Insect-eating bats use echolocation to catch moths, while these night-flying prey have evolved early sonar detection and aerobatic maneuvers to evade bats. They’ve been dueling it out for over 65 million years. Tiger moths have an additional bat evasion strategy that involves ultrasonic signals. According to a new PLOS One study, the high-intensity clicks they emit warn bats to back off by advertising that they’re toxic, not tasty.
Many animals use bright colors or contrasting patterns to signal their toxicity to would-be predators – a behavior called aposematism. But moths and their bat predators are mostly nocturnal, so they rely on acoustic signals instead. Previous lab work revealed that toxic tiger moths produce ultrasonic signals. But this has never been observed in nature, and it’s unclear if these high-frequency clicks are used to jam bat sonar or used to warn bats that they taste bad.
To test the effectiveness of tiger moth anti-bat sounds in their natural environment, a team led by Nicolas Dowdy of Wake Forest University studied two tiger moth species: Pygarctia roseicapitis and Cisthene martini (pictured above). These moths sequester toxins from the plants that hosted them as caterpillars. After removing the sound-producing organs (called tymbals) from some of the moths, the team released them into an open grassy field at the Southwestern Research Station in Cochise County, Arizona, during the summers of 2011 through 2013. Infrared cameras and ultrasonic microphones were used to examine the interactions and acoustic signaling between the moths and the area’s free-flying bats.
Compared to moths that were rendered silent, tiger moths with intact tymbals were between 1.6 and 1.8 times less likely to be captured by bats. In fact, the bats actively kept their distance from sound-producing moths. And if they did catch tiger moths, they would often reject them – releasing the unpalatable insect unharmed.
The team also found that the two tiger moth species had different evasive flight responses and clicking behaviors. P. roseicapitis respond with evasive dives and aposematic sound production in an earlier phase of bat attack. Meanwhile, C. martini were more “nonchalant,” producing clicks later and using evasion strategies in fewer interactions. Dives and spiraling flights are energetically costly, and different tiger moth species use these behaviors at different rates.
Additionally, the moth clicks rarely occurred in the critical time window required to jam bat sonar. As Dowdy explained in a statement: "This means that in evolutionary history these moths first evolved these sounds for use in warning bats of their toxicity and then sometime later, these sounds grew in complexity in certain species to perform a sonar jamming function."