Moths vs. Bats: A Rivalry Spanning Millennia

Oct 9, 2017

If you walk near the lake at night, there’s a good chance you’ll see bats swooping through the air to feast on insects. What you might not see - or hear - is the aerial warfare at play dividing the weak and the strong in a battle for survival that has spanned 50 million years.

Tiger moths - also known as Arctiinae - are a diverse subfamily of moths with around 11,000 species, including more than two dozen species which make their home in Wisconsin. For millennia, their survival has been dependent on their ability to avoid and evade bats. 

Tiger moth caterpillars are often very furry and they eat poisonous plants to use against their predators.
Credit blucolt / Flickr

Dr. Nicolas Dowdy is a behavioral ecologist and an expert on tiger moths. This Tuesday, Dowdy will give a lecture on the evolutionary arms race between tiger moths and bats as part of the Milwaukee Public Museum's Lunch and Lecture presentations

Dowdy says that because insectivorous bats are such impressive hunters, a moth's ability to survive an attack has really impacted the evolution of Arctiinae moths. 

"Can you live through a bat attack or not? That’s where these anti-bat defenses really come into play. They help the moths survive that intense predatory selective pressure from the bats," he explains. 

Over millennia tiger moths have developed a myriad of mechanisms to fend off predators, like taking chemicals from plants to use for their own defense. 

"Tiger moths have special biochemical adaptations which allow them to not only overcome those [toxic] plant defenses, but also take those defenses for themselves and use them against their bat predators."

"As caterpillars or sometimes even as adults, they will go to toxic plants and they will feed on them. And normally, toxic plants, they have these compounds to stop insects from feeding on them because they are very harmful to most insects," Dowdy explains. 

He continues, "Tiger moths have special biochemical adaptations which allow them to not only overcome those plant defenses, but also take those defenses for themselves and use them against their bat predators or even their invertebrate predators or bird predators, too." 

Sight and hearing are both very important to a moth's ability to survive, and it allows them to evade predator bats more easily. Dowdy says there are two major evasive maneuvers moths employ. One is simply spotting a bat and turning around to avoid it. The other is a interesting dance between opponents. 

Bats flying through the air, consuming insects with their mighty jaws.
Credit Ann Froschauer / USFWS

Dowdy explains, "When the bat is very close to the moth, about to make a capture, the moth will do these crazy, spiraling dives. And this allows the moth to make sort of a quick escape at the last second to get away from the bat." 

One of the more interesting deterrents tiger moths have developed is something that covers a moth's entire body: scales. Dowdy explains that scales on a moth are, "tiny pieces of exoskeleton - shingled, kind of like on the roof of your house." Inside of the shingles on a tiger moth is a microstructure that helps it hide from bats like a stealth bomber. 

"Relative to a butterfly - which shouldn't be under any selective pressure from bats, really - those particular types of scales absorbed bat ultrasound by 5-6%, so that decreased the distance by which the bats could detect them by about 5-6%," says Dowdy. 

Dr. Nick Dowdy is a postdoctoral research fellow at Purdue University and the Milwaukee Public Museum, and his research focuses on the behavioral ecology of tiger moths.