Sexual selection in the wolf spider Hygrolycosa rubrofasciata: female preference for drum duration and pulse rate
The unusual form of sexual signaling, the drumming produced by the wolf spider Hygrolycosa rubrofasciata, allows exceptionally detailed studies of female preference patterns against signal characteristics. It is easy to manipulate the signals and to use large numbers of females in playback experiments. Males of H. rubrofasciata produce drums by striking their abdomen against dry leaves on the ground. Drums travel not only as substrate-borne vibrations, but also as airborne acoustic signals. Females respond sooner to drums transferred as substrate borne, but the mode of signal transfer has no effect on female preference for different types of drums. We investigated the effects of two key components of the drums, duration and pulse rate, on female choice. Previous studies have shown that the duration of the drum is both repeatable within males and variable between males. In contrast, pulse rate shows high within-male repeatability but only little variability between males. Using playbacks of manipulated drums, we found that females preferred drums of longer duration but that pulse rate had no effect on female preference. Our results suggest that drum duration may function as an indicator of male quality for choosy females. Pulse rate, on the other hand, is less likely to be an important trait in intersexual selection. Female preference for drum duration was open-ended within the natural variation of the drum durations, but it leveled off outside the normal range. Thus supernomal stimuli would not pay for males using this energetically demanding acoustic signal.