Abstract
The function of fish sounds in territorial defence, in particular its influence on the intruder's behaviour during territorial invasions, is poorly known. Breeding Lusitanian toadfish males (Halobatrachus didactylus) use sounds (boatwhistles) to defend nests from intruders. Results from a previous study suggest that boatwhistles function as a 'keep-out signal' during territorial defence. To test this hypothesis we performed territorial intrusion experiments with muted Lusitanian toadfish. Subject males were assigned to three groups: muted, sham and unmanipulated. Males were muted by making a cut and deflating the swimbladder (the sound producing apparatus) under anaesthesia. Sham males suffered the same surgical procedure except the swimbladder cut and deflation. Toadfish nest-holder males reacted to intruders mainly by emitting sounds (sham and unmanipulated) and less frequently with escalated fights. When the nest-holder produced a boatwhistle, the intruder fled more frequently than expected by chance alone. Muted males experienced a higher number of intrusions than the remaining groups probably due to their inability to vocalise. Together, our results show that fish acoustic signals are effective deterrents in nest/territorial intrusions, similar to bird song.
