Hearing
The external ear is absent in fishes and amphibians but is present in reptiles, namely the lizards and crocodilians, which indicates that reptiles were the first animals to develop such a structure. The external ear is not to be confused with the fleshy structure that most people call the ‘ear’, which is in fact the pinna. This is typically a cartilaginous flap surrounding the external ear of most mammals, but is absent in reptiles.
The reptilian ear varies considerably from species to species, as does its capabilities but generally the ear is divided into three adjoining compartments; the external, middle and inner ear. Typically the tympanic membrane (or eardrum) is the most external structure of the reptilian ear. This structure is a thin, circular membrane located near the back of the reptiles’ head and forms the outer boundary of the middle ear cavity. The eardrum is connected to the inner ear by a single bone called the columella, which is the equivalent of the stapes in mammals and is directly involved in the conduction of sound vibrations from the eardrum to the inner ear. Interestingly, the jaw-bones of reptiles possess two additional bones known as the quadrate and articular, these bones are not found in the jaws of mammals but over the course of mammalian evolution, had lost their functional purpose in the jaw joint and were repurposed within the middle ear, forming a chain of three bones known as the stapes, incus (quadrate) and malleus (articular) (or stirrup, anvil and hammer – collectively known as ossicles). This allowed mammals greater efficiency in the transmission of sound waves resulting in more acute hearing.
The reptilian ear varies considerably from species to species, as does its capabilities but generally the ear is divided into three adjoining compartments; the external, middle and inner ear. Typically the tympanic membrane (or eardrum) is the most external structure of the reptilian ear. This structure is a thin, circular membrane located near the back of the reptiles’ head and forms the outer boundary of the middle ear cavity. The eardrum is connected to the inner ear by a single bone called the columella, which is the equivalent of the stapes in mammals and is directly involved in the conduction of sound vibrations from the eardrum to the inner ear. Interestingly, the jaw-bones of reptiles possess two additional bones known as the quadrate and articular, these bones are not found in the jaws of mammals but over the course of mammalian evolution, had lost their functional purpose in the jaw joint and were repurposed within the middle ear, forming a chain of three bones known as the stapes, incus (quadrate) and malleus (articular) (or stirrup, anvil and hammer – collectively known as ossicles). This allowed mammals greater efficiency in the transmission of sound waves resulting in more acute hearing.
Image sourced from evolution.berkeley.edu
Reptilian hearing is indeed inferior to that of mammals, even more so in snakes which lack an external opening and the tympanic membrane all together but just how much can a snake hear? Snakes lack external structures for sound conduction leading many to believe that snake were deaf but the inner ear remains well developed. Snakes are anatomically peculiar in this respect, such an evolutionary trait is likely reflective of fossorial or aquatic ancestry. It is often assumed that snakes respond to ground vibrations which transmits through the head and ultimately stimulate the receptors responsible for hearing, but studies have demonstrated that snakes are also able to respond to airborne and waterborne sounds as well. The quadrate and articular bones of the jaw in snakes may aid in transmitting sound waves to the columella and ultimately contribute to the hearing of airborne sound waves. The notion that snakes are deaf is not true but hearing may be restricted to a limited range of sound frequencies, regardless of these limitations the sensitivity of snakes to vibration and sound is remarkably good.
©2014 Cat Read