Aspects of the biology of Pilot whales (Globicephala melaena) in recent mass strandings on the British coast

Five mass strandings of Pilot whales, involving from 23 to 40 animals, occurred on the British coast between 1982 and 1985. The sex ratio in all strandings was biased towards females (62% overall), but more than one mature male was present in each group. A multi-male, polygynous social system is suggested. Growth is rapid from a mean body length at birth of 1.78 m to about 3 m at 2–3 years. Thereafter, males grow faster than females and attain a greater body length by some 18–25%. Maximum body lengths in this study were of a 6.3 m male and a 5.5 m female. The greatest ages determined were of a 20-year-old male and a 25-year-old female, but there is a possibility that readable dentine is not deposited in the teeth of older animals and that some whales are thus of a greater age than can be detected. Females become sexually mature at about seven years of age and a body length of 3–4 m. Some reach sexual senility before death. Males mature at a greater age and at about 5 m in length. Annual calf production is about 11% and no seasonality in parturition could be detected. Pollutant levels are generally within the range of those published for odontocetes, but PCB levels are higher than any yet found in other Pilot whale populations. Evidence of squid was found in three digestive tracts. Blubber thickness increases with the size of the animal, reaching 35–65 mm in adults. The existence of an annual, rigid north-south migrational pattern is unlikely.