276 DISCOVERY REPORTS 



years after conception; so anoestrum lasts about seven months. The proportion of resting whales 

 in the catches (Table 24) must therefore be very much higher than in the actual population. There may 

 be a climacteric in female sperm whales, in which case some proportion of the present sample would 

 actually have been barren and not resting, but allowance for such post-climacteric animals would be 

 unlikely to reduce the proportion in the catch of resting whales from 48% to the figure of about 12% 

 required by the cycle now put forward. It is however possible that the pregnant and nursing whales are 

 segregated to some extent, being sequestered at such a distance from the coast as not to be fully re- 

 presented in the catches (p. 280). The sexual cycle does not invariably include anoestrum, because 

 there are three reports from Horta of females pregnant whilst still lactating. One is in the whaling 

 returns for 1948; the second refers to a whale examined in 1951 (Table 24, footnote), and Senhor 

 Jacinto Silviera de Medeiros has told me of a third. 



. Weaninq 



■ 67 m- 



-5 



■4 



3 



2 



I 



1 """l 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 I I f" 



JAN.FEBMAR.APRMAY JUN.JUL. AUGSEP.OCT-NOVDECJAN FEB MAR APR.MAY JUN.JUL AUG.SEP-OCT NOV DEC JAN. FEB MAR APR.MAY.JUN JUL.AUG.SEP OCT NOV 



Fig. 10. Sperm whales in the North Atlantic. Growth curve from conception to weaning. 



The present study concludes that the sexual cycle of the female sperm whale normally lasts three 

 years. Matsuura (1936) estimated a two-year cycle, but he believed that gestation lasted only one year. 

 Matthews (1938, p. 142) tentatively suggested a cycle of nearly two years; but he assumed a lactation 

 period of at least six months by analogy with the condition in whalebone whales, and he considered 

 anoestrum to be absent or of very short duration. There was only one resting whale among the fourteen 

 adult females in Matthews' material, but they were all examined between July and September, that 

 is, during the first half of the southern sexual season, when one would not expect to encounter 

 numbers of resting whales. 



Growth and age 



The foregoing study of foetal and calf growth in the sperm whale shows that growth from conception 

 to weaning is not only absolutely but also relatively slower than growth during this period in the great 

 whalebone whales where, for instance, gestation in blue and fin whales lasts a little less than a year 

 and lactation six or seven months, the calves being weaned at about 16 and 12 m. respectively (Mackin- 

 tosh & Wheeler, 1929). 



The present material offers little information about growth after weaning. For catches between 

 1949 and 1954 the length frequency curve for the shorter whales (Fig. 2, p. 242) shows no strong 

 peaks which might suggest age groups, nor are the data sufficient for analysis by successive years to 

 see if any features of the length frequencies persist from year to year. Laws (1953) has used periodic 



