To our ears, the buzz of a mosquito is intensely irritating and a sign of itchiness to come, but to theirs, it's a lover's serenade. The high-pitched drone of a female is a siren's song that attracts male mosquitoes. And a new study shows that when the two love-bugs meet, they perform a duet, matching each other's buzzing frequency with careful precision.
The female Aedes aegypti mosquito (the carrier of both dengue and yellow fever) beats her wings with a fundamental frequency of about 400Hz, producing a pitch just slightly lower than concert A. Males on the other hand, have a fundamental frequency of around 600Hz, about one D above middle C.
Lauren Cator and colleagues from Cornell University discovered the sonic secrets of courting mosquitoes by tethering individuals to pins and moving the females past the males. On two-thirds of these fly-bys, the amorous mosquitoes harmonised. Neither took the lead - instead, both buzzers shifted their flight tones so that the male's second harmonic (the second multiple of his fundamental frequency) and the female's third had a mutual frequency of about 1,200 Hz. They synchronised in this way for about 10 seconds.
Lacking ears like ours, mosquitoes hear with their antennae and structures called Johnston's organs. But for decades, the wisdom of textbooks has said that males are deaf to any frequency over 800Hz and females are completely deaf. Cator disproved that by using miniature electrodes to show that the Johnston's organs of both sexes produced electrical signals in response to frequencies as high as 2000 Hz.
The duets depended on one partner hearing the other. If they were deafened by removing their antennae, or by gluing the antennae to the Johnston's organs, nothing happened. But seeing another mosquito wasn't necessary - individuals were all too happy to match the frequency of an electronically produced tone, even one that is set below or above the insect's natural flight tone.
Cator suggests that a male mosquito's ability to match the tone of his partner is the result of sexual selection. Perhaps females can judge the best mates by selecting those who can match their frequencies with the greatest skill. That will need to be tested in future studies but for now, one thing is clear - falling for one song makes the female less likely to fall for another.
Cator found virgin females were about three times more likely to match an electronic tone than those who had already mated. That backs up other research which suggests that Aedes aegypti females aren't keen to mate again for a fair while after they've done it once. Cator even suggests that releasing sterile males could be a way of controlling mosquito populations in the wild - it would lead to fruitless matings that would prevent females from engaging in productive ones later.
Reference: L. J. Cator, B. J. Arthur, L. C. Harrington, R. R. Hoy (2009). Harmonic Convergence in the Love Songs of the Dengue Vector Mosquito Science DOI: 10.1126/science.1166541
More on mosquitoes:
- Life-shortening bacteria vs. dengue mosquitoes
- Size matters for mosquitoes but medium-sized males do better
- Genetically-modified mosquitoes fight malaria by outcompeting normal ones
I'll try to remember this the next time I'm in mosquito country and swatting.
I heard on NPR (and I'm not saying they're an authority) that actually the 1200Hz note was an overtone. That is to say that the insects don't double (the male) or triple (the female) their wing buzzing frequency, to match each other at 1200 Hz they just subtly alter their tones around the 400 and 600 Hz ranges such that the overtone is manifested.
You know, when I read that, I thought, "Yes. That's right. That's what I said, isn't it?". Now, reading back, that's absolutely not what I said - what I wrote implied that they shift their fundamental frequency didn't it? Nuts.
Also managed to get the lead researcher's name wrong - Cator not Calor.
All round accuracy FAIL, methinks. /slaps self on hand - must do better.
Note that the text has now changed to correct my cock-ups.
"(...) releasing sterile males could be a way of controlling mosquito populations in the wild - it would lead to fruitless matings that would prevent females from engaging in productive ones later."
Unless the sterile males are less skillful in getting their frequencies right. In fact, it seems that any release of males into a wild population for mosquito control (e.g. genetically modified ones) needs to take this into account.
Yeah absolutely. It's clear that there's a lot of work to do before any approach that involves releasing genetically-engineered males can be done successfully. For example, a study last year showed that we're not even entirely clear about what criteria mosquitoes use to select mates - surely a fundamental consideration for these sorts of strategies?