442 DISCOVERY REPORTS 



compared with the estimated cumulative frequencies of ovulations, the upper limit of the shaded 

 portion representing a corpus luteum life of 15 days and the lower representing a life of 20 days. It is 

 apparent that there is a reasonably good correlation between these curves, and a cumulative curve of 

 ovulations may be taken to represent the entry of 'resting' females into the antarctic population. 



The best agreement is obtained if the centre of the shaded area, corresponding to a corpus luteum 

 life of 17-18 days, is used. Up to January there is fairly close agreement, but after this the numbers 

 of non-pregnant adult females in South Georgia waters fall off. Probably very small numbers arrive 

 in the Antarctic after March. 



1000- 



> 

 u 



z 



Ul 



a 



ui 

 a. 



500 



D J 



MONTHS 



Text-fig. 49. Thick line, catch of non-pregnant females, South Georgia 1927-35, from Text-fig. 48. Shaded area represents 

 estimated cumulative frequency of ovulation for corpus luteum life of 15-20 days. See text for explanation. 



According to these calculations 50% of non-pregnant females are south of the Antarctic Convergence 

 by mid-December, and almost all have arrived by the end of March. As regards the true monthly 

 frequency of this summer ovulation, December may, therefore, be considered as the median month. 

 This is in reasonably close agreement with the cycle of ovarian activity in pregnant females, which 

 shows a peak in November/December (see p. 348, Text-fig. 4). Pregnant females enter antarctic 

 waters about a month in advance of non-pregnant females (Text-fig. 486), and it would seem that 

 in this class also ovarian activity is correlated with the southward migration. It will also be remembered 

 that a similar correlation was demonstrated for some nulliparous females at puberty. We will return 

 to this interesting point later (p. 450). 



The main purpose of this brief discussion has been to show that the estimated cumulative frequency 

 of ovulations corresponds fairly closely with the ascending limb of the curve showing the catches of 

 non-pregnant mature fin whale females at South Georgia, and therefore supports the hypothesis put 

 forward above concerning an end-of-lactation, or post-lactation, ovulation. The catch statistics show 

 that the peak in the pelagic catch of fin whales is usually a month later than the peak numbers at 

 South Georgia. This island ' occupies a peculiar position where Antarctic conditions extend into a 



