422 DISCOVERY REPORTS 



ovulated, with a corpus luteum of ovulation in the ovaries, and have not yet acquired a diatom film. 

 The most recently formed corpora albicantia in the ovaries of other nulliparous females taken in 

 antarctic waters appear to have been derived from corpora lutea formed as late as November, December, 

 or January, and this is associated with a decline in the percentage of pubertal females with corpora 

 lutea of ovulation in the ovaries from January onwards. This subsidiary ovulatory period also repre- 

 sents a monoestrous cycle and rarely, if ever, initiates a pregnancy. A period of anoestrus succeeds, 

 and the next ovulation, which probably follows a northward migration, occurs in April-November 

 when this group of females have probably attained on average a length of about 68 ft. This ovulation 

 is usually succeeded by pregnancy. 



The type of sexual cycle described in this short summary is very different from the type of cycle 

 which has previously been assumed to apply to cetacea. Hitherto it has been assumed that whales 

 have a series of polyoestrous cycles during a single protracted ovulatory period. It has been assumed 

 that an individual female may experience several dioestrous cycles at intervals until interrupted by 

 pregnancy or anoestrus. The new evidence about the fin whale suggests that this species is not usually 

 polyoestrous but rather that it is seasonally monoestrous, and further evidence is presented in a later 

 part of this paper (p. 425) which shows that the annual cycle of multiparous female fin whales is also 

 characterized by two ovulatory periods, which probably represent two monoestrous cycles. 



If the arguments put forward above are valid then there are two seasons of the year at which fin 

 whale females may attain puberty. The majority, probably about 70%, ovulate for the first time in 

 winter during the usual pairing season (April-November) after a northward migration, and the others 

 probably attain puberty and ovulate for the first time in spring and summer (September?) (October- 

 February), after a southward migration. This is a very unexpected conclusion and it is worth looking 

 into possible exteroceptive factors which may be responsible. 



A majority of mammals (insectivores, rodents, non-ruminant ungulates and carnivores) are spring 

 breeders and, as Marshall (1922) pointed out, they appear to be sensitive to increasing light. A minority 

 (the ruminant ungulates) are autumn and winter breeders and would appear to be sensitive to de- 

 creasing amounts of light; others appear to be uninfluenced by seasonal changes in the amount of 

 light. Other factors such as temperature, humidity, food, etc. are also important. 



' The various factors which affect the time of the appearance of puberty probably act through the 

 pituitary-gonad mechanism. Such exteroceptive factors, too... as the length of daylight changes 

 during the course of the year which cause the onset of the breeding season will accelerate or retard 

 the age of puberty according as the normal age for puberty in the individual falls before or after this 

 season ' (Hammond and Marshall, 1952, p. 824). Thus for certain breeds of sheep the breeding season 

 extends from September to March and those lambs born early in the season will be longer in arriving 

 at the age of puberty than those born later in the season. In sheep there is neither a fixed age at 

 puberty nor a fixed time of year for the first heat (J. Hammond, Jr., 1944). Sexual activity begins at 

 a minimal age of about 6 months, and with increasing age the threshold of stimulation required to 

 produce the first oestrous cycle falls, until by about 10 months it reaches the adult level. The sheep 

 responds to decreasing light, and as day length decreases so the intensity of stimulation increases. 

 It is maximal at the middle of the breeding season which corresponds to the minimal age at first heat 

 (about 6 months). If this age is reached later in the breeding season the intensity of stimulation is not 

 sufficient to bring about oestrus, and puberty is not attained until the next breeding season when the 

 animal may be over a year old (Hammond and Marshall, 1952, p. 824). 



A similar mechanism appears to be operative in the case of the fin whale except that this species 

 probably responds to increasing light, attains puberty at a much greater age and, owing to the 

 migratory habits, experiences two periods of increasing day length each year instead of one (see below). 



