MULTIPAROUS FEMALES 439 



incidence of ovulations, and it is possible that a sigmoidal curve, or an irregular curve, as well as giving 

 a better fit, might be nearer to the truth. Calculations have, therefore, also been made using a sigmoid 

 curve to estimate the monthly incidence of ovulations, and also using the actual means of the samples. 

 It makes little difference to the following discussion which fit is adopted, and the straight line shown 

 in Text-fig. 47 is used below, because it probably gives the most likely value for September. 



It is supposed that the numbers of fin whales south of the Antarctic Convergence are at a minimum 

 in July and August and then build up to a maximum in say February (see Mackintosh, 1942, p. 270; 

 Mackintosh and Brown, 1956, fig. 2; and evidence from the catch statistics). The numbers of non- 

 pregnant, non-lactating females in the Antarctic will follow a similar pattern though later in time 

 (Text-fig. 48 b). According to the hypothesis under consideration, these females ovulate either shortly 

 before or soon after their entry into the Antarctic population. A small proportion of the fin whale 

 population probably overwinters in antarctic waters (Hart, 1935, p. 276; Mackintosh, 1942, p. 250), 

 and it follows that apart from these, all non-pregnant, non-lactating females present in the antarctic 



z> 



5 



111 



o 



:z 



LU 



U 



£T 

 UJ 



a. 



SONDJ FMAM 



MONTHS 



Text-fig. 47. Monthly percentages ( ± 2 s.E.) of recent ovulations in antarctic waters. 



in September should have ovulated recently. In the following months there will be an increasing 

 influx of new arrivals which are expected to ovulate just before, on arrival, or soon after arrival, but 

 in successive months there will also be an increasing accumulation of females which have been in 

 antarctic waters for some time, and have ovulated some time before. It is, therefore, to be expected 

 that the monthly proportion of recent ovulations would decline as the number of truly 'resting' 

 females in the population increases, and this is in fact what the actual figures show. 



It has been pointed out that in September all but the small proportion of non-pregnant females 

 which have remained south of the Antarctic Convergence during the winter months, or have migrated 

 into the Antarctic before September, should have recently ovulated. In fact Text-fig. 47 shows less 

 than half of these females to have a corpus luteum of ovulation. So far we have not considered the 

 length of life of the active corpus luteum of the cycle, but this is an important factor which will affect 

 the figures for the incidence of ovulations. Thus, if the corpus luteum persists in a recognizably active 

 form for a month, the figures given in Table 22 and Text-fig. 47 will represent the monthly frequency 

 of ovulation, but if, for example, the corpus luteum persists for only 2 weeks after entry into the whaling 

 grounds, then the observed frequencies of corpora lutea of ovulation must be doubled to give the true 



