THE REPRODUCTIVE CYCLE 4°9 



old females are sexually mature, while some sexually immature whales are as old as 7 years, a range of 

 5 years. The full range for the population is probably greater than this small sample indicates. 



As regards the male fin whale, Laws and Purves (1956) compared testis weights and ear-plug 

 laminations of a small sample of northern hemisphere animals. They concluded that sexual maturity 

 is attained at 4-6 years. Purves and Mountford (1959) show that the estimated length at sexual maturity 

 (taken to be 63 ft.) corresponds to nine laminations on their growth curve for body length, that is, to 

 4I years. Sexual maturity in the male appears, therefore, to be attained at a slightly lower age than 

 in the female. 



These estimates of age at sexual maturity are based on an assumed bi-annual rate of lamina forma- 

 tion. Although this appears probable for adult fin whales (see below, p. 467), it is possible that the 

 incremental rate is less regular in immature whales (Chittleborough, 1959, fig. 4), and the laminations 

 are difficult to read. This will probably not greatly affect the estimate of the average age at sexual 

 maturity, but means that the range of ages at which sexual maturity is attained may perhaps be greater. 



NEWLY MATURE FEMALES 



The mammary gland 

 Positive identification of newly mature females is made possible in the field by the characteristic 

 appearance (gross and microscopic) of the mammary gland. This is the only group of mature females 

 for which the precise reproductive status is known with certainty. The location of the mammary 

 glands of baleen whales and their anatomy and histology are described by Lillie (191 5), Mackintosh and 

 Wheeler (1929), Heyerdahl (1930), Ommanney (1932), van Lennep and van Utrecht (1953) and 

 Chittleborough (1958). 



In sexually immature female fin whales the mammary gland is usually not more than 3 cm. deep 

 (mean 2-2 cm.) at the widest part (Text-fig. 35) and is pale pinkish white in colour. It is composed 

 mainly of a mass of connective tissue in which a few small lacteal ducts and blood vessels are seen, 

 and the alveoli are only slightly developed (Mackintosh and Wheeler, 1929, fig. 135). 



The glands of females in their first pregnancy (usually with only one corpus luteum and no corpus 

 albicans in the ovaries) are very similar in gross and microscopic appearance to those of immature 

 females and are 3 cm. thick, or less, in the majority (mean 2-9 cm., range 1-6 cm.). The development 

 of the lobules may be slightly greater than in the immature gland, but less than in the resting condition, 

 and the colour remains a pinkish white. 



A preliminary study suggested that the mammary gland increases in thickness from the immature 

 level of about 2 cm. to about 4-5 cm. at each ovulation preceding the first pregnancy, and shrinks 

 again to the former level in anoestrus or pregnancy. This would account for the few thicker mammary 

 glands at puberty and the first pregnancy; in spite of their greater depth they retain the immature 

 appearance. Unfortunately the material is insufficient for statistical treatment. There is no evidence 

 for an increase in the thickness of the mammary gland during pregnancy, at least up to the time when 

 the foetus has attained a length of 4-5 m., which is thought to be about a month before parturition 

 (Laws, 19590). One first-pregnancy female with a foetus 6-4 m. long (which must have been very 

 near term) had mammary glands only 2 cm. thick. Van Lennep and van Utrecht (1953) remark that, 

 ' In contrast to many other mammals the alveoli do not develop until the end of pregnancy '. Chittle- 

 borough (1958) found that in humpback whales which were very close to the time of parturition the 

 lobules and alveoli were well developed and colostrum was present in most cases. It is considered 

 unlikely that in fin whales mammary gland development and the secretion of colostrum begin until 

 after they have left the Antarctic on the northward breeding migration. 



