Whispering Bats Catch Moths That Can Hear Bat Sonar

Sep 24, 2011
Albert Burchsted

Barbastelle Bat Sonar (Green) Reflection (Yellow) From Moth - Albert Burchsted

By emitting quiet hunting chirps, the barbastelle bat can locate and capture moths that normally evade bats hunting with loud sonar.

The detection of flying prey at night is a challenge to visual hunters. Bats have overcome this challenge by evolving the ability to use ultrasonic sonar to locate, home in on, and catch their prey. From the time that bats first began to hunt by sonar, insects have been developing methods to counteract this elegant method of hunting. As insects developed mechanisms to hear bats, bats had to develop better methods of locating the insects, and there has been an arms race between bats and insects since.

Methods of Avoiding Bats

Today many insects have the ability to hear the bats' ultrasonic pulses. Some of these insects are toxic or inedible and have evolved methods of identifying their toxic natures to bats. In this way the insects are not killed and the bats are not harmed. Many of the edible insects also hear the bats and avoid capture by turning away from the bat's approach path then dropping out of the sky if the bat begins to emit its very loud (100 – 110 decibel) zeroing in string of chirps. A few moth species will emit their own ultrasonic pulses that mask the echoes of the bat's homing in sonic pulses.

The Whispering Bat

At least one species of bat, the barbastelle (Barbastella barbastellus), found in England and Europe locates prey by emitting very low intensity pulses that are barely loud enough for the insects to hear. These “whispering bats” have hearing only a little better than that of other bats and with a sonar volume is only one percent as loud (75 to 80 decibels) as that of other bats (100 to 110 decibels), the barbastelle can not detect insects until they are 3.5 meters (11 feet), instead of 30 meters (100 feet), from the insect.

Short distance insect detection would be a disadvantage for most bats, but the barbastelle feeds almost exclusively on yellow underwing moths (Noctua pronuba) – a species that can hear the sonar of and evade normal bats first by flying directly away from the bat, then by dropping out of the air. Because the barbastelle enters its final attack path almost as soon as it detects a moth, and the bat's sonar never becomes loud enough to trigger the moth's dropping response, the moth does not react appropriately to the bat's sonar and the bat captures it.

Under most circumstances, a bat at 30 meters (100 feet) cannot detect the reflection from the moth, will alter its flight path several times in response to peripheral insect detection, and will probably not locate an insect flying directly away from it.

With it hunting using a vocalization only 1/100th the volume of a normal bat and immediately aiming for the moth when it detects prey at a distance of 10 meters, the barbastelle enjoys a high capture rate. The moth cannot determine the bat's distance by the intensity of the bat's sonar pulses and simply flies directly away from the bat, which zeroes in on the insect and never emits a vocalization loud enough to trigger the moth's final escape strategy to stop flying and drop to the forest floor.

Winning the Arms Race

The barbastelle is a rare example of a predator obtaining the upper hand in the ages old arms race between bats and moths. This is unusual because when the prey loses, it is eaten. When the predator loses, it misses a small meal and has the opportunity to look for another.

Although the bat is at a slight disadvantage because it cannot detect moths until it is almost too late to home in on them, the moths are at a greater disadvantage because if they evolve a more sensitive response to bat vocalizations in order to avoid being eaten by barbastelles, they will be responding to leaf movement and other bats at far greater distances than the barbastelle.