446 DISCOVERY REPORTS 



southern hemisphere fin whale. This estimate was based on an apparent increase in the rate of growth 

 of the baleen plates at weaning, and on the sizes of the largest suckling calves and the smallest inde- 

 pendent calves. There is no additional information to give us cause to modify this estimate, although 

 it should be noted that Chittleborough (1958) found no such increase in the growth of baleen in the 

 humpback whale. By plotting the lengths of small fin whales against the time of capture Mackintosh 

 and Wheeler were able to extend their average curve of foetal growth to the lactation period, and 

 found that it attained the level of 12 m. in the first week in December. In the same way they estimated 

 that in the blue whale weaning occurs at an average length of 16 m., which in this species also is 

 attained in December. These authors, therefore, assumed that weaning occurs on average in Decem- 

 ber, and they pointed out that in each of these species there is a regular influx to South Georgia 

 waters, mainly from January onwards, of small whales, many of which have probably just recently 

 been weaned. 



A short paper by Ash (1956) also suggests that weaning occurs on average in December. He 

 expressed the increase in fatness of fin whales by plotting the blubber ratio (average blubber thickness 

 in cm. /length of whale in feet) against time. The rate of fattening is fairly constant during the 10-week 

 period covered by the data. Males and non-pregnant females are represented by curves which are 

 almost identical; the curve for pregnant females is well above the curve for non-pregnant females, 

 but is nearly parallel to it, and lactating females are seen to be very lean. The average blubber ratio 

 of the five lactating females given by Ash is 0-279. If we assume for the reasons given above that 

 lactating females are very near to weaning, then this figure is probably very close to the average 

 blubber ratio for non-pregnant females when they first enter the antarctic population (because the 

 non-pregnant group is largely, if not entirely, composed of post-lactation females). When the fatness 

 curve for non-pregnant females is extrapolated backwards in time to a value of 0-279, tms * s found to 

 correspond to the third week of December. This evidence again suggests December as the average 

 month of weaning. 



It will also be remembered that the cumulative curve of ovulations which is held to correspond to 

 the change from lactation to the 'resting' phase reaches the 50% level in December (p. 442). 



These several estimates all suggest December as the average month of weaning for the southern 

 hemisphere fin whale. However, when we examine the incidence of lactating females on the whaling 

 grounds, we find that rather small numbers of lactating females figure in the catches for the early part 

 of the season, and are at a maximum towards the end of the season. Thus, at South Georgia, between 

 1925 and 1931, out of 459 females examined closely, 68 or 14-8% were found to contain apparently 

 normal milk in the mammary glands, but when the monthly proportion of lactating females is examined 

 there are found to be about 5 % of mature females in October and November, rising to 44% in March 

 (Text-fig. 50). If weaning occurs on average in December, then most lactating females should be 

 taken in the early part of the season, and by March very few unweaned calves should be left. The fact 

 that the bulk of lactating females in the catches are taken late in the season strongly suggests that in 

 the earlier months lactating females do not enter the whaling grounds in representative numbers, but 

 in fact wean their calves before entering colder waters. 



Mackintosh and Wheeler (1929) also commented on the fact that the behaviour of lactating females 

 as a class differs from that of non-lactating females. They also suggest that for part of the nursing 

 period the lactating females are segregated from the main herds, so that not all lactating females appear 

 on the whaling grounds. These authors suggest that lactating females are slower on migration than 

 non-lactating females, and this is borne out by Chittleborough's observations on humpback whales 

 (^SS) P- 222 )- Mackintosh and Wheeler (1929) point out that the killing of females accompanied by 

 a calf is prohibited in the Falkland Islands Dependencies; according to the whalers young calves are 



