Auditory systems are expected to be under selection to increase detection of signals that affect survival or reproduction. In the absence of physical or physiological constraints, such selection may result in correlated evolution between vocalizations and auditory tuning (Endler 1992). Indeed, correlations between audiograms and vocalization frequencies within species have been reported (e.g. Brown & Waser 1984). However, only one study on frogs has attempted to incorporate evolutionary history, and in this case correlated evolution between call frequency and hearing sensitivity was not detected (Wilczynski et al. 2001).

We used published data from species for which echolocation calls, isolation calls and audiograms were available (see appendix in the electronic supplementary material). We used median forearm length to estimate body size. For some species, only spectrograms were available for estimating echolocation or isolation call frequencies, making it impossible to determine peak frequencies. Thus, for consistency we used the median frequency of the fundamental for isolation calls and the median frequency of the entire call for echolocation calls. The fundamental frequency was used for isolation calls because it often contains the most energy (e.g. Habersetzer & Marimuthu 1986; Sterbing 2002). Isolation calls typically increase in frequency during development and older pups' calls may be intermixed with developing echolocation calls (reviewed in Altringham & Fenton 2003). We minimized inclusion of echolocation call precursors by only using calls from pups less than two weeks of age. For measures of hearing sensitivity, we did not use neural audiograms recorded from anaesthetized bats because they show reduced sensitivity to low frequencies (Neuweiler 1990). Most bat audiograms have two regions of enhanced sensitivity separated by a relatively insensitive region (figure 1a; appendix, electronic supplementary material). We used the frequency of greatest sensitivity in these regions. If two adjacent frequencies within a region showed the same hearing threshold, we used the midpoint between those two values. Many bat species that produce constant frequency echolocation calls show three frequency regions of increased auditory sensitivity. For these species, we used the lowest and highest frequency regions (figure 1b; appendix, electronic supplementary material).

This study presents evidence of correlated evolution between vocalizations and auditory sensitivity for frequency regions that correspond to echolocation and pup isolation calls. These results did not depend on the topology or branch lengths of the phylogeny. In apparent contrast to another study (Jones 1999), we did not find evidence of correlated evolution between body size and either vocalization or hearing frequency. However, Jones (1999) examined correlations on species data within families without any phylogenetic correction. Examination of his data reveals that vocalization and hearing frequencies are not correlated between families. Thus, these apparently disparate results can be reconciled if echolocation evolution has been decoupled from body size evolution among families.