Echolocation jamming

Echolocation (or sonar) systems of animals, like human radar systems, are susceptible to interference known as echolocation jamming or sonar jamming. Jamming occurs when non-target sounds interfere with target echoes. Jamming can be purposeful or inadvertent, and can be caused by the echolocation system itself, other echolocating animals, prey, or humans. Echolocating animals have evolved to minimize jamming, however, echolocation avoidance behaviors are not always successful.

Echolocating animals can jam themselves in a number of ways. Bats, for example, produce some of the loudest sounds in nature, and then they immediately listen for echoes that are hundreds of times fainter than the sounds they emit. To avoid deafening themselves, whenever a bat makes an echolocation emission, a small muscle in the bat's middle ear (the stapedius muscle) clamps down on small bones called ossicles, which normally amplify sounds between the ear drum and the cochlea. This dampens the intensity of the sounds that the bat hears during this time, preserving hearing sensitivity to target echoes.

Jamming can occur if an animal is still producing a sound when an echo returns, for example, from a nearby object. Bats avoid this type of jamming by producing short sounds of 3-50 ms when searching for prey or navigating. Bats produce progressively shorter sounds, down to 0.5 ms, to avoid self-jamming when echolocating targets that they are approaching. This is because echoes from nearby targets will return to the bat sooner than sounds from distant targets.

Another form of jamming occurs when an echolocating animal produces many sounds in succession and assigns an echo to the wrong emission. To avoid this type of jamming, bats typically wait enough time for echoes to return from all possible targets before making the next sound. This can be seen clearly when a bat attacks an insect. The bat produces sounds with progressively shorter time intervals, but always allowing enough time for sounds to travel to the target and back. Another way bats overcome this problem is by producing successive sounds with unique time-frequency structures. This allows bats to process echoes from multiple emissions at the same time, and to correctly assign an echo to its emission using its time-frequency signature.

Like electric fish, echolocating animals are susceptible to jamming from other animals of the same species emitting signals in the nearby environment. To avoid such jamming, bats use a strategy also employed by electric fish to avoid this jamming: a behavior known as jamming avoidance response (JAR). In a JAR, one or both animals change the frequency of their sounds away from that used by the other animal. This has the effect of allowing each animal a unique frequency bandwidth where jamming will not occur. Bats can make this adjustment very rapidly, often in less than 0.2 seconds.

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