Reptilian Hearing

The basic auditory structures are the same for sauropsids: bilateral tympanic membranes on the body’s surface, on the head; a middle ear filled with air and bridged by a single ossicle or other conductive tissue; and an inner ear with auditory papillae anchored in a basilar membrane (Wever, 3, 977; Tucker n.p.). Some reptiles have a meatus protecting their tympani, while in birds, the feathers cover this structure (Wever, 3). Further, not all reptiles have tympanic ears (Tucker, 5). In contrast to mammals, the middle ear has a single ossicle, the stapes, transmitting the mechanical signal from the tympani to the inner ear (Tucker, 2).

Uses and Advantages of Hearing in Reptiles:

• Reptiles use hearing as a way to aid in threat detection, baseline environmental awareness, sexual mating calls, detection of prey and avoiding predation, and for mate selection and attraction.

Evolution

In comparison to fish, reptilian hearing evolved to covers a greater frequency range, among the species that do hear. This likely reflects selection for terrestrial conditions. There is speculation that auditory canals evolved from vestibular canals via an infolding of epithelial tissues from vestibular structures (Wever, 997).

The reptilian-and-bird inner ear exemplifies parallel (convergent) evolution. The key inner ear structure for reptiles and birds are auditory papillae, which feature hair cells anchored in the basilar membrane. The hair cells themselves do not not move in the fluid, but rather the cell bodies to, transmitting that torsion to the hair cells (Wever, 977). (The analogous structure in mammals is the organ or Corti.) The basilar membrane is stretched out, and different frequencies are parsed at different locations along its length (Wever, 977-978). Wever speculates that these structures first evolved for vestibular purposes and were later repurposed in the adaptation of hearing (Wever, 978). He noted the different origins of hearing apparatus in different groups of animals thus: “It appears that the vertebrate cochlea emerged independently out of the non-auditory labyrinth in three different vertebrate groups: in fishes, amphibians, and reptiles.” (Wever, 980). These independent evolutions mean that the structures in fish, reptiles-birds, and mammals are homologous adaptations at the large-group level (and we add insects, which he omits here).

Avian Evolution

While hearing evolved in reptiles before it did in birds, birds are far more specialized for it than their older relatives. Among birds, the behavioral use of hearing and sound production is much richer. Their pitch range is generally 1-3 kHz, with some species exceeding this (Dooling, 547). In many species, detection thresholds are similar to that of mammals, but some owl species have measured detection thresholds in the range of -20 to -30 db (Dooling, 549). Dooling notes that birds have a wide variety of behavior responses to hearing, even with very similar physical hearing capacities (Dooling, 553). The birds with the most acute sound detection were predatory owls (Dooling, 549), which shows adaptation to prey detection by a long-distance sense that can operate in low-light conditions. Song birds, on the hand, use sound production socially, for mate selection, protecting nests, and warning the others in their social group. This requires a sophisticated hearing sense. The basic elements of song is species-specific, but groups of same-species birds studied show variation in different regions, indicating that a component of birdsong is social (Dooling, 553). Dooling supposed that central auditory systems could explain this difference, but we point out that studies would need conditions that could separate CNS differences from differences in learned behavior — Tinbergen’s second question about the experiences of individual animals (Urry et al. 835). It’s interesting that the same basic ear in reptiles and birds, and very similar psychophysics in birds, is used in such diverse ways across the species in this group. That makes hearing in reptiles and birds a great example of how the unity and diversity of life work in evolution.

Uses and Advantages for Hearing in Birds:

• Songbirds have socially learned songs built from basic species vocalizations. Other uses for hearing include mating, and for predatory birds, prey detection. There is some evidence of more sensitive hearing in Strigiformes (owls), which are predatory.

• Many bird species show the ability to sound-locate between birds of same species (Dooling, 1991).

• Birds use hearing to their advantage during mate attraction, threat displays, social cohesion, prey detection, and for the avoidance of predators.

Evolution of Avian Hearing

The hearing of birds evolved from reptiles and has the same basic structures. Birds hearing has evolved a wider psychophysical range and a much wider repertoire of social uses than displayed in reptiles. In addition to the differences in social use of hearing, avian middle ears have a distinctive feature: the columella, a cartilagenous rather than bony structure. Tucker reports that the common amniote ancestor lacked a tympanic middle ear, which accounts for the variety of analogous, convergently evolved features of middle ears (Tucker n.p.).