27 o DISCOVERY REPORTS 



in the month of September in Bermuda. Except for Wheeler's record, new-born calves of any species 

 of the great whales seem not to have been available previously, and estimates of neonatal length 

 have been approximations based on the length of the smallest calf and the largest foetus examined. For 

 sperm whales such estimates have been close to the present, more exact, information. Matthews (1938, 

 p. 138) gave '4 m. or a little more ' in southern whales ; Bennett (1836, p. 129; 1840, 11, p. 167) measured 

 a full-time foetus which was' 14 feet in length and 6 in girth', a record which Melville (185 1, p. 397) 

 and Davis (1874, p. 185) seem to have used in their estimates; Matsuura (1836) and Mizue & Jimbo 

 (1950, p. 127) gave 14 to 15 ft. for neonates in the North Pacific. 



Turning again to Fig. 6 it is seen that the mean curve of foetal growth shows a gestation period of 

 sixteen months. A little uncertainty about the lowest part of the curve (the beginning of pregnancy) 

 has already been noticed, but a sixteen months period agrees precisely with Matthews' findings (1938, 

 p. 142) and is very close to the seventeen months gestation period found by Mizue & Jimbo (1950, 

 p. 128). There is no doubt that Harmer (1933, p. 410) and Matsuura (1935, 1936) under-estimated 

 when they suggested that pregnancy lasted about 12 months or a little more. 



Months 

 J un 1 J u ly 



of birth 

 Aug 1 Sep 



Oct 



v o 



E Dec 'Jan 'Feb 'Mar 'Apr 'May ' Jun 'July' Aug 



J! Months of pairing 



Fig. 7. Pairing and birth frequencies from foetuses examined at Horta between 1949 and 1954. 



The extent of the pairing or sexual season has been determined by drawing lines in Fig. 6 parallel 

 to the mean growth curve but taking their origin at monthly intervals from it ; the number of preg- 

 nancies within the lines defining each interval was then counted, being the number of pairings for the 

 month of origin of the interval. Plotting the number of pairings against months gave the frequency 

 of pairing (Fig. 7). The pairing season would seem to be protracted, lasting for seven months between 

 January and July, although most activity is contained in the three months between March and May. 

 Assuming there is no substantial variation in the rate of foetal growth, the same curve plotted sixteen 

 months later represents the birth frequency, which is also shown in Fig. 7. The seven months season 

 of parturition lasts from May to November, but is mostly concentrated between July and September. 

 Matthews found that in southern sperm whales the season also lasts seven months, although it is 

 mostly contained in five months of the southern summer and autumn. Comparing the months, north 

 and south, it is clear that the peaks of calving (and, of course, of pairing) are in the same season of the 

 year in each hemisphere but of course in opposite months, a point which will be discussed later 

 (pp. 288-9). Bennett (1840, II, p. 178) observed sperm whales copulating in the South Seas in 

 August, a month which accords with Matthews' findings. Mizue & Jimbo's results in the North 

 Pacific give a season of only 2\ months, although they put the peaks of pairing and of parturition in 

 March and August respectively. 



