a relatively short period. There are several methods by which we can obtain an estimate of the 

 monthly frequency of pairing. 



The male reproductive cycle 



The mating season depends to some extent on the cycle of activity of the males. Thus if, in the male, 

 rut is short and well-marked, the season of pairing will probably also be well-defined and the season 

 of parturition limited. This relationship is very clear in the pinnipedia, in some of which the pairing 

 season is confined to two months or less (Laws 1956c). 



In investigating the male reproductive cycle of fin whales little direct evidence is available because 

 of the inaccessibility of the breeding population and we must resort to indirect methods (as in the 

 case of females). 



Histological evidence 



One method is to attempt to distinguish a cycle of activity in the gonads, but here again there are 

 difficulties. A point which has not perhaps been sufficiently emphasized by previous workers is that 

 the epithelium of the seminiferous tubules is more sensitive to post-mortem changes than most other 

 tissues. These changes often take the form of extensive desquamation, of at least the superficial 

 layers, which is similar to natural and experimentally induced testis degeneration. 



In most pelagically caught animals the time from death to the examination is about 5-10 hr. and 



