Development of the hyolaryngeal architecture in horseshoe bats: Insights into the evolution of the pulse generation for laryngeal echolocation

Abstract

Background The hyolaryngeal apparatus generates biosonar pulses in the laryngeally echolocating bats. The cartilage and muscles comprising the hyolarynx of laryngeally echolocating bats are morphologically modified compared to those of non-bat mammals, as represented by the hypertrophied intrinsic laryngeal muscle. Despite its crucial contribution to laryngeal echolocation, how the development of the hyolarynx in bats differs from that of other mammals is poorly documented. The genus Rhinolophus is one of the most sophisticated laryngeal echolocators, with the highest pulse frequency in bats. The present study provides the first detailed description of the three-dimensional anatomy and development of the skeleton, cartilage, muscle, and innervation patterns of the hyolaryngeal apparatus in two species of rhinolophid bats using micro-computed tomography images and serial tissue sections and compares them with those of laboratory mice. Furthermore, we measured the peak frequency of the echolocation pulse in active juvenile and adult individuals to correspond to echolocation pulses with hyolaryngeal morphology at each postnatal stage. Results We found that the sagittal crests of the cricoid cartilage separated the dorsal cricoarytenoid muscle in horseshoe bats, indicating that this unique morphology may be required to reinforce the repeated closure movement of the glottis during biosonar pulse emission. We also found that the cricothyroid muscle is ventrally hypertrophied throughout ontogeny, and that the cranial laryngeal nerve has a novel branch supplying the hypertrophied region of this muscle. Our bioacoustic analyses revealed that the peak frequency shows negative allometry against skull growth, and that the volumetric growth of all laryngeal cartilages is correlated with the pulse peak frequency. Conclusions The unique patterns of muscle and innervation revealed in this study appear to have been obtained concomitantly with the acquisition of tracheal chambers in rhinolophids and hipposiderids, improving sound intensity during laryngeal echolocation. In addition, significant protrusion of the sagittal crest of the cricoid cartilage and the separated dorsal cricoarytenoid muscle may contribute to the sophisticated biosonar in this laryngeally echolocating lineage. Furthermore, our bioacoustic data suggested that the mineralization of these cartilages underpins the ontogeny of echolocation pulse generation. The results of the present study provide crucial insights into how the anatomy and development of the hyolaryngeal apparatus shape the acoustic diversity in bats.

Musculoskeletal morphogenesis supports the convergent evolution of bat laryngeal echolocation

“…Bats have also been reported to learn vocalizations, including echolocation frequencies, from their mothers [36]. Furthermore, it has been reported that ultrasonic sound generation at the larynx required least two weeks after birth in bats [37]. Considering these findings, bat larynx muscles may specialize into SFM postnatally facilitated by vocal learning similar to songbirds, although laryngeal anatomical architecture is established prenatally as reported in this study.…”

“…In the future, functional morphological analyses should be done to understand the relationship between laryngeal anatomy and acoustic properties of the ultrasonic sounds produced in bats [30]. Moreover, new insights into evolutionary history of the bat laryngeal echolocation should be obtained from observation of innervation pattern of the laryngeal muscles [30,37] and studies of molecular mechanisms that regulate the laryngeal morphogenesis and SFM formation in the echolocating bat lineages.…”