Reproductive state modulates utricular auditory sensitivity in a vocal fish.

The plainfin midshipman, , is a seasonally breeding vocal fish that relies on acoustic communication to mediate nocturnal reproductive behaviors. Reproductive females use their auditory senses to detect and localize "singing" males that produce multiharmonic advertisement (mate) calls during the breeding season. Previous work showed that the midshipman saccule, which is considered the primary end organ used for hearing in midshipman and most other fishes, exhibits reproductive state and hormone-dependent changes that enhance saccular auditory sensitivity. In contrast, the utricle was previously posited to serve primarily a vestibular function, but recent evidence in midshipman and related toadfish suggests that it may also serve an auditory function and aid in the detection of behaviorally relevant acoustic stimuli. Here, we characterized the auditory-evoked potentials recorded from utricular hair cells in reproductive and nonreproductive female midshipman in response to underwater sound to test the hypothesis that variation in reproductive state affects utricular auditory sensitivity. We show that utricular hair cells in reproductive females exhibit up to a sixfold increase in the utricular potential magnitude and have thresholds based on measures of particle acceleration (re: 1 ms) that are 7-10 dB lower than nonreproductive females across a broad range of frequencies, which include the dominant harmonics of male advertisement calls. This enhanced auditory sensitivity of the utricle likely plays an essential role in facilitating midshipman social and reproductive acoustic communication. In many animals, vocal-acoustic communication is fundamental for facilitating social behaviors. For the vocal plainfin midshipman fish, the detection and localization of social acoustic signals are critical to the species' reproductive success. Here, we show that the utricle, an inner ear end organ often thought to primarily serve a vestibular function, serves an auditory function that is seasonally plastic and modulated by the animal's reproductive state effectively enhancing auditory sensitivity to courting male advertisement calls.