BREEDING AND GROWTH 43i 



with milk in the glands and Hinton mentions one or two cases recorded in the north 

 among Blue whales. Risting also refers to such cases among Humpbacks. We have 

 not examined a sufficient number of Humpbacks to express an opinion on that species, 

 but so far as Blue and Fin whales are concerned it must be supposed that such cases 

 are extremely rare, but might arise if a female were impregnated near the end of a long 

 period of lactation. 



The percentage of Fin whales pregnant (46 per cent) strongly suggests that pregnancy 

 recurs every two years. The percentage of Blue whales pregnant is lower (31 per cent) 

 but is still rather high for pregnancy every three years and one would not expect Blue 

 and Fin whales to differ in this respect. Taking everything into consideration, including 

 the uncertainty as to whether one is dealing with a representative sample of the general 

 stock of whales, it may be said that although the possibility of pregnancy every three 

 years is not finally ruled out, it may be regarded as almost certain that it recurs in the 

 majority of cases every two years. It is probable that an interval of three j^ears may 

 occasionally elapse between pregnancies, but apart from considerations of the ratio of 

 pregnant whales we have firstly the fact that gestation lasts for nearly a year, and 

 secondly, as will be shown below, the nursing period lasts until after the next pairing 

 is over. Thus it is naturally at the second pairing season after gestation that the next 

 impregnation may be expected to take place. 



We now come to the nursing period. This does not necessarily correspond to the 

 whole period during which the mother is accompanied by the calf or to the whole 

 period during which the mammary glands of the parent are in functional activity, but 

 for our immediate purpose it is required to find the length of the period from birth 

 until weaning. To find the length of the period of gestation it was necessary to ascertain 

 the size of the calf at birth and the rate of growth from conception until parturition. 

 In the same way it is now required to find the length of the young whale at weaning 

 and the rate of growth of the calf during the nursing period. 



The period of lactation is more difficult to determine than the period of gestation, 

 but a few records are available which may be plotted out in much the same way 

 as the foetuses. In the first place, the average length at which weaning takes place 

 may be found (in the same way as the length at birth) from records of the largest sucking 

 calves and the smallest young whales which are feeding independently. 



Among Blue whales examined by us there appear to be only two which were 

 being fed by the mother. These were both caught at Saldanha Bay in September. 

 The first. No. 1064, measured 13-35 n"*-' ^nd had very poorly developed baleen plates, 

 of which the longest were only 18 cm. A curve showing the rate of growth of the baleen 

 is given in Fig. 49 on p. 314. This curve shows a sudden increase in the rate of growth 

 after the calf has reached a length of i6-o m., and it has already been suggested that 

 this increase is connected with a change from a diet of milk to one of krill, and comes 

 considerably after the longest plate has reached a length of 18 cm. Whale No. 1064 

 was taken two days after a lactating whale (No. 1057) by the same boat and at the same 

 spot, and was considered by the whalers to be the calf of that whale. The stomach 



