222 DISCOVERY REPORTS 



back pairing is at a maximum in September and births in August (i.e. a little later than 

 in Blue and Fin whales), and that in Sei whales both pairing and birth reach a peak in 



J ul Y- 



The question whether whales are monoestrous or polyoestrous is discussed by 



Wheeler (1930, p. 412), and it has an important bearing on the interpretation of counts 

 of corpora lutea. There have been frequent instances of whales having a functional 

 corpus luteum but in which no trace of a foetus could be found in spite of a careful 

 search of the uterus. Wheeler also mentions the case of a Fin whale with one old corpus 

 luteum, immature mammary glands, and an unbroken vaginal band. It can at least be 

 assumed then that ovulation is spontaneous and not, as in some mammals, dependent on 

 coition. The protracted breeding season, together with the evident capacity of the 

 ovaries to produce numerous Graafian follicles, very strongly suggest that each of these 

 species is polyoestrous ; that is to say that more than one oestrous cycle may take place 

 in the course of one breeding season and that therefore more than one corpus luteum 

 may be formed if pregnancy does not follow on the first discharge of an ovum. This is 

 another point that still needs final proof, but that does not affect the inference that the 

 number of old corpora lutea in a pair of ovaries is likely to be greater than the number 

 of two-year cycles (see below) which have elapsed since the first ovulation. It might 

 sometimes of course be equal to it, for a whale might become pregnant every two years 

 at the first ovulation, and it is known that the corpus luteum of pregnancy inhibits the 

 discharge of a new ovum. If it were found, contrary to expectation, that whales were in 

 fact monoestrous, the determination of age from corpora lutea numbers would of course 

 be enormously simplified. 



In many mammals with an annual breeding season the females normally become 

 pregnant every year. This occurs, for example, in some, and probably all, Antarctic 

 seals (see Matthews, 1929, p. 236; Hamilton, 1934, p. 300; Bertram, 1940, pp. 59, 113). 

 It cannot be the rule, however, in whales, for the gestation period of about a year is 

 followed by a considerable period of lactation, and instances of females which are 

 simultaneously pregnant and lactating are very rare. Apart from this the proportion of 

 non-pregnant females in the catches during the period between breeding seasons is 

 sufficiently high to show that annual pregnancy cannot possibly be the normal ex- 

 perience of adult females. It is difficult, however, to be quite certain of what the normal 

 interval between pregnancies actually is. From the percentage of adult females pregnant 

 in the catches at South Georgia, Mackintosh and Wheeler (1929, p. 431) inferred that in 

 Blue and Fin whales pregnancy normally occurs every two years, but that the interval 

 may sometimes be three years. Laurie (1937), with additional data, re-examined the 

 question. His tables on pp. 239 and 243 give 50-60 % of pregnancies among Blue whales 

 examined at South Georgia and in two factory ships. On the face of it this would also 

 indicate a two-year cycle, but he raises the difficulty that the majority of lactating females 

 appear in the catches in the later part of the season. If a female spends the year following 

 gestation first in lactation and then in resting we should expect a high proportion of 

 lactating whales early in the summer. This is explained on the grounds that since 



