WHALEBONE WHALES—MACKINTOSH 253 
following their parents and small foetuses have been found. These 
authors conclude that birth and pairing take place on these southerly 
grounds in winter, that gestation lasts for about a year, and that im- 
pregnation takes place every 2 years. The breeding of this species, 
like its migrations, is believed to be more regular than in other species, 
but for confirmation of this it would be desirable to obtain a larger 
number of measurements of foetuses. 
The exact length of the period of gestation and the dates of maxi- 
mum breeding activity cannot be regarded as precisely established in 
any species. The important point is that in those species on which 
adequate observations have been made there is no doubt that breeding 
mostly takes place in the winter when the whales have moved into 
warmer waters. In summer the pregnant females move into the high 
latitudes where food is abundant, and return to warmer latitudes in 
the following winter to bring forth their calves. 
Normally one young is born at a time, and the female is not as a 
rule reimpregnated in the same year, for parturition is followed by 
a considerable period of lactation, and instances of whales which are 
simultaneously pregnant and lactating are rare. In the Antarctic 
catches of blue and fin whales the proportions of adult females which 
are pregnant, lactating, and resting alter during the summer months, 
but these proportions on the whole suggest that a female normally 
becomes pregnant every 2 years and that the interval may sometimes 
extend to 3 years (Wheeler, 1930; Laurie, 1937; Mackintosh, 1942). 
(3) GROWTH AND AGE 
In the rorquals the length at sexual maturity is, very roughly, three 
times the length at birth, and some estimates have been made of the 
time taken to grow to sexual maturity. Some direct evidence of rapid 
growth in fin whales is mentioned by Harmer (1920). Andrews and 
Risting believed that the gray whale reaches maturity a year after 
birth. This seems likely enough, since foetuses as long as 17 feet and 
pregnant females as short as 34 feet are recorded by Risting. The in- 
ference that they grow to maturity in a year is based on the occurrence 
of young whales of intermediate lengths, but it is not clear that the 
evidence is sufficient to be conclusive. Mackintosh and Wheeler (1929) 
estimated, from dated measurements of small calves and the relative 
growth rate of the baleen, that blue and fin whales are generally 
weaned about 6-7 months after birth, blue whales having by then 
grown to over 50 feet, and fin whales to 35-40 feet. This, together 
with some indications of length groups in immature whales, suggested 
that these whales become adult at an age of about 2 years. The calcu- 
lation cannot be regarded as fully reliable, and Ruud’s evidence (see 
below) indicates that 3 years is the more usual period. The recovery 
