Arginine vasotocin modulates a sexually dimorphic communication behavior in the weakly electric fish Apteronotus leptorhynchus

South American weakly electric fish produce a variety of electric organ dis charge (EOD) amplitude and frequency modulations including chirps or rapid increases in EOD frequency that function as agonistic and courtship and mat ing displays, In Apteronotus leptorhynchus, chirps are readily evoked by th e presence of the EOD of a conspecific or a sinusoidal signal designed to m imic another EOD, and we found that the frequency difference between the di scharge of a given animal and that of an EOD mimic is important in determin ing which of two categories of chirp an animal will produce. Type-I chirps (EOD frequency increases averaging 650 Hz and lasting approximately 25 ms) are preferentially produced by males in response to EOD mimics with a frequ ency of 50-200 Hz higher or lower than that of their own. The EOD frequency of Apteronotus leptorhynchus is sexually dimorphic: female EODs range from 600 to 800 Hz and male EODs range from 800 to 1000 Hz, Hence, EOD frequenc y differences effective in evoking type-I chirps are most likely to occur d uring male/female interactions. This result supports previous observations that type-I chirps are emitted most often during courtship and mating. Type -II chirps, which consist of shorter-duration frequency increases of approx imately 100 Hz, occur preferentially in response to EOD mimics that differ from the EOD of the animal by 10-15 Hz, Hence these are preferentially evok ed when animals of the same sex interact and, as previously suggested, prob ably represent agonistic displays. Females typically produced only type-II chirps. We also investigated the effects of arginine vasotocin on chirping. This peptide is known to modulate communication and other types of behavio r in many species, and we found that arginine vasotocin decreased the produ ction of type-II chirps by males and also increased the production of type- I chirps in a subset of males. The chirping of most females was not signifi cantly affected by arginine vasotocin.