Effects of noise on sound localization in male grasshoppers, Chorthippus biguttulus

The localization of acoustic signals is an important component of mating behaviour in many species. However, in natural environments sound localization may be hindered by background noise that masks acoustic cues that enable the determination of sound direction by the auditory system. I studied the effects of noise on the sound localization abilities of male grasshoppers, Chorthippus biguttulus, in response to female songs. Males respond to female songs with a stereotyped lateralization behaviour in which they turn towards the perceived sound direction; repetition of this behaviour allows them to approach females. Under quiet conditions, males are highly sensitive to small differences in the timing or amplitude of signals that arrive at each ear. I tested whether noise affected male lateralization accuracy by presenting males with female songs that differed in time or amplitude cues of sound direction, under different levels of masking noise. Noise sharply reduced the responsiveness of males to female songs. However, when males did lateralize the female song, they were surprisingly accurate, even at the highest noise levels. These results suggest that noise only weakly impairs males' directional hearing abilities, but strongly affects their abilities to detect and respond to female songs. Noise therefore probably increases mating costs, because both males and females must expend additional time and energy in mate attraction.