A cost-benefit analysis of public and private communication.

Understanding the diversity of animal signals requires knowledge of factors which may influence the different stages of communication, from the production of a signal by the sender up to the detection, identification and final decision-making in the receiver. We studied a Neotropical katydid (Docidocercus gigliotosi) which uses airborne sound for long distance communication, but also an alternative form of private signalling through substrate vibration. Males spend more time with private signalling under full moon conditions, when the nocturnal rainforest favours predation by visually hunting predators. For either type of signal we measured the energetic costs of producing it, its active space, and the background noise levels in both transmission channels. Signal perception was studied using neurophysiological methods under outdoor conditions, which is more reliable for the private mode of communication. Our results demonstrate the complex effects of ecological conditions, such as predation, nocturnal ambient light levels and masking noise on the performance of receivers in detecting mating signals, affecting the net advantage or disadvantage of either mode of communication.