Sexual competition in sagebrush crickets: must males hear calling rivals?

The acoustic signals produced by male crickets and katydids function in part for the maintenance of territories, broadcast areas within which males have exclusive access to sexually receptive females. Although it is widely assumed that a male's ability to hear and respond appropriately to calling neighbors is vital to his mating success, this hypothesis has never been experimentally challenged. We tested this hypothesis by experimentally deafening male sagebrush crickets (Cyphoderris strepitans) and comparing their mating success with that of untreated and sham-treated control males. Despite the fact that deafened males had a reduced ability to detect nearby rivals, we found no difference in the mating success of deafened and control males in experiments conducted over two years (1991 and 1993). We further examined the importance of signal detection to male spacing by establishing experimental populations consisting exclusively of either deafened males or hearing controls, whose nearest-neighbor distances had been experimentally compressed. Assuming that calling functions in part to repel rivals, we predicted that hearing males would space themselves out more rapidly in the nights following their release. However, in only two of four replicates were nearest-neighbor distances significantly different across treatments. We conclude that, in contrast to the mating systems of other acoustic Orthoptera: (1) male mating success is not contingent on auditory input from calling rivals, (2) signaling in sagebrush crickets may function only sporadically in territorial maintenance, and (3) calling occasionally mediates spacing of males in natural populations, but this effect may vary either over the course of the breeding season or between populations. We attribute these results to the unique mating system of C. strepitans. a short, highly synchronized breeding season coupled with a high male mating investment and a super-abundance of calling sites conspire against investment in territorial maintenance, but instead favor a form of acoustically mediated scramble competition.