Masking of an auditory behaviour reveals how male mosquitoes use distortion to detect females

The mating behaviour of many mosquito species is mediated essentially by sound: males follow and mate with a female mid-flight by detecting and tracking the whine of her flight-tones. The stereotypical rapid frequency modulation (RFM) male behaviour, initiated in response to the detection of the female’s flight-tones, has provided a means of investigating these auditory mechanisms while males are free-flying. Mosquitoes hear with their antennae, which vibrate to near-field acoustic excitation. The antennae generate nonlinear vibrations (distortion products, DPs) at frequencies that are equal to the difference between the two simultaneously presented tones, e.g. the male and female flight-tones, which are detected by mechanoreceptors in the auditory Johnston’s organ (JO) at the base of the antenna. Recent studies indicated the male mosquito’s JO is tuned not to the female flight-tone, but to the frequency difference between the male and female flight-tones. To test the hypothesis that mosquitoes detect this frequency difference, Culex quinquefasciatus males were presented simultaneously with a female flight-tone and a masking tone, which should suppress the male’s RFM response to sound. The free-flight behavioural and in vivo electrophysiological experiments revealed that acoustic masking suppresses the RFM response to the female’s flight-tones by attenuating the DPs generated in the nonlinear vibration of the antennae. These findings provide direct evidence in support of the hypothesis that male mosquitoes detect females when both are in flight through difference tones generated in the vibrations of their antennae owing to the interaction between their own flight-tones and those of a female.诸多蚊虫物种的交配行为本质上由声音介导：雄性蚊虫会在飞行途中通过探测并追踪雌性的飞行音调（flight-tones）来尾随并与之交配。这种由探测到雌性飞行音调触发的典型快速频率调制（rapid frequency modulation, RFM）雄性行为，为研究自由飞行状态下雄性蚊虫的听觉机制提供了可行途径。蚊虫依靠触角感知声音，触角会在近场声激励下产生振动。触角会在同时存在双音（如雄、雌飞行音调）的情况下，产生频率等于两音差值的非线性振动（失真产物，distortion products, DPs），而位于触角基部的听觉器官琼氏器（Johnston’s organ, JO）可探测到这类失真产物。近期研究表明，雄性蚊虫的琼氏器并非对雌性飞行音调调谐，而是对雄、雌飞行音调之间的频率差产生调谐响应。为验证蚊虫可探测该频率差这一假说，研究人员向致倦库蚊（Culex quinquefasciatus）雄性同时施加雌性飞行音调与掩蔽音，后者本应抑制雄性对声音产生的RFM响应。自由飞行行为学实验与在体电生理实验结果显示，声学掩蔽可通过衰减触角非线性振动产生的失真产物，抑制雄性对雌性飞行音调的RFM响应。上述研究结果为以下假说提供了直接证据：当雄雌蚊虫均处于飞行状态时，二者自身飞行音调相互作用会在触角振动中产生差频音，雄性蚊虫正是通过这类差频音完成雌性识别。