MULTIPAROUS FEMALES 429 



POST-PARTUM HEAT 



It has not been possible to study post-partum females directly, and we are again restricted to drawing 

 inferences from material collected in the Antarctic. Events during lactation are difficult to establish, 

 because lactating whales are now protected and therefore appear in the catches each year only in 

 small numbers. The present material includes 129 lactating female fin whales examined at South 

 Georgia (in earlier years) and on the pelagic whaling grounds. 



Females simultaneously pregnant and lactating 

 For several Balaenopterid species there are records of females which are simultaneously pregnant and 

 lactating. This appears to be of most common occurrence in the minke whale. Jonsgard (1951) found 

 that in this species lactating females were invariably pregnant, and only 4-5 % of mature females were 

 neither pregnant nor lactating. Omura and Sakiura (1956) also concluded that the majority of minke 

 whales in Japanese waters become pregnant as a result of a post-partum ovulation. Chittleborough 

 (1958) gives data obtained by Norwegian workers on the incidence of pregnancy in lactating hump- 

 back whales and Symons and Weston (1958) give a few more records. In this combined sample there 

 are 23 lactating females of which nine, or 39% were simultaneously pregnant. Chittleborough also 

 points out that there are very few females in the resting condition. There appear to be no published 

 records of blue whales which were simultaneously pregnant and lactating, but there are in the records 

 of the National Institute of Oceanography references to two female blue whales which were pregnant 

 while lactating. This is evidently a very rare condition in the blue whale. 



As regards the fin whale Hinton (1925, p. 124) refers to four fin whales which were simultaneously 

 lactating and pregnant, and Mackintosh and Wheeler (1929) stated that, although there were a few 

 records of lactating females which were pregnant, none of the lactating whales examined by ' Discovery ' 

 Investigations at South Georgia was concurrently pregnant. They concluded that such cases are 

 extremely rare, but ' might arise if a female were impregnated near the end of a long period of lacta- 

 tion ' (p. 431). Wheeler (1930, p. 414) stated that seven out of 199 pregnant whales were also lactating, 

 and suggested that a post-partum ovulation may sometimes take place. In later seasons, as we shall 

 see, about one-fifth of lactating females examined at South Georgia were found to be simultaneously 

 pregnant, and Mackintosh (1942, p. 224) stated that there was some evidence that 'the occurrence of 

 whales simultaneously pregnant and lactating is also less rare than in former years '. Ruud (1945, p. 58) 

 remarked that 'nursing females with a foetus seem to be unknown', and Brinkmann (1948, p. 36) 

 gave details of a lactating fin whale with a 77-cm. foetus. He drew attention to the fact that no such 

 case had, so far as was known, been previously recorded. This appears to be the first detailed record 

 to be published, although at that time at least ten such females had been examined by 'Discovery' 

 Investigations (Table 19). 



There are in the present material 129 lactating female fin whales examined between 1925 and 1958. 

 Of these 15, or n-6% were simultaneously lactating and pregnant (Table 19). This material has been 

 subdivided into four groups, the first two representing the work of ' Discovery ' Investigations at 

 South Georgia and the next groups representing the work undertaken by ' Discovery ' Investigations 

 and the National Institute of Oceanography in pelagic expeditions. 



Some explanation of the meaning of this grouping is called for. At South Georgia up to the end of 

 the 1926/27 season, no lactating female had been recorded which was also known to be pregnant. 

 In January 1928 the first female which was simultaneously lactating and pregnant was recorded and 

 following this a much higher number of such females were found amounting to some 20% of all 

 lactating females examined. In the pelagic operations no females pregnant while lactating were found 



