THE HUMPBACK WHALE 



69 



Returning now to Fig. 54 and combining that curve with this, the result shown in 

 Fig. 61 is produced which shows that sexual maturity is reached at the age of about 

 twenty to twenty-two months and that Humpback whales first breed towards the end 

 of their second year. The accuracy of the curve in Fig. 61 formed by joining the curves 

 in Figs. 59 and 60 is upheld by the statement of Olsen (1914-15) that on their southern 

 migration the female Humpbacks are accompanied by calves about 5 m. or more in 

 length and that calves killed off Port Alexander in August were 6-5 m. in length. Engle 



Fig. 60. Humpback whale. Average lengths of immature whales by months. 



(1927), who examined a few Humpbacks in California, suggested that these whales be- 

 come sexually mature while they are yet quite young, a suggestion which is here shown 

 to be correct. 



The age classes of Humpback whales cannot be deduced from the number of corpora 

 lutea in the ovaries, as can those of the Blue, Fin and Sei whales, because of the shortness 

 of the complete breeding cycle in this species and the indication already shown that 

 pregnancy may sometimes occur twice every three years, or at least that an extra preg- 

 nancy may be interpolated in a more usual two-year cycle. The most that can be said is 

 that the whales with the lowest numbers of corpora lutea (one or two) are in their second 

 year and aged from eighteen months to two years. The classes shown by numbers of 

 corpora lutea after this age may be separated by one and a half years in some cases and 

 two years in others, so that taking into account also the paucity of the data, no indication 

 can be given of the ages of the older whales by this means (cf. Wheeler, 1930). 



