Age determination and reproductive traits of killer whales entrapped in ice off Aidomari, Hokkaido, Japan

Abstract

On 7 February 2005 a group of 9 killer whales (Orcinus orca) were trapped in drifting sea ice and died at Aidomari, Hokkaido, Japan. We carried out age determination based on tooth growth layers and examined the reproductive organs of these whales. Growth layer groups (GLGs) in the dentine and cementum were readable, even in the old specimens, and complementary to each other in decalcified and stained thin sections of lower teeth. Reliable age determination of killer whales is feasible, and GLGs are accumulated annually. The longitudinal growth of the teeth continued until about 20 years of age, which is much longer than for the corresponding age in other delphinids. Counts of corpora lutea and albicantia increased linearly with age from 3 to 7 in whales 13–34 years old, but the oldest female (59 years old) had only 8 corpora albicantia, which could indicate a decline in the ovulation rate in old females. Photographs of the original group trapped in the ice confirmed that at least 2 whales escaped or died and did not strand with the others. Therefore, this group was composed originally of a mature male, 1 possibly postreproductive female, 5 reproductively active females, 3 calves, and 2 or 3 unidentified individuals.