War of Voices

May 29, 2021   5:30pm IST

   Bats and moths have been in an evolutionary arms race for more than 60 million years. Both being nocturnal animals play a vital role in the ecosystem as pollinators for various plants and are significant elements in the food chain; wherein moths serve as an important food source for bats. Such interactions are termed ‘predator-prey relationships’ and form a significant evolutionary force that mediates their behaviour. A fascinating fact about this evolutionary race is the usage of sound to hunt: bats use Sound Navigation and Ranging (SONAR) to navigate by producing high-frequency calls and listen to the echoes from their targets to determine their position (echolocation). They also use echolocation to identify and track their prey during their flight at night.

Over the long period of coevolution between these two organisms, moths evolved various strategies to elude bats (1). For example, many members of the family Saturniidae (silk or emperor moths) have long, twisted tails. Studies have shown that these twisted tails are useful in deflecting bat sound waves in a different direction. This allows the moths to fool the bats about their presence and also makes the bats think that there are multiple prey which leads them to attack the non-life-threatening tail of the moth rather than the body.

Butterflies and amphibians use vibrant and bright colours to signal toxicity to their predators, a phenomenon known as aposematism. Some moths use a similar strategy to signal their toxicity and bad taste. Although some species (for example, Chasmina sp. belonging to Noctuidae: Owlet Moths, Arctiidae: Tiger Moths) taste bad and convey honest signals to the bats, some moths are batesian mimics (eg. several members of Geometridae: Geometer Moths) and bluff about this and send false signals to protect themselves from bats. They transmit ultrasounds that are very similar to the model signals.

More than half of the nearly 140,000 moth species possess ultrasonically (frequency >20,000Hz) sensitive ears which help them detect bats(2). Simple ears evolved in many noctuid moth species, allowing them to perceive echolocation calls of insectivorous bats (3). Further, a very large number of moth species are known to produce sound (eg. some members of Pyralidae -snout moths, Noctuidae -owlet moths/armyworms, Geometridae-geometer moths) back at bats. There are various methods by which moths produce ultrasound; for example, Tiger moths signal a series of clicks by using ‘tymbals’ which is a vibrating membrane in their thorax. Some species have also modified their genitals on the tip of their abdomen to produce sound.

So how exactly do these little acoustic warriors protect themselves from the attack of bats by producing sound? Many hawk moth species (belonging to the family Sphingidae) go into stealth mode by producing sound to jam the bat radar. Researches have shown that the signalling ability has evolved three times in hawk moths and about a dozen times among all other moths (4). Another way through which moths save themselves from becoming a meal for bats is by “talking back”. When the moths hear the bat’s clicking intensify, they start emitting their own signal, thereby leading the bat to veer off at the last second. Possibilities from research suggest that it is likely that the bat recognises the signal produced and avoids the target on its own.

These anti-bat strategies are important to understand how evolution works on our planet. During this long duration of 60 million years where moths and bats have shared the planet, random mutations occurred in both moths and bats. Over the course of many generations, the need to escape predation gave rise to such amazing strategies in moths. Natural selection, further, favours new adaptations in bats as well, which leads to locking these predators and prey in a beautiful coevolutionary battle.

References:

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1. Nair, A. (2021, January 22). Bats and Moths: Evolution and the Relationship of Predator and Prey. RoundGlass | Sustain.   
2. KRISTENSEN, N. P. (2012). Molecular phylogenies, morphological homologies and the evolution of moth ‘ears.’ Systematic Entomology, 37(2), 237–239.   
3. Each moth escapes its own way. (2019, July). Max-Planck-Gesellschaft.   
4. PENNISI, E. (2016, January 22). Moths avoid capture by “talking back” to bats. AAAS.   
5. Kawahara, A. Y., & Barber, J. R. (2015). Tempo and mode of antibat ultrasound production and sonar jamming in the diverse hawkmoth radiation. Proceedings of the National Academy of Sciences, 112(20), 6407–6412.   
6. Barber, J. R., & Kawahara, A. Y. (2013). Hawkmoths produce anti-bat ultrasound. Biology Letters, 9(4), 20130161.   
7. HHMI Biointerative. (2016, May 10). Moth Mimicry: Using Ultrasound to Avoid Bats | HHMI BioInteractive Video. YouTube.