420 DISCOVERY REPORTS 



THE BREEDING SEASON 



The term "breeding season" is not very explicit and needs to be used with a certain 

 amount of caution, for it does not clearly differentiate between the pairing season and 

 the season at which the young are born. The term "breeding season" will probably 

 convey to most people the season at which pairing takes place, but in any case it so 

 happens that the two fall very close together and certainly overlap to some extent. 

 In many cases, however, it will be better to speak separately of the "pairing season" 

 and the "season of parturition". 



The fact that whales actually have a breeding season has been clearly established by 

 various previous authors. It is pointed out by Hinton that the first attempt to deduce 

 the time of the breeding season from a study of the lengths of foetuses at different times 

 was made by Guldberg (1886), who found from an examination of foetuses from the 

 North Atlantic that there was at least such a thing as a definite pairing season. More 

 data were subsequently collected by Cocks (1886-90) and Collett (19 12) and the question 

 was re-examined by Hinton in 1925. Similar work has been done, as already explained, 

 by Harmer (unpublished) and Risting (1928), and the results of these investigators' 

 work are mainly in agreement. 



The position from which we now have to start is as follows. From a study of the 

 recorded lengths of a considerable number of foetuses of Blue and Fin whales, several 

 authors have arrived at the conclusion that (i) the pairing season is very protracted and 

 lasts as such over two or three months, while pairing may exceptionally take place at 

 almost any time of year, (2) the maximum amount of pairing takes place about July in 

 the case of Blue and Fin whales. 



That pairing is spread over a considerable period is obvious from the diversity in 

 the lengths of the foetuses which may be taken at any one time, but these authors do 

 not claim to have proved that, say, July is the month in which the maximum numbers of 

 Blue and Fin foetuses were conceived. It is merely stated that, as far as the available 

 data goes, July appears to be the most likely month, the doubt lying mainly in the rate 

 of linear growth during the earliest stages of development. Perhaps the only way in 

 which it is possible to ascertain the length of this preliminary period, apart from guess- 

 work, is to compare the mean curve of foetal growth with the dates at which the greatest 

 numbers of whales show signs of active breeding, such as the occurrence of minute 

 foetuses, heat and ovulation among females, and the increased activity of the testis 

 among males. 



It will be convenient to start with the more direct evidence concerning the time of 

 the breeding season and to go on later to the foetal growth curve. 



In the first place, some information on the female sexual season is to be had from the 

 ovarian follicles. The fact that many follicles are to be seen in ripening ovaries is a sign 

 that many ova may be shed, but the rule is that one ovum is shed at a time, for, in the 

 first place, one follicle appears always to be definitely larger than any others in the ovaries, 

 and, in the second place, twins and multiple births are not common occurrences. 



