Effects of temporal envelope modulation on acoustic signal recognition in a vocal fish, the plainfin midshipman

Amplitude modulation is an important parameter defining vertebrate acoustic communication signals. Nesting male plainfin midshipman fish, Porichthys n otatus, emit simple, long duration hums in which modulation is strikingly a bsent. Envelope modulation is, however, introduced when the hums of adjacen t males overlap to produce acoustic beats. Hums attract gravid females and can be mimicked with continuous tones at the fundamental frequency. While i ndividual bums have flat envelopes, other midshipman signals are amplitude modulated. This study used one-choice playback tests with gravid females to examine the role of envelope modulation in hum recognition. Various pulse train and two-tone beat stimuli resembling natural communication signals we re presented individually, and the responses compared to those for continuo us pure tones. The effectiveness of pulse trains was graded and depended up on both pulse duration and the ratio of pulse to gap length. Midshipman wer e sensitive to beat modulations from 0.5 to 10 Hz, with fewer fish approach ing the beat than the pure tone. Reducing the degree of modulation increase d the effectiveness of beat stimuli. Hence, the lack of modulation in the m idshipman's advertisement call corresponds to the importance of envelope mo dulation for the categorization of communication signals even in this relat ively simple system. (C) 2001 Acoustical Society of America.