BATS use sonar, or echolocation, to navigate complex environments, and also to forage and then accurately pinpoint the flying insects on which they prey. Insects in turn have evolved various counter-measures to evade capture. Some species have ears which are in tune to the echolocation signals, while others are capable of performing complex evasive flight maneuvers in response to the clicks produced by attacking bats.
Tiger moths have evolved the ability to produce ultrasonic clicks in response to attacking bats. However, the function of these clicks was unclear, although decades of research has led to a number of hypotheses. The clicks may act to startle attacking bats, or they may be an acoustic signal which warns them that the moth is unpalatable. A study published in today’s issue of the journal Science provides the first clear evidence for the third hypothesis – that the clicks interfere with (or “jam”) the bats’ echolocation signals.
To determine whether the moths’ clicks startle, warn or jam bats, Aaron J. Corcoran and William E. Connor of the Department of Biology at Wake Forest University pitted the two species against each other in a flight room equipped with digital high-speed infrared video cameras and an ultrasonic microphone. First, they trained three juvenile and one adult big brown bat (Eptesicus fuscus) to capture tethered moths. Then, on each of nine consecutive nights after the training, the bats were presented with 16 more tethered moths. 4 of these were tiger moths; another 4 were tiger moths which had been silenced by damage to the sound-producing organ; and the remaining 8 were wax moths (Galleria mellonella), which do not emit ultrasonic clicks, but which the bats also prey upon.
If the clicks act to warn that the moths might be venomous or otherwise unpalatable, the bats should initally capture the moths and drop them, then learn to abort their attacks whenever they hear the clicks. If the bats are startled by the clicks, they should quickly habituate to them. Alternatively, if the clicks are a jamming defence, they should deter the bats’ attacks when they are being emitted, but not at other times.
The researchers used the rate of contact with the moths as a measure of the bats’ attack success. They found that the bats contacted the wax moths over 400% more often than the clicking tiger moths. By contrast, tiger moths with damaged sound-producing organs were contacted and eaten every time they were encountered, confirming that the attack success rate on the unaltered tiger moths was due to the clicks they produced. The contact rates with sound-producing moths did not change during the experiment, suggesting that the bats did not learn to avoid clicking moths. The bats also persisted in their attacks after the moths began to produce their clicks, futher suggesting that they did not habituate to the sounds. These results strongly suggest that the clicks produced by the tiger moths function not as a warning or to startle the bats, but to jam their echolocation signals.
Analysis of the film footage also revealed that the clicks led to unusual echolocating behaviour. Normally, a bat’s attack progresses through three phases. First, it approaches its target. Then, it increases the frequency and amplitude of its echolocation signals; this so-called “feeding buzz” enables it to generate a more detailed auditory image, so that it can homes on and track the target. Finally, during the terminal phase of the attack, it captures its prey. Corcoran and his colleagues noticed that the ultrasonic clicks produced by the tiger moths led to atypical echolocation behaviour in the bats. In about one third of the attacks, the bats reversed the attack phase, from tracking to approaching, or from the terminal phase to tracking, before continuing with the attack.
The sonar jamming hypothesis has been tested in another moth species which has simple sound organs that produce low frequency clicks. This earlier study was carried out under the same conditions, the clicks produced by the moths did not jam the bats’ echolocation signals. But bats produce high frequency clicks for echolocation, so it may be the case that the frequency of clicks produced by this particular species was just too low to interfere with those of the bats. The authors of the current study suggest that the sound organs of moths may have evolved to produce acoustic warnings for predators, and that these were a stepping stone to the more complex organs which can produce high frequncy clicks that are capable of jamming bats’ echolocation signals.
Corcoran, A., et al. (2009). Tiger Moth Jams Bat Sonar. Science 325: 325-327 DOI: 10.1126/science.1174096.
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