Effects of signal duration on the recognition of masked communication signals by the grasshopper Chorthippus biguttulus

The detection and recognition of acoustic communication signals masked by n oise was investigated in a grasshopper (Chorthippus biguttulus) whose audit ory system exhibits only poor spectral resolution and therefore has to oper ate in the time domain. The signals of this species consist of numerous ide ntical subunits that enable the receiver, in principle, to make repetitive measurements. We aimed at determining the maximum integration time in this species by using stimuli of different durations under increasing noise leve ls. As a criterion for recognition the typical phonotactic turning response of the males was evaluated, which is reliably triggered by a female song, and thus is a sensitive indicator for recognition of conspecific signals. W hen confronted with a long signal (1000 ms) males tolerated a 2.4 dB higher noise level as compared to a short signal (250 ms). Noise tolerance improv ed with increasing signal duration from 250 ms to 450 ms. Beyond this signa l duration, however, no further improvement was observed, indicating an upp er limit for temporal integration that corresponds to only five song subuni ts. The gain in noise tolerance had a slope of 2.7 dB per doubling duration , which corresponds to the expectation derived from an energy detector mode l (3 dB per doubling duration) rather than to the value expected from signa l detection theory (1.5 dB per doubling duration).