Listening in the bog: II. Neural correlates for acoustic interactions and spacing between Sphagniana sphagnorum males

Konstantinos Kostarakos,Heiner Römer

doi:10.1007/s00359-018-1251-7

Abstract

Males of the katydid Sphagniana sphagnorum maintain inter-male distances from one another using agonistic song interactions with a frequency-modulated song that consists of alternating audio and ultrasonic parts. We studied the neuronal representation of this song in auditory receptors and interneurons of receivers, using playbacks of songs that mimicked the absolute and relative sound pressure levels of the two song modes varying with distance. The tuning and sensitivity of both receptors and interneurons strongly determine their responses to the two song modes at different distances. Low-frequency interneurons respond preferentially to the audio mode of the song at larger distances. High-frequency (HF) interneurons respond preferentially to the HF component of the song at close range. ‘Switch interneurons’ are sensitive to both spectral song components, but exhibit a typical activity switch towards the high-frequency mode at distances nearer than 3–6 m. The activity of the latter two groups of interneurons correlates with the distance in the field at which males begin to interact acoustically with their neighbours. Important information about the rate of changes in the song mode is represented by the afferent activity despite the influence of the masking song produced by a sympatric katydid species.

Highlights

Many species of orthopteran insects form aggregations of singing males (Gerhardt and Huber 2002; Greenfield 2002)
The song of S. sphagnorum is a prolonged trill with a characteristic, regular change in two song modes that differ in pulse structure and frequency spectrum
The ranging hypothesis that has been proposed by various authors (McGregor and Krebs 1984; Morton 1986; Naguib and Wiley 2001; Morris et al 2016) suggests that receivers use frequency-dependent excess attenuation as a sensory cue to estimate their distance from a signaller

Summary

Introduction

Many species of orthopteran insects form aggregations of singing males (Gerhardt and Huber 2002; Greenfield 2002). The results of analysis of the distances between neighbours have indicated that individuals are rarely close enough to detect each other using any sense other than hearing, and that males within aggregations maintain a given distance from their nearest neighbours (Campbell and Clarke 1971; Meixner and Shaw 1979; Thiele and Bailey 1980; Morris et al 2018). A suprathreshold perceived intensity of 65 dB SPL, as measured from the focal males position, was identical in two populations studied, the mean inter-male distances differed significantly (Römer and Bailey 1986)

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