Supplementary Material for: Effects of Frequency on the Directional Auditory Sensitivity of Northern Saw-Whet Owls (Aegolius acadicus)

Many animals use sound as a medium for detecting or locating potential prey items or predation threats. Northern saw-whet owls (Aegolius acadicus) are particularly interesting in this regard, as they primarily rely on sound for hunting in darkness, but are also subject to predation pressure from larger raptors. We hypothesized that these opposing tasks should favor sensitivity to low-frequency sounds arriving from many locations (potential predators) and high-frequency sounds below the animal (ground-dwelling prey items). Furthermore, based on the morphology of the saw-whet owl skull and the head-related transfer functions of related species, we expected that the magnitude of changes in sensitivity across spatial locations would be greater for higher frequencies than low frequencies (i.e., more “directional” at high frequencies). We used auditory-evoked potentials to investigate the frequency-specific directional sensitivity of Northern saw-whet owls to acoustic signals. We found some support for our hypothesis, with smaller-magnitude changes in sensitivity across spatial locations at lower frequencies and larger-magnitude changes at higher frequencies. In general, owls were most sensitive to sounds originating in front of and above their heads, but at 8 kHz there was also an area of high sensitivity below the animals. Our results suggest that the directional hearing of saw-whet owls should allow for both predator and prey detection.诸多动物均借助声波作为媒介，以探测、定位潜在猎物，或是识别来自捕食者的威胁。北方锯鸮（Northern saw-whet owls, Aegolius acadicus）在此领域尤为值得研究：它们主要依靠声波在黑暗环境中完成捕猎，同时自身也会受到来自更大体型猛禽的捕食压力。我们据此提出假说：这两类对立的感知任务，应当会使该物种对两类声波具备更优的敏感度——一是来自多方位的低频声波（对应潜在捕食者），二是来自其身体下方的高频声波（对应地表栖息的猎物）。此外，基于锯鸮的颅骨形态以及近缘物种的头部相关传递函数（head-related transfer functions），我们推测：相较于低频声波，高频声波的敏感度随空间方位变化的幅度会更大，即高频声波的听觉方向性更强。我们采用听觉诱发电位（auditory-evoked potentials）技术，探究北方锯鸮对不同频率声波的方位特异性听觉敏感度。实验结果部分支持了我们的假说：低频声波的敏感度随空间方位的变化幅度较小，而高频声波的变化幅度则更大。总体而言，锯鸮对来自头部前上方的声波敏感度最高；但在8 kHz频段下，其对来自身体下方的声波也存在一处高敏感区域。本研究结果表明，锯鸮的方向性听觉能力，可同时满足捕食者探测与猎物定位的双重需求。