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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.
This revision fixes a typo in the Lande-Arnold phenotypic selection model: the non-linear coefficient for the soft song has a positive coefficient (it was reported as negative in the previous version). Thus the coefficient implies disruptive and not stabilizing selection. The main focus of the paper remains the negative correlational selection as before. The revision also has an updated Figure 1. The manuscript is provisionally accepted for publication at Evolution.
R-code for the analyses
the code is provided as text file, please copy and paste it into R