The Signaller's Dilemma: A Cost–Benefit Analysis of Public and Private Communication

Abstract

BackgroundUnderstanding 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. Yet, many studies on signalling systems focus exclusively on the sender, and often ignore the receiver side and the ecological conditions under which signals evolve.Methodology/Principal FindingsWe study a neotropical katydid which uses airborne sound for long distance communication, but also an alternative form of private signalling through substrate vibration. We quantified the strength of predation by bats which eavesdrop on the airborne sound signal, by analysing insect remains at roosts of a bat family. Males do not arbitrarily use one or the other channel for communication, but spend more time with private signalling under full moon conditions, when the nocturnal rainforest favours predation by visually hunting predators. Measurements of metabolic CO2-production rate indicate that the energy necessary for signalling increases 3-fold in full moon nights when private signalling is favoured. The background noise level for the airborne sound channel can amount to 70 dB SPL, whereas it is low in the vibration channel in the low frequency range of the vibration signal. The active space of the airborne sound signal varies between 22 and 35 meters, contrasting with about 4 meters with the vibration signal transmitted on the insect's favourite roost plant. Signal perception was studied using neurophysiological methods under outdoor conditions, which is more reliable for the private mode of communication.Conclusions/SignificanceOur results demonstrate the complex effects of ecological conditions, such as predation, nocturnal ambient light levels, and masking noise levels on the performance of receivers in detecting mating signals, and that the net advantage or disadvantage of a mode of communication strongly depends on these conditions.

Wing mechanics, vibrational and acoustic communication in a new bush-cricket species of the genus Copiphora (Orthoptera: Tettigoniidae) from Colombia

“…3, see also Morris et al 1994). Similar to C. (Belwood, 1988(Belwood, , 1990Belwood and Morris, 1987;Morris et al, 1994;Römer et al, 2010). Eavesdropping seems to explain why some bush-crickets use such bewilderingly high principal carriers (Belwood and Morris, 1987;Falk et al, 2015;Montealegre-Z et al, 2006;Montealegre-Z et al, 2011b;Morris et al, 1994;Sarria-S et al, 2014).…”

“…We also recorded vibrations from a single point at the base of each palm branch, and did not try other locations. Römer et al (2010) showed that the host plant of Docidocercus gigliotosi filters vibrations, depending on the location of the signaller on the plant, and that some parts of the plant enhance the transmission of the vibration. For instance, the leaves seem to act as resonators in the range of 10-15 Hz.…”

“…For instance, the leaves seem to act as resonators in the range of 10-15 Hz. We did not measure vibration from the leaves, but the frequency of the vibrations recorded from the base of the plant fall in the range reported by Römer et al (2010).…”

“…Vibrational communication, also known as seismic tremulation, is the communication between organisms by the vibration of a substrate. In bush-crickets it is speculated that tremulation functions to attract females whilst reducing the cost of acoustic eavesdropping by predators and also allowing for an alternative communication channel (Morris, 1980;Morris et al, 1994;Römer et al, 2010). Some species seem to have dramatically reduced their acoustic behaviour and adopted tremulation as a key modality for communication with females (Belwood and Morris, 1987).…”

Multimodal cues improve prey localization under complex environmental conditions

“…Multimodal signals allow animals to shift energy between signal components when costs and benefits differ between communication channels [46,50]. Tropical katydids, for example, increase the use of substrate-borne vibrations at the expense of air-borne sounds under full moon light conditions [46]. This behaviour presumably evolved to allow private communication, and thereby avoid the risk of eavesdropping bats [46,51].…”