Density Dependent Song Duration in the Grasshopper Omocestus Viridulus

Abstract

AbstractAlthough duration of grashopper song is species specific, there is also considerable variation within species. One possible reason for this is that males are adjusting their singing tactics in relation to the density of competitors. Males are expected to sing shorter songs when other males are nearby to enhance the probability of hearing female response or to listen to other males. To test this hypothesis an experiment was performed by placing different numbers of Omocestus viridulus males in large outdoor cages and monitoring their singing behaviour. The results show that males sing on average shorter calling songs as the number of males increases. It was also found that males compensate for singing shorter songs by singing more songs, so the total time spent singing stays similar at all densities. The scenario is complicated by the singing of very short songs which are mainly, but not exclusively, used in male-male interactions. The probability that males sing rivalry songs increases with the density of males and the number of these songs increases as male-male interactions get longer. The results suggests that part of the variation in the duration of calling songs reported in several field studies is caused by a change in singing tactics at different densities of males. The function of rivalry songs is not clear. These songs may function in the spacing of males or may be an extremely short form of the normal calling song used when other males are very close.