434 DISCOVERY REPORTS 



by the whaling companies the number of whales recorded is so great that it is possible 

 to glean a moderate number of measurements of whales sufficiently small to be regarded 

 as unweaned calves. These have been plotted according to their size and date in Figs. 

 149 and 150, and are taken from the statistics of the whaling stations at South Georgia 

 and of Messrs Irvin and Johnson's station at Saldanha Bay, Cape Colony. Records from 

 the South Shetlands and South Orkneys have been excluded as unreliable owing to 

 the difficulty of measuring whales from a floating factory. 



For the estimation of the rate of growth during lactation Blue whales may be taken 

 first, since for this species the material gives rather more definite results. It will be 

 seen in Fig. 149 that the plotted points representing calves are much more scattered 

 than those representing foetuses, but that the young calves are inclined to occur 

 early in the southern winter while the large ones are massed in the late winter and 

 early summer (i.e. about October, November and December). There is in fact an 

 unmistakable path of points sloping upwards through the southern winter, and the 

 mean curve of growth is drawn to represent as closely as possible the line of this 

 path. 



Two dotted curves are drawn parallel to the mean curve of growth and set on either 

 side of it at intervals of two months. Over the gestation period these curves are accurate 

 reproductions of the mean curve and represent the growth of foetuses conceived two 

 months before or two months after the height of the breeding season. They are similar 

 during the nursing period, but are drawn to diverge slightly to allow for diff'erential 

 rates of growth. It will be seen that these two curves enclose not only practically every 

 foetus but also the vast majority of the calves. In other words, although they appear 

 rather scattered and irregular, it may be supposed that the majority of recorded calves 

 were born within four months of one another. Various factors might explain the points 

 which fall outside the dotted curves, among which are exceptional pairings outside the 

 breeding season, exceptionally rapid or slow growth after birth, and faulty measurement 

 or recording by those supplying the statistics from which the plotted calves are derived. 



To return to the mean growth curve it is seen that this reaches the 16 m. level 

 opposite the earlier part of December. It may be estimated therefore that the nursing 

 period lasts on the average from May to December, i.e. about seven months. Growth 

 during this period thus appears to be very rapid and equal (in linear increase in size) 

 to the rate at the end of gestation. 



One might expect that the occurrence of lactating females, especially at South Georgia, 

 might help to throw some light on the normal time at which the nursing period closes, 

 but little help is to be found in this direction owing to the irregularity of the appearance 

 of these whales. In the 1925-6 season at South Georgia there was a comparatively large 

 number early in the season and few in the second half, a state of affairs which appears 

 to tally well with the foregoing conclusions as to the nursing period. On the other hand, 

 in the 1926-7 season, the lactating Blue whales predominated towards the end. It 

 must of course be remembered that December would be only the average of a large 

 number of times at which weaning might take place and that the process of weaning 



