Threshold assessment, categorical perception, and the evolution of reliable signaling

Animals often use assessment signals to communicate information about their quality to a variety of receivers, including potential mates, competitors, and predators. But what maintains reliable signaling and prevents signalers from signaling a better quality than they actually have? Previous work has shown that reliable signaling can be maintained if signalers pay fitness costs for signaling at different intensities and these costs are greater for lower quality individuals than higher quality ones. Models supporting this idea typically assume that continuous variation in signal intensity is perceived as such by receivers. In many organisms, however, receivers have threshold responses to signals, in which they respond to a signal if it is above a threshold value and do not respond if the signal is below the threshold value. Here, we use both analytical and individual-based models to investigate how such threshold responses affect the reliability of assessment signals. We show that reliable signaling systems can break down when receivers have an invariant threshold response, but reliable signaling can be rescued if there is variation among receivers in the location of their threshold boundary. Our models provide an important step towards understanding signal evolution when receivers have threshold responses to continuous signal variation.

The fitness consequences of honesty: under-signalers have a survival advantage in song sparrows

How honest or reliable signaling can evolve and be maintained has been a major question in evolutionary biology. The question is especially puzzling for a particular class of signals used in aggressive interactions: threat signals. Here we report a study on song sparrows (Melospiza melodia) in which we assayed males with playbacks on their territories to quantify their aggressiveness (flights and close proximity) and aggressive signaling levels (rates of soft song, a close range signal reliably predicting attack) and asked whether these traits affect individuals’ survival on territory. We found that the effect of aggressive signaling via soft song interacted with aggressive behaviors such that there was a negative correlational selection: among males with low aggression, those males that signaled at higher levels (over-signalers) had higher survival whereas among males with high aggression those that signaled at low levels (under-signalers) survived longer. In other words, males who deviate from reliable signaling have a survival advantage. These results, along with previous research that suggested most of the deviation from reliable signaling in this system is in the form of under-signaling (high aggression males signaling at low levels) pose a puzzle for future research on how this reliable signaling system is maintained.

The fitness consequences of honesty: under-signalers have a survival advantage in song sparrows

How honest or reliable signaling can evolve and be maintained has been a major question in evolutionary biology. The question is especially puzzling for a particular class of signals used in aggressive interactions: threat signals. Here we report a study on song sparrows (Melospiza melodia) in which we assayed males with playbacks on their territories to quantify their aggressiveness (flights and close proximity) and aggressive signaling levels (rates of soft song, a close range signal reliably predicting attack) and asked whether these traits affect individuals’ survival on territory. We found that the effect of aggressive signaling via soft song interacted with aggressive behaviors such that there was a negative correlational selection: among males with low aggression, those males that signaled at higher levels (over-signalers) had higher survival whereas among males with high aggression those that signaled at low levels (under-signalers) survived longer. In other words, males who deviate from reliable signaling have a survival advantage. These results, along with previous research that suggested most of the deviation from reliable signaling in this system is in the form of under-signaling (high aggression males signaling at low levels) pose a puzzle for future research on how this reliable signaling system is maintained.

Aggressiveness, reliable signaling and survival in a wild songbird

The evolution and maintenance of honest or reliable signaling has been a major question in evolutionary biology. The question is especially puzzling for a particular class of signals used in aggressive interactions: threat signals. Here we report a study on song sparrows (Melospiza melodia) in which we assayed males with playbacks in their territories to quantify their aggressiveness (flights, and close proximity) and aggressive signaling levels (rates of soft song, a close range signal reliably predicting attack) and asked whether these traits affect individuals’ survival on territory. We found that the effect of aggressive signaling via soft song interacted with aggressive behaviors such that there was a negative correlational selection: among males with low aggression, those males that signaled at higher levels (over-signalers) had higher survival whereas among males with high aggression those that signaled at low levels (under-signalers) survived longer. In other words, males who deviate from reliable signaling have a survival advantage. These results, along with previous research that suggested most of the deviation from reliable signaling in this system is in the form of under-signaling (high aggression males signaling at low levels) pose a puzzle for future research on how this reliable signaling system is maintained.

Aggressiveness, reliable signaling and survival in a wild songbird

The evolution and maintenance of honest or reliable signaling has been a major question in evolutionary biology. The question is especially puzzling for a particular class of signals used in aggressive interactions: threat signals. Here we report a study on song sparrows (Melospiza melodia) in which we assayed males with playbacks in their territories to quantify their aggressiveness and aggressive signaling levels and asked whether these affect their survival on territory. We found that the effect on survival of residual signaling (signaling above or below the level predicted by their aggressiveness) depended on aggression levels such that among males with low aggression, those with higher residual signaling scores had higher survival. The residual signaling did not have a strong effect among high aggression males. Aggressiveness by itself did not have an effect on survival. These results present a first step in understanding the fitness consequences of honest signaling in aggressive contexts.