Microbat

Differences from megabats

• Microbats use echolocation, whereas megabats do not typically. (The Egyptian fruit bat Rousettus egyptiacus is an exception, but does not use the larynx echolocation method of microbats, instead giving scientists the theory that it clicks using its nasal passages and back of its tongue.)
• Microbats lack the claw at the second finger of the forelimb. This finger appears thinner and almost bonded by tissue with the third finger for extra support during flight.
• Megabats lack tails, with the exception of a few genera such as Nyctimene, whereas this trait only occurs in certain species of microbats.
• The ears of microbats possess a tragus (thought to be crucial in echolocation) and are relatively larger than megabat ears, whereas megabat ears are comparatively small and lack a tragus.
• Megabat eyes are quite large, whereas microbat eyes are comparatively smaller.

Dentition

The form and function of microbat teeth differ as a result of the various diets these bats can have. Teeth are primarily designed to break down food; therefore, the shape of the teeth correlate to specific feeding behaviors.^[5] In comparison to megabats which feed only on fruit and nectar, microbats illustrate a range of diets and have been classified as insectivores, carnivores, sanguinivores, frugivores, and nectarivores.^[6] Differences seen between the size and function of the canines and molars among microbats in these groups vary as a result of this.

The diverse diets of microbats reflect having dentition, or cheek teeth, that display a morphology derived from dilambdodont teeth, which are characterized by a W-shaped ectoloph, or stylar shelf.^[7] A W-shaped dilambdodont upper molar includes a metacone and paracone, which are located at the bottom of the “W”; while the rest of the “W” is formed by crests that run from the metacone and paracone to the cusps of the stylar self.

Microbats display differences between the size and shape of their canines and molars, in addition to having distinctive variations among their skull features that contribute to their ability to feed effectively. Frugivorous microbats have small stylar shelf areas, short molariform rows, and wide palates and faces. In addition to having wide faces, frugivorous microbats have short skulls, which place the teeth closer to the fulcrum of the jaw lever, allowing an increase in jaw strength.^[8] Frugivorous microbats also possess a different pattern on their molars compared to carnivorous, insectivorous, nectarivorous, and sanguinivorous microbats.^[6] In contrast, insectivorous microbats are characterized by having larger, but fewer teeth, long canines, and shortened third upper molars; while carnivorous microbats have large upper molars. Generally, microbats that are insectivores, carnivores, and frugivores have large teeth and small palates; however, the opposite is true for microbats that are nectarivores. Though differences exist between the palate and teeth sizes of microbats, the proportion of the sizes of these two structures are maintained among microbats of various sizes.^[6]

Production of ultrasonic waves

Most microbats generate ultrasound with their larynx and emit the sound through their nose or mouth.^[12] Sound productions are generated from the vocal folds in mammals due to the elastic membranes that compose these folds. Vocalization requires these elastic membranes because they act as a source to transform airflow into acoustic pressure waves. Energy is supplied to the elastic membranes from the lungs, and results in the production of sound. The larynx houses the vocal cords and forms the passageway for the expiratory air that will produce sound.^[13] Microbat calls^ⓘ range in frequency from 14,000 to over 100,000 hertz, well beyond the range of the human ear (typical human hearing range is considered to be from 20 to 20,000 Hz). The emitted vocalizations form a broad beam of sound used to probe the environment, as well as communicate with other bats. At the molecular level, it has been found that CPLX1 is involved in this ultrasonic wave production.^[14]

Laryngeally echolocating microbats

Laryngeal echolocation is the dominant form of echolocation in microbats, however, it is not the only way in which microbats can produce ultrasonic waves. Excluding non-echolocating and laryngeally echolocating microbats, other species of microbats and megabats have been shown to produce ultrasonic waves by clapping their wings, clicking their tongues, or using their nose.^[9] Laryngeally echolocating bats, in general, produce ultrasonic waves with their larynx that is specialized to produce sounds of short wavelength. The larynx is located at the cranial end of the trachea and is surrounded by cricothyroid muscles and thyroid cartilage. For reference, in humans, this is the area where the Adam's apple is located. Phonation of ultrasonic waves is produced through the vibrations of the vocal membranes in the expiratory air. The intensity that these vocal folds vibrate at varies with activity and between bat species.^[15] A characteristic of laryngeally echolocating microbats that distinguishes them from other echolocating microbats is the articulation of their stylohyal bone with their tympanic bone. The stylohyal bones are part of the hyoid apparatus that help support the throat and larynx. The tympanic bone forms the floor of the middle ear. In addition to the connection between the stylohyal bone and the tympanic bone as being an indicator of laryngeally echolocating microbats, another definitive marker is the presence of a flattened and expanded stylohyal bone at the cranial end.^[10]

Microbats that laryngeally echolocate must be able to distinguish between the differences of the pulse that they produce and the returning echo that follows by being able to process and understand the ultrasonic waves at a neuronal level, in order to accurately obtain information about their surrounding environment and orientation in it.^[9] The connection between the stylohyal bone and the tympanic bone enables the bat to neurally register the outgoing and incoming ultrasonic waves produced by the larynx.^[11] Furthermore, the stylohyal bones connect the larynx to the tympanic bones via a cartilaginous or fibrous connection (depending on the species of bat). Mechanically the importance of this connection is that it supports the larynx by anchoring it to the surrounding cricothyroid muscles, as well as draws it closer to the nasal cavity during phonation. The stylohyal bones are often reduced in many other mammals, however, they are more prominent in laryngeally echolocating bats and are part of the mammalian hyoid apparatus. The hyoid apparatus functions in breathing, swallowing, and phonation in microbats as well as other mammals. An important feature of the bony connection in laryngeally echolocating microbats is the extended articulation of the ventral portion of the tympanic bones and the proximal end of the stylohyal bone that bends around it to make this connection.^[9]