4 24 DISCOVERY REPORTS 



6o° S., and so on. If it is assumed that each migration takes longer than a month, then the changes in 

 day length also show two periods of increasing photo-period. Animals remaining at any one latitude 

 in the southern hemisphere are subject to increasing lengths of day from July to December, though 

 the rates of increase are in general much smaller than those experienced on migration. 



We may now turn to the question of when females approaching the age of puberty make their 

 northern migration. Mackintosh (1942) discussed the monthly frequency of immature females on 

 the feeding grounds. He shows that both at South Georgia and on the pelagic whaling grounds the 

 average length of the catches falls, and the proportion of immature females in the catches (calculated 

 from the mean length of puberty) increases in the later part of the summer season. Although this 

 change in the composition of the catches is more marked in blue whales than in fin whales, it suggests 

 that immature female fin whales arrive at and probably depart from the antarctic feeding grounds 

 later than the adult females. A similar conclusion was reached by Hjort, Lie and Ruud (1935) and 

 by Ottestad (1938). 



Mackintosh and Brown (1956, fig. 2, table 1) show that the population of large baleen whales (blue, 

 fin and humpback) in antarctic waters is at its peak in February, but there are still large numbers 

 present in March and April. Thus, expressed as whales observed per hundred miles steamed, there 

 were 9-9 in February, 7-2 in March and 3-6 in April. Allowing for the area of ice-free water in three 

 surface temperature zones the calculated population in April is rather more than half that present in 

 February. The sampling in May and June, when the estimated population south of the Antarctic 

 Convergence is higher, is considered by the authors to be unsatisfactory and unrepresentative of the 

 Antarctic as a whole. Nevertheless an appreciable number of whales are probably still present in these 

 months and the great majority of these are likely to be fin whales both because fin whale populations 

 are larger than the stocks of the other species and because the feeding season for blue and humpback 

 whales is in advance of the season for fin whales (Mackintosh, 1942). 



If we are correct in assuming that a large proportion of this rearguard are immature females, then 

 the main northward migration of fin whale females approaching maturity probably takes place from 

 April onwards. They will be subjected to maximal stimulus from increasing day length on migration 

 during July (Text-fig. 42). This is the peak month of pairing for those females which become pregnant 

 at the first ovulation (Text-fig. 37). Animals migrating northwards in May and June and in August 

 and September will also be subject to increasing day lengths although the rate of increase will be 

 smaller. It may be significant that the curve showing the frequency of ovulations in the ' o corpus 

 albicans ' group of primiparous females closely parallels the hypothetical curve showing the rate of 

 increase of day length with migration. The small percentage of ovulations in April (2-5%), which is 

 unexpected on theoretical grounds, is possibly attributable to the working of an inherent cycle and also 

 to the effect of individual variations in the rate of foetal growth on the estimated frequency of pairing 

 (see p. 403). The small percentage of successful ovulations in October and November may perhaps be 

 attributed to this effect, and partly perhaps to animals which experience increasing day lengths only 

 on arrival at the breeding zone (see next paragraph). Similarly 95 % of successful pairings at the second 

 ovulation are estimated to take place in the 5 months May-September. 



Animals migrating north before May will experience decreasing day lengths on migration. Then, if 

 they remain in any one latitude zone, they will continue to experience decreasing day lengths until 

 the end of June, when day length will increase again. Animals which migrate northwards after 

 September (which are unlikely to be more than a very small fraction of the population) will experience 

 decreasing day lengths on migration, and then increasing day lengths on arriving at say 20 S., the 

 rate of increase decreasing until December, after which they will again be subject to decreasing day 

 lengths. By far the greatest change in day lengths occurs during the migratory phase. 



