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But a handful of conservation biologists have recently been making appeals to consider the sensory ecology, the way organisms acquire, process, and share information, of spe- cies (Dominoni et al., 2020). These investigators implore us to transition away from questions like “What do these organisms eat?” and “Who eats them?” toward questions that ask, “In what ways do these animals find food or avoid being eaten?” For many invertebrates, the ubiquitous answer to questions concerning the mechanism for find- ing food or mates, avoiding predators, choosing suitable habitats, or communicating with conspecifics is “sound.”
It just so happens that over the past century, while inver- tebrate populations have been in free fall, sound from increased human activity has been rising (Buxton et al., 2017; Goulson, 2019). For vertebrates, anthropogenic sound has been found to impact behavior and physiology, leading, in some cases, to reductions in vertebrate spe- cies abundances (Shannon et al., 2016). However, much less is known about the impact of anthropogenic sound on invertebrates. Results from recent research aiming to bridge the gap between two fields of study, invertebrate bioacoustics and anthropogenic sound, suggest that in searching for the major contributors to invertebrate decline, we might look first to anthropogenic sound.
Invertebrate Bioacoustics
Once submerged in alcohol and placed on a museum shelf, little attention is paid to the sensory life that defined the day to day of a living invertebrate. At first glance, they would appear to be too strange and void of emotion to interact with sound in a meaningful way and, therefore, disqualified from being impacted by anthro- pogenic sound. Moreover, in contrast to mammals, most invertebrates lack protruding folds that are identifiable as ears. They don’t delight us with their audible praise of the morning sun the way that birds do. Their anguish isn’t conveyed through familiar bellows. But invertebrates have rich sensory lives that are much different from our own and are often dominated by sound.
Recall the cacophony of crickets singing on a warm summer night (available at youtu.be/fh3uNUrAnss). Crickets, like humans and vertebrates, rely on airborne pressure waves to sense and convey acoustic information. However, the audibility of a cricket’s song to human ears is an exception among invertebrates. In fact, most of the invertebrate sounds you know, like those produced by
crickets, cicadas, katydids, and grasshoppers, are depar- tures from the invertebrate rules of sound production. (For more on the bioacoustics of these animals, see Pollack, 2017.) Few invertebrates are large enough to efficiently move air (Bennet-Clark, 1998) and thus resort to trans- mitting information through particle oscillations (particle motion) or vibrations of solids (substrate-borne sound), modalities of sound out of the sensory reach of humans.
Invertebrates sense and convey information through not only air but also through water, soils, rocks, leaves, and plant stems. Many marine invertebrates have hearing organs that are highly sensitive to low-frequency particle motion rather than pressure waves (Popper and Hawkins, 2018). Among terrestrial invertebrates, over 90% use some type of substrate-borne sound for communication
Figure 1. Average frequency of signals by mechanism for 97 invertebrate species that communicate with substrate-borne sound. Included are the four most common mechanisms that invertebrates use for producing substrate-borne sound: tremulation (oscillations of a body part), wing-fanning (rapid oscillation of wings), percussion (transient impacts of an appendage against another appendage or against the substrate), and stridulation (rubbing of two rigid structures against one another) (Raboin and Elias, 2019). Nearly all species that communicate with substrate-borne sound use frequencies that overlap with the highest energy anthropogenic sound (<2,000 Hz; orange box). Pink boxes, 1st to 3rd quartile; black lines, 95% confidence intervals; black circles, points outside the confidence interval.
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