Under the threat of fast-approaching bats, some moth species do exactly what you might expect: they fly quickly and erratically, in the hope of shaking off their pesky predators. But other species don’t employ these evasive maneuvers, appearing comparatively apathetic to the fanged creatures swooping overhead. Now, a study published in Frontiers in Ecology in Evolution puts forth a theory as to why certain moths don’t hightail it when bats get close: They taste so bad that they don’t need to bother.
Moths are a favorite snack for bats, which in turn has driven the insects to evolve a host of defensive strategies: ultrasonic hearing that helps them detect echolocation, the production of clicking sounds that muddle bats’ biological sonar, expendable tail wings that fall off after being bitten, and yes, erratic flight movements. Some species protect themselves by sequestering toxins from their host plants, which in turn makes them unpalatable to predators.
Researchers still don’t fully understand how these different defensive factors are linked, but the co-authors of the new report suspected that the degree of a moth’s unpalatability might be connected to its relative sluggishness in the face of incoming predators. To test the theory, Nicholas Dowdy of the Milwaukee Public Museum and Wake Forest University, along with Wake Forest Biologist William Conner, collected five different species of tiger moths and set them loose in an outdoor flight arena, which bats often visited to feed.
With the help of infrared cameras, the researchers monitored the interactions between the bats and moths. To gauge how the moths tasted, the study authors looked to see whether the bats gobbled up their prey or dropped it immediately; all of the moths in the study were surgically silenced so they did not produce ultrasound in response to bat echolocation, thus helping the researchers confirm it was the moths’ taste that was throwing the bats off. The team also studied the moths’ behavior, and whether they were “evasive or nonchalant” while under attack.
The moths seemed to fall on a continuum, with “some species enacting escape maneuvers often, some rarely, and others lying somewhere between,” the study authors write. But as Dowdy and Conner had hypothesized, there seemed to be a correlation between how bad the moths tasted and how sluggish they were when danger approached.
“Strikingly, we observed that moths with weak or no chemical defenses often dive away to escape bat attacks,” Dowdy explains. “However, moths with more potent chemical defenses are more ‘nonchalant,’ performing evasive maneuvers less often.”
This correlation seemed to exist independently of other anti-bat defenses. For instance, two of the moth species included in the analysis, Bertholdia trigona and Carales arizonensis, both produce very high-rate clicks that are believed to jam bat sonar. But C. arizonensis exhibited more nonchalant behavior under threat, and seemed to be less tasty to bats.
As Aaron Walawalkar of the Guardian points out, taking a laissez-faire approach to predators makes sense, if you have other defences to protect you. In the wild, each anti-predator strategy has its risks and its rewards; zooming off might help foul-tasting critters get out of harm’s way and avoid ending up in a bat’s mouth, even if temporarily. But flying erratically away from predators runs the risk of colliding with a spider’s nest, or travelling far from food and mates. Sometimes, it’s better to stay put and let your nasty flavor do the work.