Science of Hearing-III ( Hearing in Vertebrates)

Hearing in Fishes and Birds

Vertebrates are the animals with a developed vertebral column. They have well developed brain. The brain is situated in a box or cranium. All the vertebrates have ears for hearing. In some of the vertebrates the ears are visible and in some the ears are invisible. Majority of the vertebrates have prominent external ears. The shape and the size of the ears may vary depending on the environment where they are living.

How do fishes hear?

Fishes are the first vertebrates.  They are adapted for aquatic mode of life. Fishes don't have external ears but they do have ear parts inside their heads. They pick up sounds in the water through their bodies and in their internal ear. Among the aquatic animals, the hearing capacity of Whales and Dolphins are of interest. Whales do not have ears on the outside of their heads. Instead, they generally hear sounds through special structures in their jawbones. Dolphins hear through a sophisticated hearing sense which is located in small ear openings on both sides of the head.

Dolphins and whales use echolocation technique underwater to locate their prey. Dolphins communicate via clicks and whistles, and whales use low frequency moans or pulse signals. Echolocation helps them to determine not only the distance of an object but also the texture, shape and size of the object. It works because water is an excellent sound transmitter, which can transmit sound five times faster than air. 

How do birds hear?

Birds are adapted for aerial mode of life. They have to recognise the sound of wind as well as their predators and their group members. Therefore their hearing sense is very accurate. Their ears are funnel shaped to focus sound. The ears are located slightly behind and below the eyes, and they are covered with soft feathers for protection. The shape of a bird's head can also affect its hearing. The hearing range of birds is most sensitive between 1 kHz and 4 kHz, but their full range is roughly similar to human hearing. Among the birds, owls have the sharpest hearing ability. Pigeons are considered the best at it because they can hear sounds at exceptionally low frequencies.

How do Owls hear?

Owls are nocturnal and silent hunters. Owls, hunt at night, are able to locate even faint sounds with remarkable accuracy.  Owls can locate and capture prey by sound alone.  The owl's sensitive hearing is enhanced by its facial ruff, a concave surface of stiff dark-tipped feathers. The ruff functions as a reflector, channelling sounds into the ears. Once a sound is detected, the owl orients toward it and accurately pinpoints its location to within 1.5 degrees in both horizontal and vertical planes.

The signal used to determine whether a sound comes from the right, left, or straight ahead is the difference in time that it takes for a sound to reach each ear. When the sound source is dead ahead, no time differential occurs. Another signal is that  the difference in intensity of sound received by each ear, is used to localize a sound vertically. 

The owls' ears are linked to specialized cells contained within a discrete region of the midbrain. Each cell is sensitive to a unique combination of time and intensity differentials and responds only to sound issuing from one small area in space. The owl's brain thus contains a "neural map" of auditory space. 

How do Bats hear? 

Bats have a very sensitive hearing to cope with their nocturnal activity. The maximum hearing frequency for bats is 160KHz. Bats navigate around objects and locate their prey using echolocation. A bat will produce a very loud, short sound and assess the echo when it bounces back. Bats hunt flying insects; these insects return a faint echo of the bat's call. The type of insect and how big it is can be determined by the quality of the echo and time it takes for the echo to rebound. The pulses of sound produced by the bat last only a few thousandths of a second; silences between the calls give time to listen for the information coming back in the form of an echo. Evidence suggests that bats use the change in pitch of sound produced via the Doppler Effect to assess their flight speed in relation to objects around them. The information regarding size, shape and texture is built up to form a picture of their surroundings and the location of their prey. Using these factors a bat can successfully track change in movements and therefore hunt down their prey.