Middle ear structures in fossorial mammals: a comparison with non-fossorial species

Abstract

A large data set, comprising certain measurements of middle ear structures in mammals, was compiled both from measurements made by the author and from the literature. Parameters of the middle ear apparatus believed to be important to audition were compared between fossorial and non-fossorial species, in an attempt to identify general trends among fossorial groups. Although their tympanic membranes are not of unusual size, many fossorial mammals possess enlarged stapes footplate areas, resulting in low anatomical area ratios. Low anatomical lever ratios are also common, and the reduction or loss of middle ear muscles seems to be a consistent trend. These characteristics might be associated with poor sensitivity to airborne sound. Other features of the middle ear apparatus are more variable both between and within fossorial families. The middle ear ossicles of fossorial rodents, talpid moles and some golden moles were not found to differ in mass from those of non-fossorial mammals of similar body size. The similar ossicular morphologies of these animals suggest convergent adaptation towards a subterranean environment, but the middle ear structure alone does not seem to explain the restricted hearing range observed in certain of these species. Some genera of golden moles possess extraordinarily hypertrophied auditory ossicles, which, relative to body mass, are the largest of all mammals for which data are available. These ossicles seem to be adaptations towards a form of inertial bone conduction, used for the detection of substrate vibrations. In stark contrast, the marsupial mole Notoryctes has particularly small ossicles. The unusual middle ear structures of this animal may well be degenerate.