DUET SINGING AND FEMALE CHOICE IN THE BUSHCRICKET PHANEROPTERA NANA

In the phaneropterine bushcricket Phaneroptera nana both males and females sing, producing a duet. The male's song is answered by a short female 'tick'. A male that receives a response in a defined interval after the end of his chirp usually initiates phonotaxis toward the female. Measurements of temporal, spectral, and energy characters of male chirps indicate that individual males vary considerably in several temporal song characters. Females exhibited equivalent response rates to solitary singing males in laboratory trials regardless of their song characters. However, when simultaneously presented with two singing males, females showed consistent preferences: They responded preferentially to relatively longer chirps produced by larger males. Male chirp rate had no influence on female response, suggesting that females were not influenced by a summation of acoustic input integrated over multiple chirps. Rather, female 'decisions' to respond were probably made following individual chirps.

The selective responses exhibited by Ph. nana females contradict some previous expecta- tions that females in duet signalling systems should be indiscriminate. Selectivity may reflect the reliability with which females can assess male song at a distance and potential disadvantages of attracting multiple suitors.

Laboratory trials indicated that only males receiving a high female response rate actually approach and mate with the female. Thus, this duet signalling system may be relatively resistant to intraspecific exploitation.