Today
•
reading time 2 minutes
•
silver green owl
© R Willlemsen
There’s something mystical about them, the moths. The silver green owl is like a leaf, the hazel owl is not noticeable when it is sitting on a tree trunk: camouflage is a good protection against predators. But in addition to visual camouflage, some species of moths also have acoustic camouflage. In this way they protect themselves against an ultrasonic predator: the bat.
arms race
Cynthia Bom’s PhD research, supervised by biologist Wouter Halfwerk, revolves around the bat and the moth. Predator and prey. There is a real arms race going on between the two species: when the bat has developed a clever trick to track down its prey, the moth adapts to avoid being caught.
Cynthia Bom and Wouter Halfwerk
© Merlin Schneiders
How does the bat hunt moths and other animals? The nocturnal animal hunts by echolocation. That means the bat emits ultrasonic sound waves and listens as they bounce back. We cannot hear that. But bats hear very well; in this way they know with their ears to determine where the prey is.
Hairs and scales
You hide with camouflage from an animal that hunts by sight, but what do you do if your predator hunts by sound? That requires a different kind of camouflage, namely acoustic. Moths are equipped with acoustic camouflage. Their body is covered with small hairs and their wings with scales. Those hairs and scales scatter the bat’s ultrasonic signal, or absorb the sound.
As a result, less sound bounces back to the bat. The noise is reduced by up to fifty percent. Moths are generalized or specified, depending on the species. Some butterflies will scatter all frequencies a bit, others are extra well protected against one specific pitch, that of their main predator.
hazel owl. The hairs are clearly visible.
© Donald Hobern via Wikimedia Commons (CC BY 2.0)
bat call
Bat species have different calls. For example, the red-tailed bat, an early flyer, has the loudest bat call in the country. So loud, that the bat cuts off its own hearing when it calls. With its call, the rufous bat can search for prey over long distances. In an open field, it is able to detect a moth at a distance of thirty to forty meters.
But a well-furred moth is a bit more difficult to find. The acoustic camouflage of a moth can thus reduce the reflected sound by a few decibels. There are also butterflies with a different tactic.
For example, some species have ultrasonically sensitive ears. As soon as they hear a bat, they whirl down. Such a moth is often less hairy, presumably because they need less acoustic camouflage. There are also butterflies (not in the Netherlands) that can make ultrasonic sounds themselves and thus confuse the bat.
Research
Wouter Halfwerk and Cynthia Bom listen to the bat sounds with special equipment and record it for their research. They also catch moths for this purpose, by luring them into a trap with UV light. About one percent of the captured butterflies are taken for research into acoustic camouflage. Ultimately, the researchers want to create a database in which the camouflage properties are recorded per species and per habitat.