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 s ize? 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 co mpliant for all species and generally increases with size. The compliance o f the middle-ear air space increases with size, but the compliance of the t ympanic membrane and ossicular chain is not correlated with size. Structure -based rules are developed to represent some features of middle-ear perform ance: (I) low-frequency sensitivity increases with size; and (2) the freque ncy of a prominent notch in admittance decreases with size. Although some s pecies 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 feat ures. More speculatively, ethological pressures which might influence heari ng performance are discussed.