Abstract
The threat sensitivity hypothesis predicts that organisms will evaluate the relative danger of and respond differentially to varying degrees of predation threat. Doing so allows potential prey to balance the costs and benefits of anti-predator behaviors. Threat sensitivity has undergone limited testing in the auditory modality, and the relative threat level of auditory cues from different sources is difficult to infer across populations when variables such as background risk and experience are not properly controlled. We experimentally exposed a single population of two sympatric gull species to auditory stimuli representing a range of potential threats in order to compare the relative threat of heterospecific alarm calls, conspecific alarms calls, predator vocalizations, and novel auditory cues. Gulls were able to discriminate among a diverse set of threat indicators and respond in a graded manner commensurate with the level of threat. Vocalizations of two potential predators, the human voice and bald eagle call, differed in their threat level compared to each other and to alarm calls. Conspecific alarm calls were more threatening than heterospecfic alarm calls to the larger great black-backed gull, but the smaller herring gull weighed both equally. A novel cue elicited a response intermediate between known threats and a known non-threat in herring gulls, but not great black-backed gulls. Our results show that the relative threat level of auditory cues from different sources is highly species-dependent, and that caution should be exercised when comparing graded and threshold threat sensitive responses.
Highlights
Prey organisms are faced with the challenge of responding to varying degrees of predation threat in a way that maximizes the benefits of deterring potential predators while minimizing the costs of time, energy, and risk accrued by employing anti-predator behaviors [1]
Our results show that more direct sources of information about predation risk are not always perceived as a greater threat
The relative threat level of predator vocalizations, alarm calls, and even novel stimuli varies according to the species of both sender and receiver
Summary
Prey organisms are faced with the challenge of responding to varying degrees of predation threat in a way that maximizes the benefits of deterring potential predators while minimizing the costs of time, energy, and risk accrued by employing anti-predator behaviors [1]. The ability of organisms to assess the degree of predation risk and respond with appropriate intensity is referred to as the threat sensitivity hypothesis [2,3]. Organisms can learn to recognize predator cues through direct experience or observation of conspecifics [6,7,8], and may modify perceived predation risk based on experience over time [9]. A key question is how the source of an auditory cue influences the predation risk perceived by a receiving organism. A comparable study from the literature on chemical threat sensitivity found that greater sirens (Siren lacertian) respond cautiously to novel cues, but that cues from known predators elicit more intense responses [12]
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