Info

Figure 12-6. Phylogeny of cochlear types among amniote species. Redrawn from (Manley and Koppl 1998) with permission.

Mono- Placentals, tremes Marsupials Turtles Tuatara

Lizards,

Snakes Crocodiles Birds

2 hair cell types (outer, inner), Elongati

Mono- Placentals, tremes Marsupials Turtles Tuatara

Synapsids (mammallike) reptiles

Lepidosaurs

Amniotes Tetrapods

Cochlear : amplifier

2 hair cell types (THC, SHC) Elongation ic middle ear

Lepidosaurs

Synapsids (mammallike) reptiles

Archosaurs

Amniotes Tetrapods

Cochlear : amplifier tetrapods, fish ears are internal organs, not open to the external environment, and there is no tympanic membrane. Sound is conducted to the hearing organs through tissue and bone. The labyrinth is made up of a series of sacs and canals, as in mammals. The semicircular canals (of which there are three in most fishes, except hagfish, which have only one, and lampreys, which have two) are oriented at right angles to each other in three different planes and are fluid filled. Each canal has an ampulla and crista, and the crista is composed of sensory cells called neuromasts. Neuromasts are covered by the cupula, a gelatinous membrane as in tetrapod cupulae. Fish also have the saccule and utricle and a lagena, an appendage of the saccule in fish, amphibians, and birds. In most fishes, the saccule is most likely primarily an auditory organ and the utricle primarily serves a vestibular function (Fay and Popper 1999).

Hearing in some fish also relies on an additional pathway; the swim bladder. This buoyancy organ in the body cavity of bony fishes is a gas-filled out-pocketing of the digestive tube that helps a fish maintain its depth and adjust its buoyancy. The swim bladder pathway to hearing is indirect: the gas-filled organ, or other gas bubbles near the ears, expand and contract in response to sound pressure, and this motion is transmitted to the otoliths.

Most amphibians share all the organs of the fish ear: the semicircular canals and their cristae, the saccule, lagena, and utricle. In terrestrial amphibians, reptiles, and birds, sound is conducted from the tympanic membrane to the inner ear by a single bone, rather than the three in higher vertebrates. In fish and aquatic amphibians, hair cells are found in the semicircular canals of the ear and along the lateral line. This raises the point that no one system is required for transmission of sound to the cochlea; the bony ossicles of the middle ear of mammals evolved as part of the evolution of jaws differently hinged than those of reptiles, for example. Mammalian ossicles transmit sound from the tympanic membrane to the cochlea, but this is not the only way, and it is not particularly clear what special advantage this system had. Perhaps it was just better than relying on the duller direct through-the-skull perception pathway, as bones were remodeled and recruited for the evolution of jaws and face. Maybe it did not matter, relative to the developmental constraints entailed by the importance of making jaws, so long as sound was transmitted somehow.

Fishes have evolved a large repertoire of sound-generating mechanisms that they use for attracting mates and during spawning. These include muscular vibrations of the swim bladder, pectoral girdle, and pectoral spines rubbing in the grooves of the pectoral girdle, plucking of enhanced pectoral fin tendons, or grinding of pharyn-geal teeth (Ladich 2000).

Fish are probably poor at localizing sound sources; because of the way sound waves reach the hearing apparatus, interaural time and intensity differences are effectively nonexistent (Fay and Popper 1999). In some species the hearing organs are connected by perilymphatic spaces, and sound conducted by the swim bladder reaches both ears at the same time.

Echolocation

A few animals, including dolphins and bats, "see" their environment by emitting sound waves and monitoring the reflected echoes as the sound bounces off whatever it hits. The suborder of bat, Microchiroptera of the order Chiroptera, both navigates and finds its prey by echolocating in this way. Approximately 800 species of bats belong to this suborder, and the sound they emit ranges from biosonar pulses to clicks and other calls. These bats are able to monitor the speed of flying objects— their prey—and their size, range, and elevation among other characteristics. Bats that do not echolocate (fruit-eating bats, for example) find their prey by vision. Bats are highly speciose (richly diversified) animals as reflected in the exotic variation in their craniofacial morphology (Figure 12-6); how much of this was specifically selected for echolocation is somewhat less clear.

Bats that echolocate make laryngeal or nasal emissions (Gobbel 2002; Springer et al. 2001; Teeling et al. 2002; Teeling et al. 2000). Teeling et al. have shown that echolocating bats are probably paraphyletic, with laryngeal echolocation probably evolving first and being lost later in different lines that have modified the details of their apparatus in various ways (Teeling, Madsen et al. 2002). Dolphins have sonar, but it evolved independently of bat echolocation and uses different means. Dolphins emit clicks, receive the echo with their jawbone (panbone), and apparently begin to interpret it with the "melon" in their forebrain. They may actually stun their prey with sonar.

Bats, including those that echolocate, share the same auditory apparatus found in most mammals; the outer ear that captures sound waves and directs them to the

0 0

Post a comment