REPRODUCTION 389 



a few weeks after birth, and their mothers can conceive shortly later, or 

 even during, lactation. 



In Cetaceans, just as in Pinniped Carnivores, the situation also differs 

 from species to species. 



Thus, Common Dolphins, Porpoises, Killers and Belugas were found to 

 be capable of conceiving immediately or very shortly after they had given 

 birth, so that the cows of these species generally produce offspring every 

 year. The same is true of Little Piked Whales, although in their case, cows 

 occasionally miss a year. Black Sea Dolphins are said by Kleinenberg 

 frequently to produce offspring for three successive years, and then to 

 miss the fourth ; Belugas have a resting period after every 4-5 pregnancies 

 (Kleinenberg, i960). 



Unfortunately for whalers, the big whales, unlike their smaller relatives, 

 do not produce offspring annually, probably because they lack sufficient 

 food during part of the year. Milk which is rich in fats and proteins makes 

 heavy demands on the mother's reserves, so much so that the blubber 

 layer of lactating cows is always very thin (see Chapter 11). In these 

 circumstances, it would be fatal if a new embryo were to make further 

 demands on her strength while she was still suckling the calf, and we 

 have, in fact, clear indications that Rorqual and Sperm Whale cows do 

 not generally ovulate during lactation. Humpbacks are more generous to 

 whalers, for Chittleborough found that Australian Humpback cows could 

 conceive immediately after birth, or during the early stages of lactation. 

 The cows could therefore produce one calf a year and according to 

 Zemski N. Pacific Humpbacks do, in fact, have four calves every five 

 years. Grey Whales, too, can, according to Hubbs, conceive shortly after 

 they have given birth. All these findings are corroborated by the fact that, 

 in both species, greatly reduced populations can be brought to normal 

 strength in a relatively short space of time. 



Laws has calculated that 12 per cent of Fin Whales ovulate about two 

 months after they have given birth, while the remainder only ovulate 

 again at the end of lactation, i.e. four months later, when they are either 

 back in the Antarctic or on the way there. At that time, most of the bulls 

 are, however, sexually inactive (at least in cold waters), so that mating is 

 rare, and the ova usually remain unfertilized. Mating generally takes 

 place once the animals have migrated to warmer waters, i.e. in about 

 May. Laws found that 77 per cent of all conceptions take place from April 

 to August; 1 7 per cent in September and March, and only 6 per cent from 

 October to March. As a result of 'void' ovulations, the number of 

 corpora albicantia (see p. 360) increases by 2-8 every two years. (In 

 Humpbacks, one by every year; Chittleborough i960). Since some cows 

 conceive in the early stages of lactation, we might expect the average 



