Scientists use different methods to determine marine mammal life spans.
Teeth in cetaceans are often aged by counting lines in the dentine of the tooth. The teeth of pinnipeds are also aged by counting the dentine or cementum of the tooth. Lines in the teeth of pinnipeds and toothed whales can be read like tree rings to determine the age of an animal. Usually, one line in the tooth corresponds to one year of life for the animal. The bones of this stranded North Pacific bottlenose whale will be used to determine the whale's size and age at death.