Acoustic communication is important for determining and maintaining intermale spacing in breeding aggregations of anurans and insects. Because the number and proximity of signalling males can show extensive temporal and spatial variation, we should expect to find mechanisms that permit males to modify their signalling behaviour in ways that balance the needs to attract females and defend their calling space. We conducted two field playback experiments to investigate the role of plasticity in male aggressive signalling in intermale spacing of spring peepers (Anura, Hylidae). In the first experiment, we found a positive correlation between the amplitude of the advertisement calls of a male's nearest neighbour and the stimulus amplitude at which the male first produced aggressive calls. In the second experiment, repeated presentations of advertisement calls above a male's aggressive threshold resulted in rapid decreases in aggressive signalling and significant, but temporary, elevations of aggressive thresholds. We suggest that short-term habituation to a neighbour's calls could function as a proximate mechanism for plasticity in aggression that would allow males to accommodate nearby callers while also tracking fluctuations in the local density of calling individuals. In a third experiment, we examined female choice as an ultimate-level explanation for plasticity in male aggression. Females preferred advertisement calls to aggressive calls, but this preference was weak and was abolished by a 6-dB reduction in the amplitude of the advertisement call. We suggest that female preferences probably function as only one possible source of selection on plasticity in male aggressive signalling and propose energetic limitations as an additional source of selection.