288 DISCOVERY REPORTS 



assumed to be 4-15 m. then this part of gestation is again 15 months. For foetal weight data Huggett 

 and Widdas's (1951) arbitrary scale suggests that Wt^ is about o-o8 x (gestation time) for gestation 

 periods of 450-500 days and converting, Lto^o-oj x (gestation time). Then Lt^^ {0-07 x 457)/o-93 ^ 

 34 days and the total gestation period is 491 days or 16 months. This agrees well with previous esti- 

 mates (16-17 months) which have taken account of other evidence of the pairing and calving season, 

 and also the bimodal frequency distribution of foetal lengths. 



The steeper growth curve of the foetal sperm whale in the southern hemisphere, and also the larger 

 neonatal size of the southern animals, is in line with work on other cetaceans showing that southern 

 hemisphere adults are larger than those in the north (e.g. Jonsgard, 1952; Pike, 1953). Clarke (1956, 

 p. 289) has already suggested that both male and female sperm whales attain larger sizes in the 

 southern hemisphere. 



3 - 



a. 



t- 



2 



o 



z 



hi 



N 



M 



MJJASOND 

 ~ " ' MONTHS 



Text-fig. 3. Foetal growth in length of southern hemisphere sperm whale, Physeter catodon (points represent monthly means). 



For the porpoise, Mohl-Hansen (1954) gives data on the length of 119 foetuses from the Baltic. 

 Of the females judged to be sexually mature 84% were pregnant; there were not two foetal length 

 groups, and this together with other data gives striking evidence that the gestation period is under 

 a year. He estimates that pairing occurs in July and August and that the duration of pregnancy 

 'should be 11, or at least lo-ii, months, instead of 9-10 as usually assumed' (p. 389). The average 

 neonatal length is approximately 75 cm. These data are presented graphically in Text-fig. 4, in which 

 Mohl-Hansen's mean monthly values for November to February are shown; also three individual 

 length records from the Zoological Museum, Copenhagen, and three individual records from Holland 

 (Van Deinse, 1946). The Norwegian material for the porpoise (Grieg, 1898; Mohl-Hansen, 1954, 

 fig. 7) shows a higher growth rate than the material from Denmark and Holland; it is not used here. 

 The line fitted by inspection attains 75 cm. at the end of June and intersects the abscissa in the second 

 half of September giving an estimate of 9^ months (or 282 days) for {t„-Q. For gestation periods of a 

 little over 300 days, using Huggett and Widdas's estimate converted for length, Lfo-o-i5 x (gestation 

 time) and therefore L^q- (0-15 x 282)/o-85=^ 50 days. 



The total gestation period estimated in this way is therefore 332 days or nearly 11 months, which is 

 in close agreement with Mohl-Hansen's (1954) estimate. The peak time of conceptions is the beginning 



