BREEDING, GROWTH AND AGE 223 



breeding takes place in warmer waters, females accompanied by calves are much later 

 in their southward migration than other whales, and do not reach the southern whaling 

 grounds until the later part of the season. The fact that there is a substantial proportion 

 of lactating whales as late as February and March, when the majority of births are 

 believed to take place in April and May, might seem to imply that the suckling of the 

 calf may last longer than the seven months roughly estimated by Mackintosh and 

 Wheeler. The breeding season, however, is very protracted, and although it is possible 

 that seven months is not a very accurate estimate of the period of lactation, there may 

 well be sufficient late births to account for these late instances of lactation. Mothers who 

 give birth at or before the normal time may, as Laurie suggests, find the southern waters 

 too cold for their calves early in the season, or they may travel more slowly, so that 

 lactation may be over by the time they reach the southern grounds. We have in fact little 

 means of knowing what is the true proportion of lactating whales in the stock in the 

 early summer months. Laurie's fig. 7 shows an increase in the percentage of resting 

 whales from November onwards, and this is in accordance with expectation if there is a 

 two-year breeding cycle, the supposition being that the summer stock in high latitudes, 

 consisting at first of a high proportion of pregnant whales, becomes progressively diluted 

 or displaced by whales which have recently finished lactation. Laurie points out that 

 there is still a by no means negligible proportion of resting whales at the beginning of the 

 season, and that there is reason to believe that these have stayed south all the winter. 

 They would thus have missed a breeding season and would next become pregnant three 

 years (instead of two) after their last pregnancy. This may well be correct, but I do not 

 think it necessarily suggests that the normal interval between pregnancies is more than 

 two years. Table 27 (p. 277) shows such high percentages of pregnant females on the 

 pelagic whaling grounds, which it must be supposed harbour the bulk of the adult stock 

 in the summer months, that even allowing for the absence of many non-breeding females 

 in the early part of the season, it is difficult to believe that the interval between preg- 

 nancies is often more than two years. It should be possible to examine the evidence 

 more fully when the data on ovaries have been properly analysed, but in the meantime 

 it is necessary to point out that Table 27 is in certain respects misleading. A preliminary 

 examination of the data indicates that the percentage of adult females which are pregnant 

 has been increasing in a remarkable degree year by year, as if the actual rate of breeding 

 were becoming faster. This curious phenomenon might alone be sufficient to account for 

 the fact that in Table 27 there is a higher percentage of pregnant females in the pelagic 

 catches in the period 1932-41 than at South Georgia in 1925-31. It could also account 

 for the higher percentage of pregnant Fin than pregnant Blue whales in the pelagic 

 catches, for the Fin whale data are nearly all from the two most recent seasons. At South 

 Georgia, however, the percentage of pregnant Fin whales does seem in each year to be 

 higher than the percentage of pregnant Blue whales. The additional data now available 

 does not seem to suggest any revision of the original inference that most females become 

 pregnant every two years, but it is evident that the whole question is more complex and 

 difficult than at first appeared. No doubt the interval varies, and if the rate of breeding 



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