Any sensory strategies that prey take to avoid eavesdropping predators will depend on the behavioral decisions of eavesdroppers. As these decisions are guided by the sensory processing of communication signals, accurate measurements of sensorimotor output will provide insights into signal preferences, parameters evaluated for signal recognition, and the perceptual and cognitive capacity of receivers. A number of techniques have been proposed for measuring walking phonotaxis (and taxis behavior more generally). Consistent limitations of such measures are (1) that some animals cannot discriminate alternative signals when they occur simultaneously (i.e., overlapping in the spectral and temporal domain), or (2) some animals respond with low selectivity to stimuli presented in isolation, and (3) identifying appropriate dimensions of response variability is not straightforward. Here we document an approach to develop a sensitive phonotaxis performance index to quantify pulse rate selectivity in two distinct populations of the acoustic parasitoid fly Ormia ochracea. Using a spherical treadmill to measure tethered walking phonotaxis, we examined the ability of flies to track a switch in the broadcast location of test songs with varying pulse-rates. By applying an information-theoretic approach, we identified a set of response parameters that best predict a previously described pulse-rate preference. These parameters were incorporated into an index to describe temporal pattern selectivity during walking phonotaxis. Our study also revealed that in Floridian Ormia ochracea, the pulse rate preference function is not affected by the locomotor mode (walking vs. flying) used in phonotaxis. Furthermore, we describe for the first time, pulse rate selectivity in Californian Ormia ochracea. Both populations have pulse rate preference functions with peak selectivity between 50 and 60 Pulses/s (pps). Previous studies demonstrating natural differences in host song preferences (Floridian O. ochracea preferring Gryllus rubens and Californian O. ochracea preferring Gryllus lineaticeps calling songs) may be based on other temporal parameters aside from pulse rate. Finally, we discuss the advantages and limitations of our approach in quantifying signal selectivity. This approach can be applied broadly to study signal preferences in other acoustic parasitoid flies and potentially other eavesdroppers that exhibit taxis behaviors in response to the communication signals of prey.

Receivers of acoustic communication signals evaluate signal features to identify conspecifics. Changes in the ambient temperature can alter these features, rendering species recognition a challenge. To maintain effective communication, temperature coupling—changes in receiver signal preferences that parallel temperature-induced changes in signal parameters—occurs among genetically coupled signallers and receivers. Whether eavesdroppers of communication signals exhibit temperature coupling is unknown. Here, we investigate if the parasitoid fly Ormia ochracea , an eavesdropper of cricket calling songs, exhibits song pulse rate preferences that are temperature coupled. We use a high-speed treadmill system to record walking phonotaxis at three ambient temperatures (21, 25, and 30°C) in response to songs that varied in pulse rates (20 to 90 pulses per second). Total walking distance, peak steering velocity, angular heading, and the phonotaxis performance index varied with song pulse rates and ambient temperature. The peak of phonotaxis performance index preference functions became broader and shifted to higher pulse rate values at higher temperatures. Temperature-related changes in cricket songs between 21 and 30°C did not drastically affect the ability of flies to recognize cricket calling songs. These results confirm that temperature coupling can occur in eavesdroppers that are not genetically coupled with signallers.

Receivers of acoustic communication signals evaluate signal features to correctly identify conspecifics. Environmental variation such as changes in the ambient temperature can alter signal features that may render species recognition a challenge. In field crickets, song temporal features evaluated for species recognition vary with temperature, and the intended receivers of these signals exhibit signal preferences that are temperature-coupled to maintain effective communication. Whether eavesdroppers of communication signals exhibit similar temperature-coupled preferences is unknown. Here, we investigate whether the parasitoid flyOrmia ochracea, an eavesdropper of cricket calling songs, exhibit song pulse rate preferences that are temperature-coupled. We use a high-speed treadmill system to record walking phonotaxis at three ambient temperatures (21, 25, and 30 ℃) in response to songs that varied in pulse rates (20 to 90 pulses per second). Total walking distance, peak steering velocity, angular heading, and the phonotaxis performance index varied with song pulse rates and were affected by ambient temperature. The peak of phonotaxis performance index preference functions became broader and exhibited a high-pass shape, shifting to higher pulse rate values at higher temperatures. Temperature related changes in cricket songs between 21-30 ℃ will not drastically affect the ability of flies to recognize cricket calling songs.

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