Insectivore bats are perfectly adapted to hunt for insects at night using an elaborate biosonar system for echolocation. To avoid sonic interference in situations where many bats fly together, some bats modulate the frequencies of the emitted acoustic signals. Mexican free-tailed bats (Tararida brasiliensis) live in huge colonies with more than a million individuals. The bats communicate amongst themselves with the help of at least 15 social calls that have different meanings, but not all of them have been identified. To decipher the function of one of the unexplored calls, named ‘sinusoidal frequency modulated (sinFM)’, Aaron Corcoran and William Conner from Wake Forest University and the University of Philadelphia, USA, performed field observations and elaborate playback experiments. They recently published their discovery that these bats send out jamming signals deliberately to paralyse the biosonar of their bat competitors in Science magazine.
As it was only possible to record sinFM signals when another bat was generating ultrasonic sounds during the final stage of an attack (known as the ‘feeding buzz’), the scientists speculated that the sinFM signals may be emitted to jam the calls of other bats to increase hunting success – some insects, such as the tiger moth, make ultrasonic clicks to confuse bat echolocation systems. To test this hypothesis, the duo recorded bats hunting at two foraging sites in Arizona and New Mexico using highly sensitive cameras and ultrasonic microphones to reconstruct the complex 3D flight paths of the bats and correlate them with the emitted calls. Indeed, they observed that hunting bats were usually off target when another bat was sending the jamming sound: the jamming call appeared to have disrupted the attacker's final approach. Interestingly, sinFM call duration and the number of sound intervals within a call (syllables) correlated with the duration of the competitors' feeding buzz. This finding suggested that the bats modulate their sinFM signals to optimize sonic interference and stimulate FM rate-sensitive auditory neurons in the rival's brain that process information about prey localization.
Next, the scientists performed sophisticated playback experiments to test the effect of the jamming signals during an attack. They encouraged bats to capture moths hanging on very thin fishing line and played different ultrasonic sounds while the bats pursued the insects. These artificially synthesized jamming calls deranged the bats only when they were emitted at precisely the right time and frequency, and prevented insect capture only when they overlapped with the rival's feeding buzz.
Corcoran and Conner have provided compelling evidence supporting the existence of a novel strategy in food competition that involves sonic interference. This type of jamming has only been observed in Mexican free-tailed bats so far. However, other animals that rely on echolocation, such as other bat species or toothed whales, may also employ jamming sonar to trick their rivals.
