252 © ANNUAL REPORT SMITHSONIAN INSTITUTION, 1946 
Whales become sexually mature at a length which normally varies 
within rather narrow limits. The average length for blue whales 
is about 74 feet in males and 77 feet in females. The corresponding 
lengths in other species, so far as they have been investigated, are esti- 
mated to be as follows: fin, 63 and 65 feet; humpback, about 39 and 
41 feet; sei, about 44 and 4714 feet. This is a matter of considerable 
practical importance, since it allows an estimate to be made of the 
percentage of immature whales in statistics of catches which give only 
the species, sex, and length of each whale. 
(2) THE SEXUAL CYCLE 
Estimates of the time of pairing, the rate of linear growth of the 
foetus, and the length of the period of gestation, depend primarily on 
measurements of the lengths of foetuses at different times of year. 
Records of the largest foetuses and the smallest calves indicate that 
in blue and fin whales the length at birth is rather more than 20 feet. 
The average length of foetuses increases rapidly from spring to au- 
tumn, and extrapolation of the curve of growth indicates that pairing 
and calving for the most part take place in winter and that the period 
of gestation is about a year. The diversity in sizes of foetuses meas- 
ured at any one time of year shows that pairing and calving must both 
be spread over a period of several months. This, together with the ap- 
parent capacity of the ovaries to produce numerous Graafian follicles 
(Wheeler, 1930), suggests a prolonged polyoestrous breeding season. 
These conclusions have been reached by various authors. The work 
of earlier investigators is summarized by Hinton (1925), and Harmer 
(1920) ; and Risting (1928) obtained similar results from an analysis 
of large numbers of foetal measurements provided by various whaling 
companies. Risting calculated that in the Southern Hemisphere pair- 
ing mostly takes place in blue whales from June to August and in fin 
whales from June to September. From the increased activity of the 
testis, the evidence in the ovaries of ovulation about the same time or 
a little later, and the occurrence of very small foetuses, Mackintosh and 
Wheeler (1929) inferred that the height of the pairing season in 
blue and fin whales is about June and July, and estimated that gesta- 
tion lasts about 10 months in blue whales and perhaps slightly longer 
in fin whales. Matthews (1937, 1938b) found that in humpbacks the 
season is probably a little later (August to October) and in sei whales 
about June to August. Right whales also are believed to pair in win- 
ter, and the period of gestation is probably not much different from 
that of the rorquals. According to Andrews (1914) and Risting 
(1928) the female gray whales arriving off California and Korea 
from the north in autumn mostly carry foetuses whose size suggests 
imminent birth. On the northward migration young have been seen 
