142 DISCOVERY REPORTS 



the proprietor bull (or less frequently bulls) which, it is said, jealously guards it, points 

 to the probability that few, if any, females escape impregnation as soon as they come 

 into oestrus. The indications of a dioestrous cycle of four ovulations show the possibility 

 that ovulation and pairing do not always result in pregnancy, a phenomenon known in 

 other animals, e.g. the hedgehog (Deansley, 1934). If that is the case it may also be 

 possible that oestrus is sometimes entirely unsuccessful and that the whale then ex- 

 periences a period of anoestrus. This is shown by the dotted line in the diagram of the 

 oestrous cycle in Fig. 45. 



The period of gestation has been shown to be about 16 months ; the period of lactation 

 is likely to be not less than about 6 months, judging by analogy from the balaenopterid 

 whales in which its length is known to be from 5 to 7 months in various species. If that 

 is so, pregnancy and lactation will together last at least 22 months. This will bring the 

 whale into its next pairing season at the end of lactation, a conclusion which is in agree- 

 ment with the facts found in the examination of the series of female whales. 



The records of commercial whaling have been examined to find if possible any in- 

 dication of a period of anoestrus. Those from Natal, which is the only whaling ground 

 where Sperm whales formed an important part of the catch from which data are 

 available, are given for 3 years in Table XII. These figures are, however, disappointing 

 when examined in this connexion, because the statistics are only roughly collected at 

 the whaling stations and only the obvious pregnancies are noted. Consequently, it is 

 impossible to state whether the female whales not noted as pregnant were lactating, 

 ovulating, resting, or in the earlier stages of pregnancy. The technique of dismembering 

 the whale carcass at the Natal whaling stations differs considerably from that at the 

 southern whaling stations, and consequently an examination of the abdominal contents 

 is difficult, and doubtless many pregnancies are overlooked. And it must further be 

 remembered that, surprising as it may seem, the whalers in general cannot tell a male 

 whale from a female, and that many a male which does not happen to have the penis 

 extruded when it is drawn up on the flensing plan is recorded as a female. However, the 

 very low numbers of pregnancies recorded may indicate the occurrence of a period of 

 anoestrus, the possibility of which is discussed above. To set against this is the fact that 

 the whalers at Durban say that after September practically every female Sperm whale 

 contains a foetus, a statement that is widely at variance with the figures supplied by them 

 to the British Museum and shown in Table XII. 



The sexual cycle of the female Sperm whale may, therefore, be summarized as in 

 Fig. 45, breeding occurring every other year, gestation lasting 16 months, lactation at 

 least 6, and anoestrus being either absent or of very short duration. The scheme of the 

 sexual cycle outlined here must be taken as the best that can be done until further evi- 

 dence is forthcoming, and it is put forward tentatively on that basis. 



