Sperm whales of the Southeast Pacific. Part VII. Reproduction and growth in the female

Abstract

This report on the reproduction and growth of the female sperm whale Physeter catodon is Part VII of our work on this whale in the Southeast Pacific. There were 1105 female sperm whales in our sample collected from two whaling stations in Chile and two in Peru. Since Clarke and Paliza (1972) have shown that they belonged to a single stock, we have worked them together. A second Graafian follicle develops more than the others in each ovary so to improve the possibility of fertilization in case the first ovum fails to be impregnated. We consider the size of the Graafian follicle at, or near, ovulation to be around 100mm, larger than what has been found in sperm whales from other seas. The corpus luteum of pregnancy is significantly larger than the corpus luteum of ovulation. The corpus albicans reduces in size throughout the life of the whale and probably does not disappear. There is a highly significant correlation between the total number of corpora and age: therefore we use the number of corpora as an indication of age. The corpora atretica are more frequent in older female sperm whales reflecting less fertility in this group. The sexual cycle in sperm whales of the Southeast Pacific has been revised to last 4yrs. Sexual maturity in female sperm whales is attained at 8.2m long and 6.5yrs of age, being both values lower than in sperm whales from other seas. The female sperm whale is born at 3.90m. The incidence of twins, 0.91%, is higher than in other seas. Fertility is low in very young whales (1-2 ovarian corpora) and it is at its lowest in the older group (over 12 corpora). The highest fertility is when females have 3-10 ovarian corpora and they are 15 to 35yrs old. The proportion of active females in pre oestrus during the months of pairing is significantly higher than during the other months. Accessory ovulations during oestrus are represented by the small groups of lactating-and-recently ovulated and lactating-and-pregnant whales. Unsuccessful ovulations are more frequent in late lactation and late resting periods, being post-partum ovulation rare. Female sperm whales in the Southeast Pacific may ovulate up to four and possibly five times during an oestrus. Physical maturity is attained at 11.2m long and 33.5yrs old. Fusion of the vertebrae begins at both ends of the vertebral column and finishes between the posterior thoracic and the lumbar vertebrae. Female sperm whales of the Southeast Pacific may live, at least, up to 50yrs of age. The age at recruitment between 1959 and 1962 was 20-21yrs of age when they had accumulated 4–5 corpora in their ovaries.