This unassuming moth is a greater wax moth (Galleria mellonella). Don’t let its drab appearance fool you, friends. This is a record-setting animal, with one of the most extreme sensory systems yet found. Its speciality? Hearing.
When you listen to anything, there are two main properties inherent in the sound: loudness and tone. The volume is determined by the size of sound waves; the tone is set by the frequency of sound waves. Humans hear tones where the sound waves vibrate back and forth at several thousand times a second. Something that moves back and forth once a second has a frequency of one Hertz (Hz); a thousand times a second is one kiloHertz (kHz).
People differ in how well they hear sounds at the high end. In particular, you lose the high frequency sounds as you get older. You can test how high you can hear at this website. Note that it stops at 22 kHz, because very few people can hear that high.
Animals, of course, have different limitations than humans. Cartoons often reference a dog whistle, with a pitch that humans can’t hear, but dogs can.
(Note: “Dog whistle” is not to be confused with “wolf whistle.” Know the difference!)
Moir and colleagues did two experiments to show the wax moth’s superior high-end hearing. First, they used a technique to show whether the ear drum (tympanum) was vibrating. If you can’t vibrate something at at the same frequency as the sound, you can’t detect the tone of the sound. They found the ear drum was able to keep up with every frequency they tested.
The critical experiment, though, is the neurophysiology. It doesn’t matter what the ear drum does if the neurons don’t convert anything into a signal. The wax moth has an ear with a grand total of four neurons devoted to picking up sound. Thus, analyzing the signals is fairly straighforward.
They found the moth’s ear could pick up sounds all the way up to 300 kHz. That’s twice as high as the previous record holder:
Sorry, Lymantria dispar. You had a good run.
The wax moth doesn’t hear equally well across the range. It is particularly good at picking up sounds in the 60 kHz range. For the wax moths to hear the end frequency sounds, they have to be much louder. At 60 kHz, the wax moths can pick up sounds of a volume about 50 decibels of sound pressure level (dB SPL); at 300 kHz, the sound has to be more like 90 dB SPL. That’s a loud sound. And at the very high end (280-300 kHz), some of the moths don’t respond at all to even loud sounds, suggesting this is near the upper limit of their hearing.
Why does the wax moth need such amazing hearing? The general explanation for why insects can hear at these high frequencies is because of these:
Bats hunt insects using high frequency sounds, and many insects have evolved ears that can hear the sounds bats make. This does not seem to be coincidence. The bats are thought to be exerting extreme selection pressure on insects, so hearing predators approaching is an adaptive advantage.
In this case, there is just one little puzzle. No bat makes a sound that hits 300 kHz. Why does the greater wax moth ear reach way up that high in the frequency spectrum? The authors suggest that this highly responsive ear allows the moth to react faster to sounds. After all, if your ear can vibrate at 300,000 times a second, and it takes 300 vibrations for the ear to pick up the sound, you could pick up the sound in a thousandth of a second, compared to about a hundredth of a second for an ear vibrating at 20 kHz, like our crappy human ears.
Reference
Moir HM, Jackson JC, Windmill JFC. 2013. Extremely high frequency sensitivity in a 'simple' ear. Biology Letters 9(4): 20130241-20130241. DOI: 10.1098/rsbl.2013.0241
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Photo by dhobern on Flickr; used under a Creative Commons license.
5 comments:
Hang on. In a human ear the ear drum only transmits vibrations mechanically to the cochlear. Hair cells in the cochlear turn vibrations into a signal, not neurons. Neurons "don’t convert anything into a signal". They interpret the signal from the cochlear. No doubt our ear-drum will vibrate well beyond 22KHz - it's the cochlear that determines the frequencies that we 'hear'.
So what is the physiology of moth hearing and how many ear cell equivalents do the moths have to go with their four neurons?
Thank you for the knowledge that moths can hear.
That last paragraph is a bit odd, though. Humans, unlike electronic pitch detection devices, need less than a period to detect the of a tonal sound. Also, the muscle reaction to impulse sounds is blazingly fast, due to the fact that the neurons between the hearing and the brain are short and efficient.
I would assume a moth doesn't need 300 cycles to detect a sound. Also, listening in that long would not help, as most bats produce short bursts of sounds, around 40k to 60k or so. I doubt these will last 300 cycles. I would guess the moth is on the lookout for these bursts of sounds.
The upshot of an ear that is sensitive to high frequencies might be that a small animal (with their ears close together) is still capable of doing some localisation of sounds. But with 4 neurons, I would guess the reaction is limited to "Blast, I hear something! Let's fly even more unpredictably than usual for now!"
Since bats 'transmit' around 50kHz, I'm skeptic that there is any evolutionary pressure to evolve such really high frequency response. I do think that the size and the construction of the hearing can allow for such capabilities.
I also have a question. Since traffic can produce high frequency content as well (it's loud and clear in my electronic bat detector), does traffic disturb flight patters of moths, other than due to drag?
Jayarava: Hm. I have always considered hair cells to be sensory neurons.
Dave: The last paragraph may be my mangled interpretation of the authors’ hypothesis. The “300 cycles” was a convenient random number. I just picked to try to illustrate what I thought the authors were hypothesising: the advantage of the high frequency is reaction time.
As for traffic, it could well be. There probably is research on what kinds of sounds trigger moth behaviour. I’m not sure whether continuous ultrasound would do it.
Thanks both for the excellent comments!
Hi , I found some supplemental information about ultrasound and also wolf whistles .
I thought some of you people reading this might possibly find these links interesting .
S. Berliner, III
Consultant in Ultrasonic Processing
"changing materials with high-intensity sound"
http://berliner-ultrasonics.org/
Red Hot Riding Hood -
http://www.youtube.com/watch?v=YtcJ7gvJP0Q&list=PL41157B5CD0427B63
Anyway , while reading about ultrasonics a while back(possibly at http://berliner-ultrasonics.org/ ) that cleaning involving megasonics , which deals with higher frequencies , has more of a 'direction' to it than ultrasonic cleaning baths . With ultrasonic cleaning , you put the object in the cleaning bath , and it hits from all directions , but megasonic cleaning only works on the side of the object facing the megasonic wave source . Perhaps the hearing of the moths is affected similarly by the angle of the source / wave front of the higher pitched frequencies which appear to be in the megasonic range (If I remember correctly , megasonics are in the 100kHz to 500kHz range .).
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