428 DISCOVERY REPORTS 



THE SEXUAL CYCLE AND GROWTH OF THE CALF 



The length of the period of gestation can be found from the curve of growth of the 

 foetus, provided the length of the calf at birth is known, a point which can be ascertained 

 with a fair degree of accuracy. One way of finding the average length at which the calf 

 is born is to compare the largest foetuses with the smallest calves which have been 

 recorded. In the course of our work two very large foetuses were found. One was that 

 of a Blue whale (No. 154) and measured 6-3 m. (20-6 ft.), the other was of a Fin whale 

 (No. 173) and measured 6^05 m. (19-85 ft.). In both cases, but especially in the latter, 

 the condition of the mother suggested that parturition would have shortly taken place. 

 Before No. 173 was flensed, it was noticed that the genital region was greatly swollen and 

 that the genital groove was stretched open to a remarkable extent. A sample of the 

 mammary gland was preserved and sectioned and it was found that the condition of the 

 gland suggested that secretion would shortly commence. The external genitalia of 

 No. 154 were in a similar condition but sections were not made of the mammary glands. 



These cases, so far as they go, suggest that in Blue whales the calf is born when rather 

 more than 6-3 m. long and in Fin whales when a little over 6-o m. Haldane (1905) records 

 Blue whale calves of 6-7 m. (22 ft.) and 6-i m. (20 ft.). Harmer mentions a Fin whale 

 foetus of 6-4 m. (21 ft.) and a Blue whale foetus of 8-8 m. (29 ft.), but these are quoted 

 from statistics supplied by the whalers and their precise accuracy cannot be regarded 

 as very reliable. Records of the smallest calves are of course very hard to come by, for 

 very small ones are rarely seen and hardly worth shooting. There is also a regulation in 

 the Dependencies of the Falkland Islands prohibiting the whalers from attacking females 

 accompanied by calves. However, Hinton (1925) mentions records of three Blue whale 

 foetuses from 5-3 m. to 7-0 m., and the statistics supplied by Messrs Irvin and Johnson, 

 whose whaling station is at Saldanha Bay, include a record of a Blue whale calf of 7-7 m. 



(25-3 ft-)- 



On the whole it seems probable that Blue whales are born on the average at about 

 7-0 m. (23-0 ft.) and Fin whales at about 6-5 m. (21-3 ft.). It is at least certain that the 

 length at birth is not far from this. No doubt some variation occurs. It has been sug- 

 gested, for instance, that the length of the calf at birth bears a definite ratio to the size 

 of the mother, but this is a statement which it would be extremely difficult if not 

 impossible to test. In any case if birth may take place at lengths other than 7-0 m. and 

 6-5 m. in the two species the diff'erence would not materially affect the estimation of the 

 length of the period of gestation especially as the rate of growth is fastest at the end of 

 gestation. 



By reference to Figs, 145 and 146 it is seen that the curve for Blue whales ends at 

 7-0 m., representing the close of the period of gestation. The point reached at the 7-0 m. 

 level is opposite the beginning of May. In Fin whales the curve reaches 6-5 m. 

 opposite the middle of June. This gives a period of gestation of slightly over ten 

 months in the case of Blue whales, and of eleven and a half months in the case of 

 Fin whales. 



