45Q DISCOVERY REPORTS 



occurrence of lactating females in the catches. The temperature curve for South Georgia has been 

 constructed from information given by Mackintosh (1946) ; the average latitude of the pelagic whaling 

 grounds is assumed to be about 62 S. (see Mackintosh, 1942, fig. 2), and the monthly sea surface 

 temperatures for this latitude are taken from Mackintosh (1946, fig. 11). 



Comparison of these sea temperature curves with the curves showing the relative monthly frequencies 

 of lactating females in the catches strongly suggests that the movements of females with suckling 

 calves are closely dependent on environmental temperatures. It appears that the influx of lactating 

 females to the whaling grounds may begin when sea surface temperatures rise above about o° C, as 

 suggested by the broken vertical lines in Text-fig. 5 1 . This apparently occurs about 5-6 weeks earlier 

 in South Georgia waters than on the pelagic whaling grounds. The figures suggest that over 50% of 

 females enter South Georgia waters after they have weaned their calves, mostly before January, and 

 that probably over 80% wean their calves before they reach the pelagic whaling grounds further south. 

 It was concluded above, that the majority of those lactating females which do penetrate to the whaling 

 grounds later in the season are either very near to the time of weaning, are in process of weaning, or 

 have just weaned the calf. 



It is not possible to demonstrate a significant difference in the percentages of lactating females in 

 the pelagic sample at different latitudes, because of the variation in the position of the isotherms in 

 different sectors of the Antarctic. Thus, south of the Atlantic Ocean in January the o° C. isotherm 

 lies at about 58 S., but south of the Indian Ocean this isotherm is at about 65° S. in January. If the 

 material is subdivided into sectors then the samples are rather too small for statistical analysis to yield 

 significant results. 



The conclusions reached in this section are of some importance in connexion with the occurrence 

 of the post-lactation ovulation, because if for the majority of females weaning of the calf and ovulation 

 occur north of the whaling grounds, then the chances of finding very early post-ovulatory stages in 

 the development of the corpus luteum are greatly reduced (see p. 443). They are also important in 

 relation to the supposed danger to the stock of killing lactating females. If the majority of lactating 

 females do not enter the whaling grounds until the calf is weaned, and if those which do are very 

 close to the time of weaning, then the effect on the stock of killing lactating females will not be so 

 drastic as was previously thought. 



The Sexual cycle and its relation to the migratory cycle 

 In an earlier paper, on foetal growth in whales, it was suggested that the explanation of the con- 

 spicuous differences in the gestation periods and curves of foetal growth of baleen and toothed whales, 

 was to be found in the discontinuous feeding and the migratory cycle of baleen whales (Laws, 1959 a, 



PP- 304-5)- 



In a preceding section of this paper (p. 421) it was also shown that the sexual cycle of newly mature 

 females was rather closely related to the migratory cycle, and a possible causative and regulatory 

 factor was suggested. The further evidence which has now been presented shows that a similar 

 correlation is found in multiparous females. This may now be discussed in detail. 



The main pairing season for multiparous females is found to extend from April to July, as compared 

 with the period May-September for primiparous females, and the median dates of conception are 

 respectively 8 June and 21 July. Another point of difference is that the frequency curve of multiparous 

 conceptions is skewed, with a long ' tail ' from July to December, whereas the primiparous curve is 

 more symmetrical. 



If we accept the hypothesis that the sexual cycle in the fin whale is regulated by the change in day 

 lengths related to and caused by the long migrations between low and high latitudes (p. 423), the 



