64 



DISCOVERY REPORTS 



against total lengths of the whales examined. From these the data in Table XVIII are 

 obtained ; and from them, though they are scanty, there is some indication of a grouping 

 of numbers near i, 5 and 10, which points to the possibility of an average of four di- 

 oestrous cycles with five ovulations for each sexual cycle succeeding the first. 



The evidence hitherto available indicates two possibilities, either (a) that the Hump- 

 back whale breeds every year or, (b) that it breeds once every two years. If it breeds 

 once every two years lactation in some individuals would be expected to overlap early 

 pregnancy in others, and this in fact is recorded, for the only two lactating Humpbacks 

 examined occurred in December and March respectively when other individuals were 

 in early pregnancy. Moreover, if lactation takes about five months, and pregnancy lasts 



Table XVIII. Humpback whale. Females. Frequency of 

 numbers of corpora lutea 



about eleven months, the breeding cycle will obviously take more than a year for its 

 completion and breeding can only take place either once every two years with a gap of 

 anoestrus between sexual seasons, or, if anoestrus is reduced in duration, twice every 

 three years. The latter possibility would explain the simultaneous occurrence of ovu- 

 lating, pregnant and lactating whales. 



On the other hand, lactation is shorter in this species than in the rorquals, and growth 

 of the calf is more rapid, probably owing to the large size of the mammary gland when 

 compared with that of other species. It would thus be possible for an early parturition 

 in July to be followed by five months' lactation and a further pregnancy in the following 

 January. This possibility is further upheld by a consideration of whale no. 516 taken in 

 February at South Georgia. This whale was pregnant, but the mammary gland, though 

 not lactating, contained a "brown sticky fluid" which may have represented the last 

 stages of involution after the previous lactation. Further, whale no. 3206, taken in early 

 March 1930 at South Georgia, was not pregnant but had just ovulated, and may thus 

 have been in oestrus after several unsuccessful dioestrous cycles, an assumption which 

 is strengthened by the presence in the ovaries of eleven corpora lutea, the largest 

 measuring 5-3 cm. in diameter. This whale appears more likely to have been a late 

 breeder of the 1925-6 season than a very early one of the 1926-7 season. 



On the whole, the evidence from the female Humpback whales points to the possi- 

 bility of breeding occurring twice every three years in some, if not all, individuals. But 



