Male hyraxes increase song complexity and duration in the presence of alert individuals

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The goal of vocal communication is the efficient delivery of signals to a target audience. Long and complex vocalizations are especially challenging because they are subject to environmental interference and may incur significant costs for the signaler. One of the various ways of increasing the efficiency of signal delivery is enhancement of communicational efforts only when potential audience is perceived nearby in an attempt to maximize the benefit over the cost of signaling (audience effect). Another way is signaling subsequently to events that intensify individual alertness in order to ensure audience attentiveness and sensitivity to the transmitted information. Rock hyraxes (Procavia capensis) are social mammals that use complex acoustic signals to communicate. Adult male hyraxes produce elaborate vocalizations (known as songs) that serve as honest advertisements of their quality. By combining analysis of male hyrax songs recorded in the field over a period of 11 years and playback experiments, we found that songs performed during and following attention-grabbing events (agonistic interactions, alarm calls, predator presence, and songs performed as a reply to conspecific singing) have an increased structural and syntactic complexity in comparison to spontaneous singing. Male hyraxes demonstrate a cognitive ability to optimize their advertising efforts in response to multiple types of events. This signaling strategy exploits the effective communication window created by attention-grabbing events and by the presence of an alert audience to delivers structurally and syntactically enhanced signal. To our knowledge, this is the first report of syntactic complexity of vocal signaling being altered following various triggers that change conspecifics mental state in terrestrial mammals.