THE REPRODUCTIVE CYCLE 401 



The follicular cycle in females 



The cycle of follicular activity in female fin whales has been discussed in an earlier section of this 



paper. In adult females there are usually large numbers of resting or regressing follicles, often of 



large size, and this makes any attempt to demonstrate a cycle of follicular activity difficult owing to the 



limited period for which data are available. 



This difficulty can be overcome by considering only the size of the largest follicle in fin whale 



females approaching puberty. In the present material there are 62 pairs of ovaries from immature 



females at this stage of the life-cycle, and the results obtained were described on pp. 346-47, Table 2, 



and Text-fig. 3. It was concluded that immature females ovulate for the first time between June 



and November (possibly also December) and that from January to May the ovarian activity is at a 



minimum. It will be shown (p. 411) that the annual cycle of immature females is retarded relative 



to that of mature females and this evidence, therefore, supports the inferences about the season of 



pairing drawn from evidence from the male cycle and from a consideration of foetal lengths (see 



below). 



Pregnancy and foetal growth 



If the rate of foetal growth is known with reasonable accuracy then it should be possible to obtain an 

 estimate of the frequency distribution of pairings and conceptions by examining the foetal length 

 records, as was done by Mackintosh and Wheeler (1929, pp. 426-7) for blue and fin whales. However, 

 small errors in the estimated curve of foetal growth can lead to quite large errors in the calculation 

 of the frequency distribution of conceptions. For this reason it is necessary to obtain an average curve 

 of foetal growth which is reasonably accurate, before going on to use this to determine the monthly 

 frequency of pairing. It is also necessary to show to what extent the length frequencies of foetuses 

 in the sample examined may be taken to be representative of the foetal length frequencies in the 

 population sampled, and the extent of the differences, if any. 



In a recent paper (1959 a) I reviewed earlier work on the problem and went on to examine the rates 

 of foetal growth in three Odontocete species and five Balaenopterids. The material for the fin whale 

 included length records of 956 foetuses from South Africa (5), South Georgia (268) and from the 

 pelagic whaling grounds (683). I found that the method developed by Huggett and Widdas (195 1) 

 for dealing with foetal length and weight data appears to hold good for the Odontocete species 

 studied and to a limited extent for the Balaenopterid species. In particular this method provides 

 for an objective estimate of the length of the initial period of very slow growth (before the placental 

 circulation is fully established?) which is impossible by means of freehand extrapolation. In the 

 Balaenopterid species growth after this initial period is linear as in the Odontocetes, but unlike this 

 group, the phase of linear growth appears to be superseded by a phase of exponential growth in the 

 second half of pregnancy. My (1959 a) conclusions about foetal growth in length in the fin whale are 

 presented and summarized in Table 10 and Text-fig. 30, and the original paper should be consulted 

 for further details. The average duration of gestation in this species is estimated to be n J months 

 (early June to mid-May). This evidence of the mean date of pairing agrees very well with the evidence 

 from the male sexual cycle and reasonably well with the ovarian cycle in females approaching puberty. 

 It is shown below (p. 41 1) that the beginning of pregnancy in primiparous females is later than the 

 average date of conception in multiparous females. 



The explanation of the relatively large variation in the foetal lengths in the monthly samples is 

 undoubtedly that the pairing season both for primiparous females and multiparous females extends 

 over several months, as will be shown below. 



The frequency distributions of foetal lengths in the monthly samples were examined in the above 



