How Moths Outwit Bats With Their Super Hearing

green, white and grey moth on a green plant

Biological Strategy

Moths

Anika Hazra

Image: Albert Thongchai / Shutterstock / Some rights reserved

Respond to Signals

To interact with its environment, a living system must not only sense a variety of signals, but also respond to them. To be energy- and material-efficient, those responses must be appropriate to the signal. This generally requires sensing thresholds to trigger an appropriate level of response (for example, hiding under a shrub versus running away to avoid a predator). Response strategies are tied to a specific signal and often have a response threshold, which determines how strong a signal must be to warrant expending energy to respond. One example is a plant that lives in arid regions in South Africa. Its seed capsules remain closed until rainfall triggers them to open to release the seeds. But the plant only responds to a second rainfall, thus protecting against releasing its seeds before there is enough sustained water for them to grow.

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Sense Sound and Other Vibrations From the Environment

For living systems, sensing sound and other vibrations is important for communicating and detecting conditions within their environment. Living systems must locate a signal’s source so that they can move toward it (such as when it is food or a potential mate) or away from it (such as when it is a predator). To prompt an appropriate response, living systems must sense these signals, recognize their amplitude or volume (which is sometimes very low), and determine their direction. Living systems must be attuned to signals relevant to them and able to distinguish these from irrelevant sounds to avoid expending unnecessary energy. For example, owls’ ears are asymmetrically placed. This enables them to detect sounds more accurately, which helps them locate small prey at night and avoid wasting energy chasing down irrelevant sounds.

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Protect From Animals

Animals–organisms that range from microscopic to larger than a bus–embody a wide variety of harms to living systems, including other animals. They threaten through predation, herbivory, defense, and parasitism, and they compete for resources such as water, nutrients, and space. Any given living organism commonly faces threats from a variety of animals, requiring strategies that effectively defend from each. Trout and other bony fish, for example, escape predators by having scales made of very thin, flake-like pieces of bone covered with slippery mucus. They also have behavioral strategies such as camouflage, fast swimming, and twisting and turning to achieve release from a predator’s grip.

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Insects

Class Insecta (“an insect”): Flies, ants, beetles, cockroaches, fleas, dragonflies

Insects are the most abundant arthropods—they make up 90% of the animals in the phylum. They’re found everywhere on earth except the deep ocean, and scientists estimate there are millions of insects not yet described. Most live on land, but many live in freshwater or saltwater marshes for part of their life cycles. Insects have three distinct body sections: a head, which has specialized mouthparts, a thorax, which has jointed legs, and an abdomen. They have well-developed nervous and sensory systems, and are the only invertebrate that can fly, thanks to their lightweight exoskeletons and small size.

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The tympanal organs of moths can "hear" bat biosonar and trigger behavioral responses.

Introduction

Almost all species on Earth fall into the category of predator or prey–some fall into both. A battle of wits determines who wins: the one who needs to eat or the one who needs to not be eaten. Evolution has refined the strategies of insect-eating bats and their moth prey to the point where what was once perceived as an advantage for the bats can be considered their downfall in this case.

Insect-eating bats use biosonar to detect their prey in the air, while moths have evolved the ability to hear bats coming towards them. Exceptional hearing has been useful for reproductive and predator avoidance strategies among nocturnal species. Millions of years of predation pressure from bats on nocturnal flying insects like moths have resulted in their heightened acoustic awareness.

The Strategy

Bats inadvertently warn moths of their approach through their very efforts to locate their prey. They get the information they need about their prey’s location from how and when their biosonar signals are echoed back to them after hitting their target. Moths can successfully avoid oncoming bats in the dark by hearing their chirps before the echoes bounce off the moths and return to the bats.

Low-frequency sound signals are detected by a moth’s receptor hairs, while the tympanal organs detect high-frequency signals. As such, the ultrasonic frequencies of bat biosonar, ranging from 20 to 200 kHz, are detected with the tympanal organs, found in various locations on the body, depending on the family. Most moths can only detect frequencies that cover a portion of the bat’s biosonar range (20 to 50 kHz), but some moths can hear frequencies of over 200 kHz.

Similar to timpani in an orchestra, the tympanal organs consist of a membrane covering an air sac. The sac in turn is attached to sensory cells that translate the vibrations into a neural signal, which is then processed and triggers the appropriate behavioral response. Tympanal organs are so useful that they have evolved independently across multiple families of moths.

The tympanal organs of two different moths

Image: Commonwealth Scientific and Industrial Research Organisation (CSIRO) / Wikimedia Commons / CC BY - Creative Commons Attribution alone

A visual comparison of tympanal organs of two moth species (left: a moth belonging to the Tineidae family, right: a moth belonging to the Pyralidae family) reveals the different forms the development of the organs have taken ("m" indicates the membrane).

The Potential

This kind of frequency-targeted, distributed sound detection could be useful for medical devices such as hearing aids. It could also be useful for sensor development in general, improving security systems on property, collision avoidance for vehicles, or wildlife monitoring.

Last Updated January 1, 2022

References

“Bats utilize ultrasonic frequencies in their biosonar, ranging from 20 kHz to 200 kHz, depending on species. In all cases studied to date, insects detect the high frequencies of bat biosonar with tympanal hearing organs. These organs are highly variable in placement on the body (wings, abdomen, thorax, legs, head) and sensory complexity (ranging from a single auditory receptor to over a hundred).” (Hoy 1992:116)

Book section

The Evolution of Hearing in Insects as an Adaptation to Predation from Bats

The Evolutionary Biology of Hearing | 1992 | Ronald R. Hoy

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Reference

“Insect tympanal organs, or ears, consist basically of an external, thin membrane (the tympanum) and associated internal air sacs, or tracheae. The auditory (sensory) cells attach to the tympanum or to an internal membrane.” (Miller & Surlykke 2001:570)

Journal article

How Some Insects Detect and Avoid Being Eaten by Bats: Tactics and Countertactics of Prey and Predator

BioScience | 2001 | Lee A. Miller, Annemarie Surlykke

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