The ears of anurans (frogs) consist of a tympanum, a middle ear and an inner ear. The tympanum vibrates in response to sound waves and transmits these movements to the middle ear. Touching the tympanum is a bony ossicle callQd columella or stapes, the opposite end of which touches the membrane of the oval window, which stretches between the middle and inner ear. High frequency (1000 — 5000 Hz) sounds strike the tympanum and are transmitted via stapes and cause pressure waves in the fluid of the semicircular canals. These waves in the inner ear fluid stimulate receptor cells. A second ossicle, the operculum, also touches the oval window. Substrate transmitted through the front appendages and the pectoral girdle causes operculum to vibrate. The resulting pressure waves in the inner er stimulate a second patch of sensory receptor cells that is sescitive to low frequency( 100 to 1000 Mz) sounds Muscles attached to the operculum and columella can lock either or both of these ossicles, allowing a frog to screen out either high or low frequency sounds. This mechanism is adaptive because frogs use low and high frequency sound in different situations. For example, mating calls are high frequency sounds used for only part of the year (during breeding season). At other times, low frequency sound may wam of approaching predators. Fig. 2.23
Give an account of salient features of ear in amphibia, reptiles, birds and, mammals.
The ear in anurans (frogs, toads) is composed of a tympanum, a middle ear, and inner ear. Caudate amphibians (salamanders) lack a tympanum and middle ear (like fishes). They have no mating calls, and the only sound they hear are probably transmitted through the substratum and skull to the inner ear. The inner ear in fishes and amphibia is similar in structure.
The structure of reptilian ears varies. The ears of snakes lack a middle ear and a tympanum. A bone of the jaw articulates with the stapes and receives vibrations from the substratum. In other reptiles (lizards and crocodiles), a tympanum may be on the surface or in a small depression in the head.
Hearing is most developed in birds. Loose, delicate feathers cover the external ear opening. Middle and inner ear are similar in structure to those of mammals.
Sense of hearing was also important to early mammals as is in present day mammals. Adaptations include an ear flap (auricle or [Anna) and the auditory tube (external auditory canal), leading to the tympanum or ear drum that directs sounds to the middle ear. In mammals the sensory structure of the inner ear is long and coiled, called cochlea, providing more surface area for receptor cells. Fig. 2.24.
Figure 2.24Anatomy of the Human Ear. Note the outer, middle and inner regions. inner ear includes the semicircular canals which are involved with equilibrium, and the cochlea, which is involved with hearing