An evolutionary battle fought over the air waves

1218moth.jpgResearchers from the University of Bristol have discovered a hitherto unknown behavioural adaptation which enables a species of moth to detect the echolocation signals used by bats to prey on it.

Bats use echolocation (or biological sonar) to navigate and to forage for prey. Echolocation signals consist of emitted pulses of ultrasonic waves. When foraging, bats emit signals of specific frequencies during the search phase. Once potential prey has been located, the bat increases the amplitude (loudness) and frequency (pitch) of its signals as it homes in on its target, so that it can assess the prey.

Many bats are insectivorous, feeding on moths and other insects. The yellow underwing moth (Noctua pronuba, left) is one insect species preyed upon by bats. N. pronuba has extremely simple ears, consisting of just two vibration-sensitive neurons attached to the tympanic membrane (eardum). The ear is most sensitive to sounds with a frequency of between 15-25kHz (kiloHertz, thousands of cycles per second); this is the frequency range of the ultrasonic signals generated by most bat species and, in fact, the moth ear appears to have evolved for a single purpose – to detect bats’ signals. In response to the signals, moths behave in one of two ways – they either fly directly away from the source of the sound, or initiate a series of complex manoeuvres, consisting of loops, spirals and dives, in an effort to evade the predator.

The new work, led by Professor Daniel Robert of Bristol University’s School of Biological Sciences, shows that the moth’s ears can mechanically tune up, changing the frequency to which they are most sensitive. This makes the bat’s high frequency ultrasonic pulses audible to the moth, which can then detect when a bat is homing in on it. The finding is somewhat surprising, because it shows that an apparently simple structure such as the insect ear is actually remarkably sophisticated. Robert’s team also found that, following the initial detection of a bat’s signal, the moth’s ears remain tuned to that frequency for several minutes, in case the bat should return.

“Because the moth cleverly tunes its ear to enhance its detection of bats, we must now question whether the bat in turn modifies its calls to avoid detection by the moth. In view of the vast diversity of bat calls, this is only to be expected,” says James Windmill, lead author of the Current Biology paper in which the findings are published. “To date, this phenomenon has not been reported for insects or, in fact, for any other hearing system in the animal kingdom. These findings change our understanding of the co-evolution of bats and moths and have implications for the hearing of many other animals.”

Other research, published recently in the Proceedings of the Royal Society B, shows that bats can indeed alter the frequency of their signals, but for another reason. Nachum Ulanovsky and his colleagues at the University of Maryland presented bats with recordings of echolocation signals. They found that, in response, the bats increased the frequency of their emitted ultrasound signals, so that it was different from that of the signals being played back to them. This constitutes a ‘jamming avoidance response’; the frequency of the bat’s call is modulated to prevent interference between the signals produced by other bats in the immediate area. This occurs within less than 200 milliseconds (one-fifth of a second).

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