In 2020, the World Health Organization (WHO) estimated that nearly half of the world’s population was at risk of malaria, while some 627,000 people died from the disease.
Although a malaria vaccine will soon be available (the WHO recommended one for children last year), malaria is only one of many mosquito-borne diseases. And the total number of mosquito-related infections is set to rise as climate change expands mosquito populations.
Thus, to reduce the disease burden from malaria and other mosquito-borne diseases, we must continue to develop effective tools to control mosquito populations.
A favored target is their copulation in the air. The mosquito mating ritual involves a male identifying and pursuing a flying female by sensing her low flight tone.
If the male cannot hear the female properly, the chase fails and they do not mate. The reproduction of mosquitoes is essentially based on their sense of hearing.
We studied the behavior of mosquitoes responsible for malaria (the Anopheles gambiae species) to better understand how males listen to females to find a mate. Our results were recently published in the journal Scientists progress.
But first, a little background. The mechanism of hearing in mosquitoes is unique, but poorly understood.
The ears of both sexes are almost deaf to the flight sounds of the other, the frequencies of which are simply too high to be heard. To get along, they borrow a trick from physics.
When male and female flight tones combine in a mosquito ear, they create low frequency – and therefore audible – “phantom tones” called distortion products.
Distortion products exist only inside the mosquito’s ear and cannot be heard or recorded outside of it.
A male mosquito needs to fly to hear a female fly. And its own flight tone must be within a specific frequency range to generate audible distortion products with a given woman.
We listened to the mozzies flying tones
We recorded the flight sounds (or “wing beats”) of mosquitoes in incubators equipped with highly sensitive microphones.
Our experiments included the examination of 100 males and 100 females in separate incubators, individual mosquitoes (one male or one female, separately), as well as a mixed incubator, with 50 mosquitoes of each sex.
In the incubators, we sought to mimic the conditions of their natural environment with lighting, and controlling temperature and humidity.
We were able to measure the frequency of mosquito wing beats over several days and at different times of the day.
We found that male mosquitoes, but not females, changed their flight tones on a daily basis. By flapping their wings about 1.5 times faster than females, males optimize their ability to detect a single female in crowded swarms.
More than a decade ago, scientists proposed and described an acoustic interaction between males and females as “harmonic convergence”. Although they identified the same ratio of wing beats that allows mosquitoes of the opposite sex to hear each other (the equivalent of 1.5 male wing beats for one female wing beat), we found that this happens by default and does not actually require any interaction. between the sexes.
Notably, we found that males beat their wings faster at dusk than at other times of the day. This makes sense because in Anopheles gambiae mosquitoes, males mostly fly around dusk when they form mating swarms, often of 1,000 or more mosquitoes.
These swarms are sporadically visited by a few virgin females. As you can imagine, finding a mating partner is not easy.
The increased frequency of male wing beats at dusk alters the frequency of distortion products, which become more audible to the male ear than those created at other times of the day. Thus, by adjusting their wingbeat in the swarm, they are better able to hear the females and increase their chances of finding one to mate with.
The flight tone adjustment of males is partly determined by their circadian clocks. Faster wing flapping is likely to consume a lot of energy for males, so they limit this behavior when swarming.
What do our findings mean?
It will be important to replicate similar experiments outside the laboratory, especially among mosquito swarms in their natural habitat. We have already started working on this in Tanzania.
Yet these findings open up new avenues of research into the evolutionary ecology of hearing, the unique mosquito auditory system, and mosquito behavior more broadly.
They could also help with mosquito control efforts. As part of vector control programs, mutant males will be released into the wild to reduce local mosquito populations. Mutant male mosquitoes are genetically modified so that when they mate with a female, the offspring are not viable and will die.
Mating efficiency in this context is highly dependent on the ability of released males to hear “resident” females. Our results suggest that to create a successful program, it may be important to assess male and female flight tone distributions, alongside male auditory ranges, before releasing mutant mosquitoes.
This would bolster any intervention by ensuring the mating efficiency of mutants is optimal – essentially that they can compete with resident male mosquitoes to identify and mate with resident females.
Joerg T Albert, professor of sensory biology and biophysics, UCL; Alex Alampounti, researcher, biophysics, UCL, and Marcos Georgiades, doctoral student, neurobiology and biophysics, UCL.
This article is republished from The Conversation under a Creative Commons license. Read the original article.