Bats and Moths: Evolution and the Relationship of Predator and Prey

At the end of July 2019, we celebrated National Moth Week in Dehradun. The week is an occasion to create awareness amongst the public, forest department, and other stakeholders, about moths and their importance in our ecosystems. In the Kaduapani Reserved Forest, on the outskirts of Dehradun, we set up a moth screen at night, consisting of a white sheet with a special light and crowded around it to observe the moths attracted to the light. Out of the corner of my eye, I watched as a couple of bats, a small one, possibly a pipistrelle, and a larger one, perhaps a horseshoe bat, swooped above the moth screen. While the bats must have been watching us felicitate moths with envy, it is more likely that these insect eating mammals were smacking their lips in the hope of an easy moth meal.

Moths are primarily nocturnal insects that play a crucial role in our ecosystems. Not only do they help pollinate flowers, but they are also food for a variety of creatures, the most prominent of them being bats. Bats are an important part of the web of life. Bats too, are pollinators. Fruit eating bats help in seed dispersal, while the insect eating bats assist in controlling insect populations.

Bats are amongst the foremost predators of moths, and over time, each of these species groups has employed intriguing strategies of protection and avoidance. Bats hunt by echolocation, i.e. they emit sound waves that reflect off the moths, permitting the bats to locate and predate on moths. Bats and moths have evolved strategies to predate and evade respectively, over many millions of years. And these processes of co-evolution are continuing.

Many silk or emperor moth species, belonging to the family Saturniidae, have long twisted tails. Experiments have shown that these twisted tails are used to deflect bat sound waves in a different direction, in order to fool bats. In addition, the twisted long tails of emperor moths confuse the bat into thinking there are multiple prey, leading it to attack the non-life threatening tail of the moth, instead of its body.

A test conducted with tailed emperor moths, compared with a group where the same species had their tails broken off, showed that the former group had much higher survival rates when faced with bat predation.

Another group, tiger moths, which are bitter tasting to predators, issue a series of clicks to warn bats of their toxicity. Tiger moths produce these sounds using tymbals, a vibrating membrane on their thorax. Many hawkmoth species, another family of moths, also produce sounds to jam bat radar. Males of these hawkmoths produce a series of clicks using their genitalia, jamming the bat’s echolocation sounds, thereby confusing the bat’s predatory foray.

Even more interesting, the predator-prey relationship of bats is co-evolutionary. Just as many moth families developed ears to detect bats over time, many bat species evolved over time to counter moth evasion. When bats are hunting moths or other insect prey, the volume of sound clicks they produce warns the prey of their presence. This leads moths to evade bats by flight evasion, or simply dropping to the ground for concealment. Studies have shown that the barbastella bat, found in Europe, has evolved stealth echolocation. In order to fool moths, the series of sound clicks it produces actually reduces in volume as it gets closer to the moths, in order to confuse the moth. This reduces the range at which it can effectively hunt moths, but provides it an effective countermeasure to evade detection by its prey.

As yet another bat swooped over my head at Kaduapani, I was left wondering what strategies the bats were now coming up with, and how the moths were going to respond.