274 DISCOVERY REPORTS 



testes of North Pacific whales, one collection made between July and October, and the other between 

 March and June, found ' more spermatozoa . . . with less effort ' in the second collection, and they 

 suggested that some cyclic activity might exist in the testis. 



Female sexual cycle 



The season of oestrus in female sperm whales in the North Atlantic has been estimated to last for 

 seven months, between January and July with most activity contained between March and May 

 (p. 270). The present data give no information on the pattern of the oestrous cycle. Matthews (1938, 

 p. 141) found indications of periodicity in corpora lutea frequencies from fourteen southern whales, and 

 he suggested details of a dioestrous cycle. Corpora lutea frequencies are available for 154 whales 

 examined at Horta between 1949 and 1954, but their frequency curve shows no regular periodicity. 



Regarding ovulation, it may be said that one calf is normally produced at a birth. There is no in- 

 stance of multiple ovulation in the present material, although twinning is known to occur in sperm 

 whales and was first mentioned by Bennett (1840, p. 179) and later by Beneden & Gervais (1880, 

 p. 308). Matthews, in his Table X, recorded twins once in seventy-four foetal records, and Matsuura 

 (1936) mentioned another instance among 228 records. Combining these gives a twinning incidence 

 of o-66%, which is the same as Paulsen's figure for twinning in foetal blue whales, discussed by Slijper 

 (1949, p. 418). Whether, or how often, such twins survive after birth is not known. 



Table 24. Adult females. Pregnant, lactating and resting whales examined 



at Horta from 1949 to 1954 





One whale, both pregnant and lactating, has been entered twice. 



It will be remembered that gestation is estimated to last for sixteen months in sperm whales of the 

 North Atlantic. Judging from what little is known of other toothed whales, the subsequent period of 

 lactation is likely to be protracted. Lactation apparently lasts for eight months in the common porpoise 

 (Mohl-Hansen, 1954,), for twelve months in the pilot whale {Rep. Fish Res. Bd. Can. 1954, p. 55), 

 and for as long as eighteen months in the bottlenosed dolphin in captivity (McBride & Kritzler, 1951, 

 p. 266), although the period is likely to be less for this species at large. Allen concluded that the calf 

 of the pigmy sperm whale, 'stays with the mother during the first year' (1941, p. 26). The present 

 study estimates the lactation period in the sperm whale by first examining the proportions of whales 

 lactating, pregnant and resting among the catches at Horta ; and then by determining the growth of the 

 calf to weaning. 



Table 24 shows the 'classes' of adult females among whales examined at Horta between 1949 and 

 1954. With one doubtful exception (Whale F268, examined on 1 July 1954) all whales were pregnant 

 whose ovaries bore a functional corpus luteum. The absence of recently ovulated whales in the Fayal 

 catches is to be expected, because scarcely any females arrive off the islands before June, when the 

 sexual season is mostly over (Fig. 7). If the classes of females in the catch are considered to be re- 

 presentative of those in the actual population, then the proportions of whales pregnant, lactating and 

 resting should be roughly proportional to the duration of each phase of the sexual cycle. Since gesta- 

 tion is estimated to last sixteen months, the percentages in Table 24 suggest that lactation should last 



