High detectability costs select weak warning signals in chemically defended gregarious prey

Abstract The evolution of costly chemical defenses in cryptic prey is puzzling, as conspicuousness should be adaptive for defended prey by enhancing predator avoidance learning. Here, we explore three hypotheses that may promote weak visual signal strategies: (1) Low prey conspicuousness is favored in diverse predator communities; (2) Conspicuousness is less critical if color patterns or behavior allow defended prey to be distinguished from cryptic prey; and (3) in gregarious prey species, aggregation itself could act as a deterrent signal. We used chemically defended and gregarious European pine sawfly (Neodiprion sertifer) larvae, exhibiting green-black-grey coloration, as our study system. First, we manipulated the conspicuousness of the larvae by placing them either solitarily or in groups and marking them with either cryptic or conspicuous coloration, then testing their survival against predators in the field. Second, we analyzed how conspicuous the ventral side (which larvae reveal during a defense display) of N. sertifer appears to avian predators and how its conspicuousness correlates with the larvae's chemical defenses. Our results indicate that increased conspicuousness was costly and decreased larvae survival. The ventral side of larvae making the defense display was more conspicuous (showed higher luminance) than that of larvae not displaying; however, this was not associated with chemical defense traits. The signaling function of synchronous displays involving defensive secretions in sawflies presents a promising avenue for future studies on the mechanisms by which weak visual signals operate in nature.