THE HUMPBACK WHALE 



59 



South Georgia. The latter whale, lactating in March, shows that the breeding season in 

 this species is, like that in other whales, considerably prolonged, so that all females are 

 not in the same stage of pregnancy at the same time. In other words the course of 

 pregnancy is not to be indicated in Fig. 53 by the dotted line but by a wide area lying 

 on each side of it. 



The period of lactation is about five months ; this value is derived partly from facts 

 which have already been given and partly from facts discussed later on. Fig. 42 



Fig. 54. Humpback whale. Average monthly foetal length. 



(baleen lengths) shows that weaning takes place at a length of 7-5-8 m. Fig. 61 shows 

 that the calf reaches this length at an age of about five months after birth, which period 

 is, therefore, the duration of lactation. 



If, in Fig. 53, lines are drawn parallel to the average line but taking their origin at 

 monthly intervals on each side of it, the points are divided up into a number of groups 

 representing pregnancies starting in each month. When these pregnancies are counted 

 and plotted by months the curve in Fig. 55 is obtained, which shows the frequencies of 

 pairing by months. This indicates that the breeding season covers the months of August 

 to November inclusive, with its highest intensity in September, and that a small amount 

 of breeding occurs throughout the year. If the curve is replotted with the time scale 

 shifted eleven months later, the curve showing the frequency of births in Fig. 56 is 

 obtained, which shows that the majority of births occur in August. It should be noted 

 that some, but not all, of the minority of pregnancies starting out of the regular breeding 

 season may be those of whales, belonging to the schools of the northern hemisphere. 



