Cues for acoustic detection of prey: insect rustling sounds and the influence of walking substrate

Abstract

SUMMARYWhen insects walk, they generally produce sounds. These can reveal the walkersʼ presence and location to potential predators such as owls, bats and nocturnal primates. Additionally, predators might extract information on taxon, palatability, size or profitability from the rustling sounds. In contrast to ear morphology, hearing physiology and psychoacoustics of acoustically oriented predators, little attention has hitherto been paid to the acoustic structure and information content of prey sounds. An important element in the ecology of acoustic prey detection remained virtually unexplored: the influence of the substrate type on rustling sounds. In this study, we analysed amplitude and frequency parameters from insects walking on various natural substrates, in both Germany (Carabus beetles) and Madagascar (various beetles and cockroaches). The data show that rustling sound amplitude and frequency content depend on substrate type. On moist substrates arthropods produced less intense and less broadband rustling sounds than on dry substrates. Sound pressure level was reduced by about 6 dB, halving the detection range for the predator. For a given insect, rustling sound amplitude increased with walking speed. Finally, we found that the previously established correlation of arthropod size and rustling amplitude holds across multiple substrates. Based on these data, we provide for the first time estimates of realistic detection distances in the field. These distances range from below 1 m to over 13 m, depending on the substrate, insect mass, walking speed and background noise level. These estimates are crucial for an understanding of the foraging ecology, foraging efficiency and sensory ecology of acoustic predators.

Foraging bats avoid noise

“…Measured at 10 cm distance, the loudest peaks ranged from approximately 45 to 62 dB SPL. The mealworm rustling was thus roughly similar to the sounds produced by a carabid beetle (typical greater mouse-eared bat prey) walking on soil, meadow or moist leaf-litter (Goerlitz et al, 2008). Rewards were not placed on the same platform location (front, middle, back) within the two compartments to achieve a homogeneous distribution of the rewarded dishes within the sound field of the speakers.…”

“…This supports our second hypothesis, predicting that the frequency-time structure of the noise will affect its deterring intensity. The vegetation noise consisted of a series of transient, broadband signals, not unlike the clicks produced by walking arthropods (Goerlitz and Siemers, 2007;Goerlitz et al, 2008). This similarity to prey sounds might render the vegetation noise an effective masker that reduces the bats' ability to detect insects.…”

“…Arthropod rustling sounds are a series of broadband clicks; they contain frequencies of up to 100 kHz and above. The main energy is concentrated between 3 and 40 kHz, however (Goerlitz and Siemers, 2007;Goerlitz et al, 2008).…”

Inferred L/M cone opsin polymorphism of ancestral tarsiers sheds dim light on the origin of anthropoid primates

“…Our suggestion that the selective advantages of visually mediated foraging are relaxed under drier conditions is consistent with field observations in Sulawesi. During the dry season, tarsiers preferentially attend to the rustling of invertebrate prey in leaf litter [48], a broadband acoustic cue [57] to which at least one species is exceedingly sensitive [1].…”

Influence of human footprint and sensory disturbances on night-time space use of an owl

“…Mice select routes to minimize visibility and production of sound in these frequencies (Barnum et al 1992) and in some situations travel on quieter substrates (Roche et al 1999). Consequently, predators that use acoustic cues may avoid hunting in noisy areas if such noise affects their ability to detect the specific sounds made by moving prey (Hübner & Wiegrebe 2003, Goerlitz et al 2008, Siemers & Schaub 2011. When noisier areas are not avoided, predators that rely on acoustic cues for hunting are less successful in capturing prey than when hunting in quieter areas (Schaub et al 2008).…”