388 WHALES 



Female seals -which are, of course, much smaller -only reach maturity 

 at four, and male seals possibly later still. Like these Pinnipeds, 

 Cetaceans continue to grow after that, but only to a small extent; 

 the length at sexual maturity being about 85 per cent of their final 

 length. 



Another question which interests whalers is how often Cetaceans are 

 in season, and how many ova are liberated during their oestrous phase. 

 It is said that Narwhal, False Killer and N. Pacific Killer bulls and cows 

 may be in season throughout the year. The same appears to be true for 

 Bryde's Whales, because they live exclusively in warm waters and do 

 not have a definite migration (Best, i960). In other species, the bulls show 

 signs of slight sexual activity throughout the year, but their activity 

 becomes far greater when the cows are in season, as well. Even when the 

 oestrous phase of cows is prolonged, this phase is divided into periods of 

 minimum and maximum activity, of which the latter usually lasts for two 

 or three months. In most Cetaceans, mating takes place in winter or early 

 spring - our summer or autumn in the case of southern species - i.e. at a 

 time when migrating species are always in warmer waters. Belugas and 

 porpoises alone mate in May and in the middle of the summer respectively, 

 probably because they never travel over large distances. 



Most observers think that Humpbacks, Common Dolphins, Pilot Whales 

 and False Killers conceive after the first ovulation, or else ovulate again 

 a few weeks later, and so on, so that all cows of these species are likely to 

 be pregnant by the end of the season. On the other hand. Laws' most 

 recent investigations of Fin Whales (confirmed by Naaktgeboren, 

 Slijper and van Utrecht; i960) show that cows ovulate a single time 

 during May to July, and that, if fertilization has not taken place, another 

 ovulation takes place five months later. However, the chances of the ovum 

 being fertilized are so good that we may take it that practically all cows 

 which were on heat during the southern winter season have conceived 

 before they migrate back to Antarctic waters. 



Another important question is whether cows can conceive immediately 

 or shortly after they have given birth to a calf, or whether no ovulation 

 occurs during lactation. In some terrestrial mammals, e.g. hamsters, the 

 female cannot conceive while she is suckling her young, wild cattle cannot 

 conceive during the first three months of lactation, and in man this 

 'sterile' period often lasts no longer than six weeks. Other mammals, 

 however, ovulate immediately or shortly after giving birth - in fact, the 

 situation differs from species to species. In the case of Pinniped Carnivores, 

 walruses, for instance, ovulate one year after they have weaned their 

 calves, while sea-lions, which suckle their young for ten months, can be 

 fertilized one month after they have given birth. Young seals are weaned 



