Anatomically, the inner ear consists of the osseous labyrinth and membranous labyrinth.
Osseous labyrinth
The osseous labyrinth consisted of a series of cavities within the petrous part of the temporal bone (Figs. 4, 5, and 6) that included the vestibule (Fig. 4), semicircular canals (Fig. 4), and the cochlear canal (Figs. 4, 5), which communicated with the vestibule via the internal acoustic meatus (Figs. 4, 6).
Cerebellar surface of the petrous part of the temporal bone
The inner ear was separated from the cranial bone, and its cerebellar surface was relatively concave and characterized by two large depressions (Fig. 5/A). The first was the shallow internal acoustic meatus (Fig. 5/B), which was caudal and ventral overlying the cochlea. The second depression was the subarcuate fossa, which was lateral and caudal to the internal acoustic meatus (Figs. 4; 5/A).
The internal acoustic meatus was located on the cerebellar surface of the petrous part of the temporal bone and acted as a link between the cranial cavity and the inner ear (Fig. 5/B). At the bottom, at the fundus of the internal acoustic meatus, four foramina for facial and vestibulocochlear nerves were detected (Figs. 5/C,H; 6/2,3). A clear transverse crest appeared oblique, caudal, and dorsal, which divided these four foramina into two groups: two placed dorsal and rostral, and two placed ventral and caudal (Fig. 5/E). At the restoral foramen of the dorsal foramina, the facial nerve passes through the mastoid part of the temporal bone (Figs. 2B/A; 5/C; 6/4), which is the largest. The other foramen was situated caudally to the utricle and the ampulla of the anterior SC nerve (Figs. 5/D; 6/2,3). The cochlear nerve passed through the restoral foramen of the ventral foramina (Figs. 2B/B; 5/H) and the other foramen, which was situated caudal to the ampulla of the posterior semicircular duct nerve (Fig. 5/J). A canal for the greater petrosal nerve was detected (Fig. 6/5).
Cochlea
The cochlea was a bony cone positioned within the petrous part of the temporal bone; it consisted of the osseous cochlea surrounding the cochlear membranous duct (Fig. 6/1), which was formed spirally upward around a central column of the bone, the modiolus. The modiolus had a thin bony spiral lamina (Fig. 2B/C). The primary and secondary osseous spiral lamina were located in the petrous part of the temporal bone (Fig. 7/1, 2). The primary osseous spiral lamina was present throughout the basal turn (Fig. 7/1), extended to the entire cochlear length, and projected from the cochlear canal’s inner wall. The secondary osseous spiral lamina was projected from the cochlear canal outer wall (Fig. 2B/D, E).
The length of the basilar membrane was 40.5 mm. The radius of curvature of the camel cochlea at the base was 3.894, the radius at the apex was 0.429, and the radii ratio was 9 (Figs. 1; 2A). The cochlea number of turns was about three turns, a rotation of 1080° (Fig. 2A). The cochlear width at the lowest cochlear turn was 11 mm, the height was 6 mm, and the cochlea shape index was 0.55; these parameters made the cochlea of the camel flat type cochlea (Fig. 2B). The diameters of the whorl and the tube of the cochlea in front of the round window were 4 mm.
Semicircular canals
The camel had three semicircular canals: the anterior, lateral, and posterior (Figs. 8/A, B, C; 8/2, 3, 4), which were located in the petrous part of the temporal bone. Each semicircular canal formed two-thirds of a circle. The anterior semicircular canal had the most circular overall shape, the lateral semicircular canal was nearly circular or irregular, and the posterior semicircular canal was oblong (Figs. 3/A, B, C; 8/1, 2, 3).
This study showed the orthogonality of the three semicircular canals (Fig. 8/1, 2, 3). There were limited specimens for this measurement, so the variation in the same species needs more investigation. These canals are approximately 90 degrees apart. The anterior duct was oriented transversely, the caudal duct sagittally, and the lateral duct horizontally. In the same horizontal plane, the anterior and lateral ampullas connect to the vestibule (Fig. 8/D, E). However, the caudal ampulla is located significantly below or ventral to the common crus (Fig. 8/F, J). The radius of curvature of the anterior semicircular canal was 4.75, that of the posterior canal was 4.13, and that of the lateral canal was 3.63. The average radius of curvature of a semicircular canal was 4.17. The average radius of curvature of a semicircular canal was 4.17.
A confluence appeared between the caudal arm of the lateral semicircular canal and the inferior arm of the posterior semicircular canal (Fig. 8/H). This confluence did not result in a secondary crus commune where the two semicircular canals became close. The anterior semicircular canal was fitted to the outer restoral edge of the fossa (Fig. 8/2) and was directed laterally, and the fossa extended through the anterior semicircular canal into the mastoid and vestibular labyrinth regions of the petrosal bone. The vestibule occupied a relatively large volume in the labyrinthine cavity (Fig. 4). Although the relation between bony and membranous size is not 1:1, a large utricle and saccule were indicated in proportion with a large elliptical and spherical recess (Fig. 9/A, B, C, D).
