Anatomy and homology of the caudal auricular muscles in greater short-nosed fruit bat (<i>Cynopterus sphinx</i>)

Abstract

Bats can be phylogenetically classified into three major groups: pteropodids, rhinolophoids, and yangochiropterans. While rhinolophoids and yangochiropterans are capable of laryngeal echolocation, pteropodids lack this ability. Delicate ear movements are essential for echolocation behavior in bats with laryngeal echolocation. Caudal auricular muscles, especially the cervicoauricularis group, play a critical role in such ear movements. Previously, caudal auricular muscles were studied in three species of bats with laryngeal echolocation, but to our knowledge, there have been no studies on non-laryngeal echolocators, the pteropodids. Here, we describe the gross anatomy of the cervicoauricularis muscles and their innervation in Cynopterus sphinx by using diffusible iodine-based contrast-enhanced computed tomography and 3D reconstructions of immunohistochemically stained serial sections. A previous study on bats with laryngeal echolocation reported that rhinolophoids have four cervicoauricularis muscles and yangochiropterans have three. We observed three cervicoauricularis muscles in the pteropodid C. sphinx. The number of cervicoauricularis muscles and their innervation pattern were comparable to those of non-bat boreoeutherian mammals and yangochiropterans, suggesting that pteropodids, and yangochiropterans maintain the general condition of boreoeutherian mammals and that rhinolophoids have a derived condition. The unique nomenclature had been previously applied to the cervicoauricularis muscles of bats with laryngeal echolocation, but given the commonality between non-bat laurasiatherians and bats, with the exception of rhinolophoids, maintaining the conventional nomenclature (i.e., M. cervicoauricularis superficialis, M. cervicoauricularis medius, and M. cervicoauricularis profundus) is proposed for bats.

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

“…To visualize the cartilage and muscle for postnatal individuals (P0, 14, 21, 30, 45, and adult), the diffusible iodinebased contrast-enhanced CT technique [51] was conducted. This method has been shown to effectively allow us to non-destructively investigate the morphology of bat soft tissues in detail [36,37,[85][86][87][88]. Specimens were stained with 1% iodine in ethanol for at least 48 hours and up to 14 days.…”

Caudal auricular muscle variations and the evolution of echolocation behavior in pteropodid bats

“…The 3D reconstruction of adult C. sphinx previously reported by Chi et al . [ 5 ] is shown in Fig. 3

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“…1 , 2 , and 3 ). We previously reported three cervicoauricularis muscles in the common ancestor of laurasiatherians [ 5 ]. Other studies have reported four muscles in rhinolophoid bats [ 19 , 20 ].…”

“…Other studies have reported four muscles in rhinolophoid bats [ 19 , 20 ]. As C. sphinx and yangochiropteran bats have three muscles, we proposed that the ancestral condition of bats is three and that the four-muscle condition is only a derived condition [ 5 ]. As only one pteropodid species has been studied so far [ 5 ], further investigation of other pteropodids is necessary to confirm this tentative hypothesis.…”

“…In this study, we used the muscle nomenclature proposed by Chi et al . [ 5 ]. M. cervicoscutularis , which attaches to the scutiform cartilage, was omitted from this study because of its negligible role in the ear movements of bats [ 19 , 20 ].…”