Discovery reports (1929) Discovery reports discoveryreports01257540inst Year: 1929 BREEDING AND GROWTH 429 The time of the season of parturition may be worked out in the same way as the pairing season. If we continue on the assumption that all the foetuses of a species grow at the same rate the season of parturition will of course be identical in duration and intensity with the pairing season. All that needs to be done is to reproduce the pairing curves (Fig. 147) ten months later in the case of Blue whales and eleven and a half months later in the case of Fin whales. The result is shown in
Discovery reports (1929) Discovery reports discoveryreports01257540inst Year: 1929 BREEDING AND GROWTH 429 The time of the season of parturition may be worked out in the same way as the pairing season. If we continue on the assumption that all the foetuses of a species grow at the same rate the season of parturition will of course be identical in duration and intensity with the pairing season. All that needs to be done is to reproduce the pairing curves (Fig. 147) ten months later in the case of Blue whales and eleven and a half months later in the case of Fin whales. The result is shown in Fig. 148. The details of the shapes of these curves of course mean nothing, but they suggest in general that Blue whales are mostly born in April and May and Fin whales in June and July. APR. ' MAY â JUNE ' JULY ' AUG. ' SEPT. ''' OCT. ' NOV. Fig. 148. Curve of frequency of births. Blue whales. Fin whales. As explained above it is being assumed that the foetuses grow at the same speed during gestation. If, however, some foetuses grow faster than others the result would be that the season of parturition might be more protracted than the pairing season. On the other hand it is possible that a whale which was impregnated early might retain the foetus slightly longer than one which was impregnated later, since birth apparently takes place mostly when the parent migrates northwards to warmer waters, and the attainment of the proper environment might have the effect of stimulating slightly premature parturition in a whale in which the foetus was later than the average. There is still another assumption involved in the construction of the foetal growth curves, and that is that the earliness or lateness of a pregnant whale's visit to the neighbourhood of South Georgia is not affected by the age of the foetus, the time at which impregnation took place. But it seems quite possible that a female which had not been impregnated before the later part of the pairing season might dela
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