394 DISCOVERY REPORTS 



THE REPRODUCTIVE CYCLE 



Introduction 



The framework of the study of the reproductive cycle of the fin whale was laid down by Mackintosh and 

 Wheeler (1929) in their classic work on blue and fin whales, and most of their conclusions still stand. 

 They showed that the season of pairing and parturition extends over a protracted period in the southern 

 winter with a peak in June and July ; that pregnancy occupies about 1 1 months ; and that a lactation 

 period of about 7 months was usually followed by a resting period lasting until the next pairing 

 season, so that there is usually only one pregnancy in 2 years. They did, however, note that exception- 

 ally two pregnancies may follow in quick succession as a result of a successful post-partum ovulation 

 in early lactation. 



Mackintosh (1942), in summarizing work up to that time, found no reason for altering these initial 

 conclusions as to the interval between pregnancies, but drew attention to a marked increase in the 

 percentage of pregnant females and suggested that this might represent a reaction to whaling. He also 

 stated that there was some evidence that females simultaneously pregnant and lactating were less rare 

 than formerly. He discussed the growth to sexual maturity and concluded that ' while the estimate of 

 two years as the normal period from birth to sexual maturity is not proved, it is unlikely to be more 

 than three years ' (p. 225). The evidence on which this conclusion was based was mainly the incidence 

 of unweaned calves and modes in the length frequency distribution of immature whales (Mackintosh 

 and Wheeler, 1929) ; on the distinction of separate sets of scars indicating migrations in young whales 

 (Wheeler, 1930) ; and on the recovery of a single female fin whale marked as a calf, which 3 years later 

 was much larger than the mean length at sexual maturity. 



This estimate of the average age at sexual maturity was later revised to 3 and then 4 years as a result 

 of age-determinations made on baleen plates (Ruud, 1940, 1945; Nishiwaki, 1952; Hylen et al. 1955) 

 and as a result of the study of the ear-plug to 5-6 years (Purves and Mountford, 1959). 



There is now reason to believe that the average age at sexual maturity, although about 5 years 

 in this species is not fixed, but varies slightly with the condition of the stock (Laws, in press) 

 perhaps acting through the food supply. Among wild mammal populations this is perhaps best 

 documented for deer on ranges of differing carrying capacity (Morton and Cheatum, 1946; Cheatum 

 and Severinghaus, 1950). Laws (19566) has suggested that this may be the result of a higher 

 level of nutrition stimulating bodily growth so that the size threshold for reproduction is reached at an 

 earlier age. It is, therefore, unwise to assume that either the average age or the average length at 

 sexual maturity are unvarying. As regards the length at sexual maturity, Mackintosh (1942) revised 

 slightly the original estimate of Mackintosh and Wheeler (1929) of 65-57 ft- f° r area H females to 

 65-24 ft., mainly for samples also from area II. Brinkmann (1948) whose material was also mainly 

 from area II obtained a figure of 65-35 ft- These estimates are all in close agreement. Mackintosh 

 (1942) gave 63-0 ft. as the mean length of male fin whales at sexual maturity. 



Mackintosh (1942) discussed the important question as to whether whales are monoestrous or 

 polyoestrous. He concluded that ovulation is spontaneous and that, although the evidence is not 

 conclusive, it strongly suggests that whales are polyoestrous. He remarks : ' If it were found, contrary 

 to expectation, that whales were in fact monoestrous, the determination of age from corpora lutea 

 numbers would of course be enormously simplified' (p. 222). 



Chittleborough's (19556) data on the humpback whale suggest that in the great majority of indivi- 

 duals there is only one ovulation during the ovulatory period, but that this is a case of later poly- 

 oestrous cycles being suppressed because the first ovulation is successful, so that it is effectively 

 monoestrous. However, there is evidence (p. 393) that in the humpback whale there are on average 



