ECHOLOCATION IN BATS AND WHALES
Echolocation (biosonar) is a method used by animals (mammals) like bats, whales and dolphins to locate objects or other organisms. These animals emit sounds (calls) out to the environment and then listen to the echoes returning from various objects in the environment. They use these echoes to locate, range and identify the objects1. The word BIOSONAR comes from the words BIO-life and SONAR -"SOund Navigation And Ranging"2. It’s used for in underwater for location of other vessels (objects) and navigation2. For the mammals it’s used for navigation and hunting1.
The first fossil of fully developed bats was dated 50 million years ago, meaning that the evolution of bats was earlier during the evolution of placental mammals4. Bats are mammals that have their fore limbs modified to become wings and are the only flying mammals. Most bats (70%) feed on insects while others feed on fruits and their juices while others are carnivorous3. Most bats are nocturnal (fly at night) and use echolocation to be able to navigate4. A flying bat has the ability to use sounds of between 50 000 and 200 000 vibrations per second sending out emitting the sounds at short bursts4. They send out these sounds every twenty or thirty times in every second and has so great hearing power that from the echo produced by the signals bouncing on an object, the bat can judge the position of the object(s) around it and how fast the prey is moving if flying4. Most bats first receive the echo of a signal before emitting another one. As the distance between the bat and an object reduces, the duration of the echo bouncing back reduces. By increasing and reducing the number of sounds emitted the bat is able to track down the prey with great accuracy1, 4.
Food in the mouth posses a challenge to the bat as it can lose the use of its senses and be not able to emit sounds4. Bats have evolved a way of overcoming this by emitting sounds through their noses that have structures which concentrate the stream of sound acting like small megaphones4. Bats ears are quite sensitive and are adjusted in a way to pick signals. The face of a bat thus looks like sonar equipment, nose has large protrusions for sending and picking signals, plus elaborate ears to pick the signals4. The patterns in the nose and the ear are conspecific so each an every species produce its own unique call. The receptors are able differentiate particular sounds being able to pick out signals from other bats and be able to avoid them even in the dark4. Even though bats have managed to use this method to hunt there are other animals that are able to counter this mechanisms and escape them. In
Whales use biosonar as they live in water where vision can be impaired by water turbidity and absorption of light rays1. Toothed whales produce a focused beam of high-frequency emissions towards the direction that their heads are facing. Sounds are generated by blowing air out of the bony nares via the phonic lips. These sounds bounce off the dense concave cranium bone in addition to the air sac at its base. It’s modulated by the large fatty organ referred to as the 'melon'1. The clicks in most toothed whales are usually in a series, with the sperm whale producing individual clicks1. In a similar way as the bats the whales are able to not only identify an object but also with the quality of the echo, the type of obstacle ahead. Echo is picked by the lower jaw and transmitted by a fat layer to the inner ear1. Like bats, when whales approach a target they reduce the noise and so reduce the echo’s strength to protect themselves from strong echoes1.
This way bats and whales are able to adapt to find direction and hunt for prey in an environment that would otherwise be unfriendly to them.
1. Wikipedia contributors. Animal echolocation [Internet]. Wikipedia, The Free Encyclopedia; 2006 May 7, 18:16 UTC [cited 2006 May 9]. Available from: http://en.wikipedia.org/w/index.php?title=Animal_echolocation&oldid=52012189.
2. Wikipedia contributors. Sonar [Internet]. Wikipedia, The Free Encyclopedia; 2006 May 9, 03:58 UTC [cited 2006 May 9]. Available from: http://en.wikipedia.org/w/index.php?title=Sonar&oldid=52261714.
3. Wikipedia contributors. Bat [Internet]. Wikipedia, The Free Encyclopedia; 2006 May 8, 03:58 UTC [cited 2006 May 9]. Available from: http://en.wikipedia.org/w/index.php?title=Bat&oldid=52191376.
4. Knight, R. BCB Biodiversity chapter 2 Theme and Variation
(Cited 2006 May8) http://planet.uwc.ac.za/nisl/biodiversity/Chapter2/page_202.htm
VINCENT MUCHAI WAIRIMU
Private Bag X17 Bellville