240 DISCOVERY REPORTS 



The figures for South Georgia have been added to since the publication of Mackintosh 

 and Wheeler's report. The number of pregnant, lactating, and resting whales found 

 during the whole of the investigations at South Georgia and Saldanha Bay from 1925 to 

 1 93 1 are now available. From the mass of this material only those whales are shown in 

 the preceding table whose condition was definitely ascertained. The months are arranged 

 to begin from the mean birth time of Blue whales (May). 



The sequel of events in the cycle postulated that whales lactate first and recuperate 

 afterwards. In a two-year cycle, nearly the whole of the first year being taken up with 

 gestation, the second year would be divided into a lactation and a recuperation period. 

 While it is not desired to emphasize the numerical importance of the table, it is interesting 

 to note that from October to March, when most of the observations were made, the 

 lactating and resting whales are fairly evenly distributed in relation to each other. In 

 order to make a direct comparison with the suggested seven months' nursing period 

 which should be at its maximum from May to the end of November, the percentages of 

 lactating and resting whales in the catch have been computed for the two periods. 

 During May to November 22 per cent were lactating and 28 per cent resting ; during 

 October to March 22 per cent were lactating and 27 per cent resting. 



The average date of capture of lactating whales in the above table is February 15, 

 with the highest incidence in a single month in March. This would at first sight imply 

 that lactation continues substantially longer than seven months, i.e. to the end of 

 November, as was previously suggested. It is possible that the original estimate of the 

 lactation period may be revised in the light of these figures, but there are several reasons 

 why the figures may be misleading. Some mothers who gave birth at the normal time 

 (May) may find the southern waters too cold for their calves early in the season, or they 

 may travel more slowly than other adult females. In either case they would arrive at 

 the whaling grounds towards, or even after, the end of lactation. On the other hand, 

 those mothers whose calves were born late might find the water warm enough for their 

 young soon after birth, and would arrive to swell the numbers of lactating whales taken 

 late in the season. 



What is less likely to be misleading is the fact that a significant proportion of the catch 

 were resting in the six months following the mean time of birth. It is difficult to believe 

 that so many whales had given birth so early in the season that they had finished lactating 

 by November, in which month thirteen resting whales out of a total of fifty-nine were 

 taken, or as early as October, when three resting whales out of thirteen were found. One 

 possible explanation is that some Blue whales take practically one year in gestation, and 

 an unknown period, possibly more than seven months, in lactation, and by reason of 

 their low condition after lactation are not able to resume breeding in the second winter 

 after the previous pregnancy so that they occur in the catch the following summer as 

 resting whales. The breeding cycle would then sometimes take three years, but the 

 validity of this theory is discussed below (p. 243). 



It is interesting to note that Hart (1935, p. 278) has shown that there are whales which 

 appear to have stayed in the cold region all the winter and not migrated with the majority 



