Humming in bears: a peculiar sustained mammalian vocalization

摘要

A peculiar sustained vocalization has long been known under various names in several bear species but not been studied in sufficient detail. Based on a critical survey of the relevant literature, analyses of tape recordings and pertinent own observations we tried to clarify the presence of this vocalization in the species of the Ursidae as well as its structural characteristics and specific mode of sound production. To avoid confusion with other vocalization types we introduce the term humming for it. Furthermore we discuss its communicatory and functional significance and formulate hypotheses as to its evolutionary origin against the background of acoustic communication signal repertoires known in the terrestrial Carnivora. Humming is present in all extant species of the Ursidae with the exception of the giant panda. It has similar structural characteristics and the same mode of sound production in all species. It is also known in adult bears but its occurrence is largely restricted to cubs. Humming is a synapomorphic vocalization type of the Ursidae which is not phylogenetically related to another vocalization type known in the terrestrial Carnivora. It is a rapid sequence of very short single sounds; long bouts of sustained exhalatory sound production are interrupted by very short inhalatory phases without sound. Both the sound and the body vibration accompanying its production are highly likely to be communication signals. Yet, controlled physiological experiments on bear mothers and cubs are still necessary to formulate and test specific hypotheses as to the communicatory function and adaptive significance of humming.