MULTIPAROUS FEMALES 453 



cases). This is probably because there is a single restricted sexual season in the male fin whale. 

 Evidence given above shows that the male sexual season extends in the main from April to July, and 

 is reflected by the data on the frequency of pairing. It agrees with the hypothesis put forward to 

 explain the factors regulating the female cycle. Almost invariably the seminiferous tubules of males 

 are still regressing on arrival in antarctic waters, but full anoestrus is attained soon after arrival. 



It is not immediately clear why the male should not experience a sexual season during or following 

 the southern migration as do the females, but there is an important difference between the sexes 

 which may go a long way towards explaining this. The female fin whale is monoestrous and produces 

 ova at infrequent intervals, but in the males it is probable that, as in some other animals, production of 

 sperm is continuous over a period of several months. If we assume a ' refractory period ' just after 

 the sexual season (as in fact occurs in female dioestrous cycles), then increasing day lengths would not 

 initiate a further sexual season if they occurred during this ' refractory period '. 



Such a refractory period has been shown to characterize the testis cycle of seasonally breeding male 

 birds (Marshall, 1950, 1951). It is defined as ' that period of the avian testis cycle when the tubules are 

 in a state of post-spermatogenetic lipoidal metamorphosis and before the newly regenerated Leydig 

 cells of the interstitium have become sufficiently lipoidal and mature to respond to neuro-hormonal 

 influences initiated by natural factors in the environment'. 

 ' This seems to be the most likely explanation, but it is also possible that the testes of male fin whales, 

 stimulated by increasing day lengths on migration, undergo a short recrudescence of activity. If this 

 were so, then it might be expected that more females would become pregnant following the post- 

 lactation ovulation. In juvenile birds, the testis tubules have not produced spermatozoa, and the 

 lipoid Leydig cells are still developing and receptive to stimuli ; it is possible that some pubertal male 

 fin whales are similarly receptive to day-length changes during the southern migration, as are pubertal 

 females. In this case some pubertal females might be expected to mate successfully with pubertal 

 males at this time. The disadvantage of such matings is clear; it would mean that larger numbers of 

 calves would be born at a time when the females must seek out food in the colder southern waters. 

 If it is disadvantageous for several-months old unweaned calves to enter antarctic waters then it is 

 clearly very disadvantageous for births to occur in antarctic or sub-antarctic waters. 



Marshall (1942) has reviewed the evidence bearing on the role of exteroceptive factors in sexual 

 periodicity. He concludes that in all species of mammals there is an internal sexual rhythm ' which 

 is presumably dependent upon an endocrine cycle but that this is usually, though not always, adjusted 

 to external seasonal change. That the recurrence of the sexual periods is not due entirely to endocrine 

 factors is shown especially clearly by those individuals which belong to species or breeds that ordinarily 

 have only one sexual season annually, yet can be induced to have two seasons by transferring them 

 across the equator from one hemisphere to the other. ' 



The evidence which has been presented strongly suggests that the ovulatory cycles of fin whales 

 are monoestrous cycles. This conclusion is strongly supported by the evidence as to the limited 

 variation in the rate of accumulation of corpora albicantia presented earlier. Further support for this 

 view will be demonstrated below, when the average annual rate of ovulation, estimated from just 

 such a seasonally monoestrous cycle as has now been described, is found to be in very close agree- 

 ment with the independent estimate of the average annual rate of ovulation given earlier in this 

 paper (p. 385), and with the rates of ovulation suggested by comparison with other methods of age- 

 determination. 



