Searching among felids for evolution’s influences on middle ears

Abstract

The 36 cat species display moderate structural variations that may be adaptive, involving species ethology, size, and lineage. Measurements of skull dimensions, acoustic responses in live and cadaver ears, and auditory brain-stem responses in anesthetized specimens have been collected. Of special interest are interspecies variations in the acoustic properties of the felid middle-ear air space, which has two coupled cavities separated by a bony septum. The acoustic effect of the cavities is to decrease middle-ear response uniformly for low frequencies, and to produce a narrow-band reduction at a mid frequency determined by the cavities configuration. We address two issues: (1) The volume of the cavities increases with species size. A physics-based model shows that the cavity-related loss in low-frequency middle-ear transmission decreases from 10 dB for the smallest species to 1 dB in lions. Is this size-dependent variation adaptive? (2) The configuration of the cavities-dividing septum is unusual in eight species, all of which have relatively wide facial skulls and live in open habitats; as these species represent six of nine lineage groups, this constellation of habitat, skull, and ear features apparently evolved several times. Can we hypothesize a testabIe, adaptive mechanism? [Work supported by NIH and NSF.]