SEXUAL COMPETITION IN SAGEBRUSH CRICKETS - MUST MALES HEAR CALLING RIVALS

The acoustic signals produced by male crickets and katydids function i n part for the maintenance of territories, broadcast areas within whic h males have exclusive access to sexually receptive females. Although it is widely assumed that a male's ability to hear and respond appropr iately to calling neighbors is vital to his mating success, this hypot hesis has never been experimentally challenged. We tested this hypothe sis by experimentally deafening male sagebrush crickets (Cyphoderris s trepitans) and comparing their mating success with that of untreated a nd sham-treated control males. Despite the fact that deafened males ha d a reduced ability to detect nearby rivals, we found no difference in the mating success of deafened and control males in experiments condu cted over two years (1991 and 1993). We further examined the importanc e of signal detection to male spacing by establishing experimental pop ulations consisting exclusively of either deafened males or hearing co ntrols, whose nearest-neighbor distances had been experimentally compr essed. Assuming that calling functions in part to repel rivals, we pre dicted that hearing males would space themselves out more rapidly in t he nights following their release. However, in only two of four replic ates were nearest-neighbor distances significantly different across tr eatments. We conclude that, in contrast to the mating systems of other acoustic Orthoptera: (1) male mating success is not contingent on aud itory input from calling rivals, (2) signaling in sagebrush crickets m ay function only sporadically in territorial maintenance, and (3) call ing 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 ca lling sites conspire against investment in territorial maintenance, bu t instead favor a form of acoustically mediated scramble competition.