THE HUMPBACK WHALE 65 



to set against this there is the fact that none of the adult males examined, admittedly few, 

 showed any signs of sexual activity in December, January or March at South Georgia, 

 while those examined in South Africa in July and August did. 



The curves produced from the large collection of foetal lengths must be relied upon, 

 rather than the information relating to the small series of males, and the conclusion 

 reached from a consideration of these is that the majority of female Humpback whales 

 breed once every two years, with a period of anoestrus between lactation and the fol- 

 lowing pregnancy, but that a minority go into oestrus and are impregnated after lacta- 

 tion during the months when the majority are in anoestrus. This conclusion is further 

 supported by a consideration of Figs. 55 and 56, which show a small proportion of pair- 

 ings and births in all months apart from the main seasons. Some of these may be cows 

 that have lost their calves after birth and have come into oestrus again. The occurrence 

 of the minority breeding twice every three years has probably given rise to the belief, 

 widespread amongst whalers and others, that all Humpbacks breed every year. Risting 

 (1928) mentions a female Humpback 46 ft. long that was brought into Hangklip Whaling 

 Station (Cape Province) on 15 November 1913, and was accompanied by a calf 29 ft. 

 long, and contained a foetus 5 in. long. This is the only case of which he is aware which 

 indicates with certainty that Humpbacks can breed in successive years. 



Further, Ommanney (1933) states that he was informed that off the Bay of Islands, 

 New Zealand, many of the females on their southward migration are accompanied by 

 newly born sucking calves about 15 ft. long and are also carrying foetuses about 2 m. in 

 length. In addition Hinton (1925) records from South Georgia in the season 1913-14 

 one pregnant female that was nursing and three pregnant females in which some milk 

 was present in the mammary glands. 



Olsen (1914-15) also records two females taken at Saldanha Bay in October 191 1 

 which contained foetuses about 70 mm. long and were in full lactation, from which 

 he rather hastily concludes that Humpbacks produce a young one every year. He 

 further states that the females are exhausted by parturition and remain quietly on 

 their breeding grounds for about a month before pairing and starting the southern 

 migration. He thus gives a period of about four weeks between parturition and pairing. 

 This is probably far from correct, for as Hinton (1925) states "no female [Humpback 

 whale] normally experiences oestrus during the essential part of the sucking period ; on 

 the approach of the time for weaning the calf oestrus may happen and an ovum be shed : 

 and if impregnation takes place at this time the calf is quickly weaned. There is, so far as 

 I know, no evidence at all of pregnancy during the early stages of suckling." 



An examination of the statistics from whaling stations shows that a period of an- 

 oestrus normally occurs in many female Humpbacks. Table XIX shows the numbers of 

 male, female, and pregnant female humpback whales taken at Natal in the years 1924-8. 

 Even allowing for hasty examination and the overlooking of many of the smaller foetuses 

 the very low proportion of pregnancies recorded points towards the likelihood of many 

 of the females being in anoestrus. This view is further strengthened by the data given 

 on pp. 60-63, in which the records of a careful examination of female Humpback whales 



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