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Acoustic camouflage

Bats are important predators of nocturnal and crepuscular moths. Many bats use echolocation, emitting ultrasonic squeaks, to detect their prey, so an important defence strategy for moths is ultrasonic hearing. Some primitive moths don?t have ultrasound hearing organs at all, so are they defenceless against the bats? Jens Rydell, of Göteborg University, Sweden, writing in Proceedings of the Royal Society, finds that they have another line of defence.

The deaf species of moth that Rydell studied is the ghost swift moth (Hepialus humuli). The earliest appearance of hearing organs in the fossil record of moths dates from the Eocene and Oligocene, around 40-50 million years ago. But the order that Hepialus belongs to dates from the early Mesozoic, more than 200 million years ago, pre-dating any echolocating predators, and the ?modern? members of this order still have no ears.

Hepialus also seems to lack other obvious predator defenses such as warning coloration, erratic flight patterns or good flight maneuverability. The beats of the fore- and hindlimbs are independent in this primitive group, rather than coupled like the more advanced forms. It is also large (with a wingspan of 5-6 centimetres) and a conspicuous silvery white colour.

But despite this seeming lack of defenses, the species survives. How does it avoid predators?

One way is the limited time that the moth chooses to fly. It is conspicuous for just two weeks around the time of the summer solstice, and males display in groups for just half an hour or so around dusk. In this way it avoids insectivorous birds and many bat species. But the early-emerging aerial hawking bats are still a threat - especially the northern bat (Eptesicus nilssonii).

The moths seem to deal with the threat by hiding in the grass. They fly among the tips of tall panicle-bearing grass stems, staying above the grass leaves, so that they are still visible to females, but seemingly taking advantage of the clutter that the vegetation will introduce into the echo signals. This ?acoustic crypsis? seems to hide them from the bats to some extent, reducing the bats? predation rate to less than 10 per cent of the success rate with other prey items.

But the moths are still at risk when they choose to change to a different patch. They must balance the risk of predation against the need to attract a mate. Even using the acoustic crypsis strategy, up to one-fifth of male moths still fall victim to the bats each evening, making it important to make a quick impression on the females.

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Phillips, H. Acoustic camouflage. Nature (1998). https://doi.org/10.1038/news980827-6

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  • DOI: https://doi.org/10.1038/news980827-6

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