Inter-male spacing in choruses of the spring peeper, Pseudacris (Hyla) crucifer

Abstract

Field measurements and playback experiments designed to assess mechanisms of spacing between calling males of the spring peeper were conducted in central Missouri. In 10 choruses of typical density, mean inter-male distances ranged from 21 to 124 cm and usually varied more than the sound pressure level (SPL in decibels (dB) re 2×10 −5Pa) of nearest neighbours' calls (overall range: 88–106 dB SPL; range at mean inter-male distance: 92–98 dB SPL). In playback experiments, conducted in choruses of unusually low density, the lowest sound pressure levels at which males vocally responded to a synthetic advertisement call of 2875 Hz ranged from 48 to 71 dB SPL; the median of 63 dB SPL is a conservative estimate of the effective threshold for antiphonal calling. The sound pressure levels of the calls of each frog's nearest neighbour ranged from 0 to 36 dB SPL (median 11 dB SPL) above its estimated threshold. These values are conservative because background noise precluded testing one-fourth of the forgs at sound pressure levels below 64 dB SPL. Thus, over a wide range of chorus densities, males of the spring peeper usually spaced themselves at places where the call amplitude of their nearest neighbour was well above their auditory threshold.