Relating middle-ear acoustic performance to body size in the cat family: measurements and models.

Is the acoustic performance of the mammalian middle ear dependent on body size? We focus on the cat family, because of its qualitatively uniform (and distinctive) middle-ear structure, large size range, and the extensive data available from domestic cats which provide a framework for relating middle-ear acoustics to structure. We report measurements of acoustic admittance in 17 live adult ears of 11 exotic species, ranging in size from sand cat (3 kg) to tiger (180 kg). For low frequencies, the middle-ear response is compliant for all species and generally increases with size. The compliance of the middle-ear air space increases with size, but the compliance of the tympanic membrane and ossicular chain is not correlated with size. Structure-based rules are developed to represent some features of middle-ear performance: (1) low-frequency sensitivity increases with size; and (2) the frequency of a prominent notch in admittance decreases with size. Although some species deviate from the rules, the data generally support the idea that in larger felids the middle-ear response is shifted to lower frequencies. Thus, in the cat family, body size partly describes variations in auditory features. More speculatively, ethological pressures which might influence hearing performance are discussed.