The evolutionary success of mammals can in part be attributed to remarkable sensory diversification e.g.. While some lineages show evidence of changes across multiple sensory modalities e.g. others have evolved one highly specialised system e.g.. Mammalian auditory systems are particularly well-developed compared to those of many other vertebrate groups, furthermore, mammalian hearing can be characterised by high sensitivity and selectivity and in particular by broad frequency ranges with high upper frequency limits[4, 5]. Multidisciplinary evidence suggests that these auditory adaptations in mammals e.g.[6, 7] can be linked to three principal adaptations: the evolution of three ossicles in the middle ears (malleus, incus and stapes), elongation of the basilar membrane in the cochlea which provides a supportive base for the sensory hair cells and the evolutionary innovation of outer hair cells (OHC)[8–10]. However, genetic studies suggest additional molecular changes have occurred in the motor protein of the OHC, known as Prestin, that are probably related to the acquisition of high frequency hearing seen in extant therian mammals. Therefore, it has recently been summarized that true high frequency hearing (i.e. >20 kHz) likely evolved approximately 125 million years (Ma) ago within the therian lineage and required additional structural modifications of the inner ear (as reviewed in).
The mammalian cochlea is a coiled cavity in which high and low frequency sounds are perceived by the basal and upper turns respectively, and this tonotopic organisation is partly achieved by a decrease in basilar membrane stiffness from base to apex[14, 15]. Mammals exhibit considerable variation in hearing capabilities and cochlear morphology, although some consistent relationships link these two traits. Basilar membrane length is positively correlated with body mass, and absolute basilar membrane length is negatively correlated with both high and low frequency hearing limits e.g.[18, 19]. It has been hypothesised that cochlear coiling evolved in response to selection pressures relating to the accommodation of elongated auditory sensory membranes of the inner ear, however, a previous study found no significant relationship between the number of cochlear turns and basilar membrane length. Recent evidence suggests that coiled cochleae may play a mechanical role in low frequency hearing limit. Across auditory ‘generalists’ high frequency hearing limits correlate with inter-aural distance, whereas auditory specialists such as subterranean mole rats and echolocating cetaceans deviate from this relationship. Other factors determining the morphology of auditory systems include physical and mechanical constraints, as well as phylogeny e.g.[16, 22, 23].
Although several mammalian taxa, including some rodents, carnivores and primates are capable of either detecting or producing ultrasonic sounds (>20 kHz)[4, 24] the most highly developed auditory systems for perceiving ultrasonic sound are seen in toothed whales and laryngeal echolocating bats[16, 25]. Bats possess some of the widest frequency ranges of vocalisations and, therefore, assumed associated hearing sensitivities of any mammal group, with recorded vocalisations ranging from below 20 kHz to over 200 kHz across the order (as reviewed in[26, 27]). Of 19 currently recognised bat families, all but one (the Old World fruit bats) use laryngeal echolocation for orientation, obstacle avoidance and, in most taxa, prey detection. The inner ears of laryngeal echolocating bats show several structural adaptations for detecting ultrasonic echoes; in particular, their cochleae are often enlarged and contain 2.5 to 3.5 turns compared to only an average of 1.75 in non-echolocating fruit bats[29–31]. Furthermore, different forms of echolocation appear to have led to different and sometimes convergent inner ear adaptations[32, 33]. For example, both Old World horseshoe bats and the New World moustached bat possess a greatly enlarged cochlear basal turn, which allows exquisite tuning of the inner ear to the echoes of the specialised constant frequency (CF) calls produced by these taxa[34, 35]. Previous studies suggest specific adaptations of the anchoring system and the width of the basilar membrane in echolocating bats.
Well-supported phylogenies of bats show that laryngeal echolocation is distributed across two highly divergent suborders of bats, termed Yinpterochiroptera and Yangochiroptera, the former of which also contains the non-echolocating Old World fruit bats. To account for this pattern, two main evolutionary scenarios have been proposed; first, that echolocation evolved once in the common ancestor of all modern bats with subsequent loss in Old World fruit bats, and two, that laryngeal echolocation evolved multiple times across the order[38, 39]. While fossils bats from the early Eocene have been taxonomically classified as falling outside of the modern bats, and thus might not inform this issue[40, 41], recent reports that echolocating members of the two suborders have undergone convergent amino acid replacements in several ‘hearing genes’ would appear to support the multiple origin hypothesis see[42–45].
Evolutionary modifications of the inner ear are likely to have arisen from selection acting on numerous loci, and thus a comparative analysis of morphology might offer a powerful means of reconstructing the origins of ultrasonic hearing and thus laryngeal echolocation in bats. Here we reconstructed three-dimensional bat inner ear volumes of a range of bat species from 16 families, encompassing the broad diversity of echolocation call types and ecological traits seen in the order. We compared patterns of cochlear morphological variation - as defined by relative basilar membrane length and number of turns - among bats, and also between bats and other mammals for which data were available. Correlations were investigated between high and low hearing frequency limits and also echolocation call parameters and morphological characters of the cochlea. We predicted that echolocating bat species would show specific adaptations in aspects of cochlear gross morphology compared to both non-echolocating bats and other mammal species, due to the particular demands associated with receiving the high frequency sounds produced during laryngeal echolocation.
If significant inner ear adaptations for ultrasonic hearing do occur, then these patterns might help us to distinguish between the two most parsimonious alternate scenarios of the evolution of echolocation. For example, it may be possible to determine whether Old World fruit bats show evidence of a loss of echolocation. Alternatively there may be evidence of functional adaptation in the inner ears of the two groups of echolocating bats. Specifically, if Old World fruit bats have lost echolocation, then we might expect to find signatures of this in their cochleae such as intermediate forms between those of echolocating bats and non-echolocating mammals or increased morphological variability, consistent with morphological relaxation.
The two alternate evolutionary scenarios that have been proposed to account for the distribution of laryngeal echolocation across divergent clades of bats might also be expected to have left different traces of inner ear morphological evolution across the bat phylogenetic tree. Firstly, given a single origin in the bat common ancestor we might expect to see increased rates of morphological change in the ancestral bat branch, possibly coupled with a subsequent rate shift in the non-echolocating Old World fruit bats corresponding to a loss of structures associated with sophisticated echolocation capability. In contrast, multiple origins might be expected to leave a signal of morphological rate shifts on specific branches after the point that the two main bat suborders diverged.