Moths Hold Their Own in an Acoustic Evolutionary Arms Race

Jabale Rahmat

Junior

School of Life Sciences

Independent University, Bangladesh

September 10th, 2018

Many of us are familiar with the scene of a speedy chase between a cheetah and its prey, for example, a deer, from runs on National Geographic. Cheetahs can reach up speeds up to 96.5 kilometers per hour in a few seconds, but this speed lasts for less than a minute. With this lung-busting speed, a cheetah can make sharp turns and catch its prey by surprise. None of its cousin species can achieve this speed let alone make such turns. This turning for the cheetah is stabilized or balanced by its muscular long tail. Alongside its speed and unique tail, cheetahs have numerous other adaptations that arose to keep up with the prey in their habitat. Similarly, prey species do not sit back and resolve to get outmaneuvered forever; they also adapt to avoid or evade predators, resulting in an adaptive arms race between predators and prey over evolutionary time.

Rubin et al., in a recent paper [1], describes some interactions between predator and prey where the prey appears to be a step ahead its predator. The predator is the famous nocturnal animal, the bat, which locates its prey by echolocation. With this technique, the bats produce ultrasound that rebounds from various objects including its prey and by reading the reflected ultrasound waves, bats can deduce the location of its prey. The prey is the moth.

Bats and moths offer interesting perspectives on coevolution. Several adaptations have been reported on either side. There are moths that produce clicks to confuse the bats by interfering with their echoes. There is a species of bat that has been shown to lower the intensity of its echolocation calls as it approaches its moth prey, helping to provide an illusion of increased distance from the prey. A recent study reported greater absorption of echolocation calls by the wings of nocturnal moths (that tend to be hunted by bats) compared to diurnal moths, providing support for the idea of adaptation to avoid predation.

A long-tailed wild silk moth. Focus on Boise State

In the paper from Rubin et al., the authors demonstrate that wild silk moth species with specific hindwing shapes and morphologies more effectively escape bat attacks. It built upon previous studies showing that spinning hindwing limbs are able to deflect sonar.

The researchers first utilized a dataset of several hundred genes found across species of wild silk moths to create a phylogenetic tree to identify patterns of relatedness. They discovered four shapes that have convergently evolved multiple times – twisted and cupped long hindwing tails (tails are extensions that emerge from the hindwing), a short hindwing tail, and a long hindwing lobe (a longer lobe means the entire hindwing structure is longer; think droopy earlobes). This was consistent with modification of the hindwing being adaptive or beneficial.

They became interested in how the length of the hindwing tail or lobe impacted escape from bat attacks. To that end, they pit big brown bats against the moths with different types of hindwings (that were tethered using strings) and observed how well the moths escaped attacks. They pit the bats against intact moths, sham moths that they experimentally created by cutting part of the hindwing and regluing it with the moth (this is a control to show that the cutting procedure itself does not bias results), and moths with experimentally increased or decreased hindwing tail and lobe lengths.

By analyzing the data from these experiments – collected using slow motion camera – they discovered that moths with longer hindwing tails or lobes, whether or not they were natural or experimentally manipulated, had the highest chances of escape.

To elucidate how the hindwing helps the moth to avert bat attacks, the authors carried out further tests. By looking at what parts of the moth the bats were being able to strike, they were able to conclude that that the hindwing structures very likely create an illusion of multiple targets, prompting the bats to aim for the lower end of the moths rather than the center. The hindwings of moths are disposable for flight, so it is plausible that moths would present them as alternative targets to allow escape.

This paper presents evidence of a powerful form of convergent evolution that arose several times in different species of moths, indicating a strong selection pressure. Much, however, remains to be done. Besides big brown bats, each moth may be attacked by other or more bats in their natural habitats. Could it be possible to find evidence for coevolution between specific species of bats and moths? This research must be put in context with other known adaptations undergone by bats and moths in their increasingly well-documented evolutionary arms race.

Bibliography:

[1] J. J. Rubin, C. A. Hamilton, C. J. W. McClure, B. A. Chadwell, A. Y. Kawahara, and J. R. Barber, “The evolution of anti-bat sensory illusions in moths,” Sci. Adv., vol. 4, no. 7, pp. eaar7428–eaar7428, Jul. 201

Jabale is a Junior in the School of Life Sciences at IUB, majoring in biochemistry. He is a future scientist who is crazy about everything related to biology, especially genetics.