Subarcuate fossa
The volume of the subarcuate fossa has measured the endocast approximated with the skull size, where our study found that the camel had a large-sized subarcuate fossa (Figs. 4; 5/A; 10/1; 11A). This fossa was located at the petrosal lobule of the cerebellar paraflocculus (Figs. 4; 11B). This fossa was situated above the internal acoustic meatus (Fig. 5/B) and separated from the latter by a raised bony ridge-like area (Fig. 5/E).
Discussion
The cochlea, vestibule, and three semicircular canals make up the inner ear of animals. The cochlea is responsible for hearing, while the vestibule and three semicircular canals are responsible for balance [7]. Although the camel shares the same inner ear outline structure and perception as other mammals, there are differences in the parameters compared to those of the other species. The camel's auditory and locomotor physiology is comparable to other mammals [7].
The length, number of turns, and curvatures of the cochlear spiral affect the efficacy of hearing in animals [8, 9, 24, 25]. Taking body size into account, the camel’s cochlea has a higher score in those parameters than the ground-dwelling mammals studied in previous studies [8, 9]. Therefore, those parameters help the camel to cope with sound propagation that characterizes the desert environment, such as spreading and frequency- and humidity-related attenuation of abiotic noise.
There is a strong relationship between basilar membrane length and high—and low-frequency hearing limits for different species. The ability to perceive at a low frequency is necessary due to the physical characteristics of sound propagation and abiotic noise in the desert [1]. Long basilar membranes have been linked to a decrease in audible frequencies [8, 26]. A high radii ratio improves low-frequency sensitivity [9]. With a basilar membrane length of 40.5 mm, the camel has high measures of both the basilar membrane and radii ratio among various animals, compared to the cow’s 38 mm and the cat’s 22–23 mm [8]. Camel’s radii ratio is 9, cat’s is 6.2, cow’s 8.9, and human’s 8.2. With the characteristics of the desert, this morphometrics qualifies the camel for high perception at a low-frequency of demand [1].
The current study noted that, in comparison to other animals, the camel has 3.0 cochlear turns with a rotation of 1080°; carnivores have 3.0 turns and horses have 2.5 turns, pigs have 4.0 turns, and ruminants have 3.5 turns [27]. The camel's cochlear coil degree has been rated as high, which helps to increase its octave range of hearing [8]. We contend that this satisfies the camel’s need to distinguish between wide ranges of pitches to deal with the abiotic cacophony of the desert. The cochlear width of the camel was 11 mm, compared to 7.5 mm for sheep and 5.5 mm for calves [28].
Like every mammal, the camel has anterior, lateral, and posterior semicircular canals [29]. The function of these canals is the sense of balance [18], and there is a sizable variation in their size among animals associated with functional variations in locomotion [30]. This study supports this hypothesized association by relating the camel’s semicircular canal shape to the need for movement in the environment and comparing the results to those of other species.
Due to the force of sandstorms and its large body mass, it is difficult for the camel to maintain its balance of movement in the desert. We can provide information on the camel's speed, vestibular sensitivity, and equilibrium based on the measurement of the radius of curvature of the semicircular canals performed in this study.
The correlation between agility and the lateral semicircular canal was the lowest. The camel’s lateral semicircular canals have a radius of curvature of 3.63 mm. It is the largest in animals, as evidenced by the sizes in horses (3.23 mm), cows (2.35 mm), gorillas (3.05 mm) [31], bovines (2.15 mm), and cats (1.39 mm) [32]. Additionally, the camel’s semicircular canals are oriented at 90 degrees to one another, enabling the vestibular system to function at its best [13]. The radius of curvature of the semicircular canals has been linked to canal sensitivity and animal agility [11].
The junction of the inferior arm of the posterior canal with the caudal arm of the lateral canal was a notable characteristic of the camel semicircular canals. Until now, the role of this union was unknown. This phenomenon, called semicircular canal dehiscence, can be present in humans. Normally, this condition is not present in extant animals but it exists in extinct mammals, and therefore researchers hypothesized that the bony confluence of the area mentioned above is accompanied by a confluence also in membranous ducts based on bony specimens available [33]. The development of a secondary crus commune is linked to the all other living mammals [34,35,36]. However, the camel's confluence does not produce a secondary crus commune. It was hypothesized that this feature in the camel is supported by a unique form of equilibrioception that is not present in any other living mammals.
The current study found that the camel is distinguished from other ruminants and large mammals by its large subarcuate fossa. This fossa housed the cerebellum's folloculonoduolar lobe, which is thought to be a component of the “vestibulocerebellum” concept [37]. As a consequence, more complex motor coordination may be associated with the large subarcuate fossa and a larger folloculonoduolar lobe of the cerebellum [16]. Furthermore, the semicircular canal size may be affected by the subarcuate fossa, which is located between the canals [38, 39].
The cochlea of the camel is believed to be flat, as indicated by the cochlear shape index [20], with an aspect ratio implicated in the animal’s agility, and in fast taxa, the cochleae are broader than those of slow taxa [17].
In challenging hearing situations, the saccule aids the cochlea’s function and can contribute to the perception of loud, low-frequency tones [19]. In light of this, a camel’s big saccule indicates a highly sensitive hearing perception. These findings are supported by the relatively large size of the vestibule and the subsequently big size of the saccule.