The evolution of novel animal signals: silk decorations as a model system

Contemporary animal signals may derive from an elaboration of existing forms or novel non-signalling traits. Unravelling the evolution of the latter is challenging because experiments investigating the maintenance of the signal may provide little insight into its early evolution. The web decorations, or stabilimenta of some orb web spiders represent an intriguing model system to investigate novel animal signals. For over 100 years, biologists have struggled to explain why spiders decorate their webs with additional threads of silk, producing a conspicuous signal on a construction whose function is to entangle unsuspecting prey. The numerous explanations for the maintenance of this behaviour starkly contrast with the absence of a plausible explanation for its evolutionary origin. Our review highlights the difficulties in resolving both the evolution and maintenance of animal signalling, and inferring the causative arrow-even from experimental studies. Drawing on recent research that focuses on physiological processes, we provide a model of the evolutionary progression of web-decorating behaviour.