The Breeding Cycle of the Female Humpback Whale, Megaptera nodosa (Bonnaterre)

Abstract

The gestation period in the humpback whale is of almost 12 months' duration, most conceptions occurring early in August with parturition at the beginning of the following August. The modal length of calves at birth is 14 ft. The sex ratio at birth is 51.4 per cent. males to 48.6 per cent. females. Twin foetuses are occasionally recorded. There is no preference towards either right or left ovary or uterine cornu with regard to ovulation or pregnancy. The breaking of the umbilical cord at birth is described. The histological appearances of the mammary gland during advanced pregnancy and during lactation are described and illustrated. Upon the evidence presented it is concluded that lactation in this species extends over 104 months, suckling generally commencing in mid August and terminating at the end of the following June. The composition of the milk has been determined during various stages of lactation. Oestrous cycles rarely occur during pregnancy, but examples of ovulation following shortly after parturition, while the females are suckling their calves, are cited. It is shown that when the calf is lost at or soon after birth, oestrous cycles usually recommence at once. Where the calf is suckled for the normal period (10½ months) there is no anoestrous period following the end of lactation as oestrous cycles recommence in July, immediately after (occasionally just before) weaning at the end of June. Three types of breeding cycle are shown for the humpback whale. In the most common cycle, absence of the post-partum ovulation, or its occurrence without conception, results in one calf in 2 years. On the other hand, a successful postpartum ovulation results in two calves in 2 years. In the third type of cycle, loss of the first calf at or just after parturition is balanced by a second pregnancy succeeding immediately afterwards. This results in two pregnancies in 2 years with only one calf reared.