Page 34 - Summer 2021

INAUDIBLE NOISE POLLUTION
(Cocroft and Rodriguez, 2005). Using a variety of mecha- nisms, substrate-borne signalers produce sounds with frequencies below about 2,000 Hz because low frequencies experience little attenuation in solids (Figure 1) (Bennet- Clark, 1998).
Sound in Survival and Reproduction
In his 1881 publication The Formation of Vegetable Mould Through the Action of Worms, Charles Darwin provides
one of the earliest recognitions of an inaudible inverte- brate acoustic world, one in which sound is communicated through substrates. He reports, “...if the ground is beaten or otherwise made to tremble, worms believe that they are pursued by a mole and leave their burrows” (Darwin, 1881, p. 28). Worm charming, a popular technique for col- lecting earthworms by transmitting vibrations through the soil with a metal rod or machine engine, repeated Darwin’s observation by the thousands throughout the twentieth century. When faced with human-produced low-frequency vibrations (<1,000 Hz), earthworms race to the soil’s surface to avoid being eaten by an imaginary subterranean rodent, allowing worm charmers the ability to quickly collect hundreds of them to sell for fish bait (Catania, 2008) (see video at youtu.be/IGviTYCFksE) Like many invertebrates, earthworms rely on substrate-borne vibrations, in part, to avoid predators.
As prey, invertebrates use sound to identify, avoid, and deter predators. As predators, they use sound to distinguish, local- ize, and lure prey. A single web in an East African forest may include an orb-weaver spider listening intently to the vibra- tions of each silken strand of a web so that it might reveal the capture and identity of a flying insect (Landolfa and Barth, 1996). But this web might also attract another kind of inver- tebrate predator. Portia, a jumping spider that prefers to hunt other spiders, employs a creative vibratory technique in its pursuit of a meal (see youtu.be/U2WHZGpdghc). On enter- ing the web of an unwitting spider, Portia plucks the strings so that they vibrate with frequencies that mimic an ensnared and struggling insect, thereby catching the attention of the web’s owner and luring them to investigate (Tarsitano et al., 2000). When the resident spider approaches, Portia seizes and eats it. For each spider, their ability to hear and translate vibratory pulses into useful information has critical conse- quences for survival.
The overall fitness of an individual organism is, in part, deter- mined by its ability to reproduce. Crucial to this endeavor
is communication with conspecifics about reproductive status, quality, and location. Invertebrates as diverse as crick- ets, lobsters, and mosquitoes, among many others, rely on sound to attract and find mates. Just as crickets, katydids, and grasshoppers advertise their location and willingness to mate through airborne song, wolf and jumping spiders (Figure 2) (see youtu.be/AZszAaJyVTc), treehoppers, and crabs encode their ability to produce quality offspring through composed arrangements of substrate-borne syl- lables channeled to potential mates (Hill, 2009). Mosquitoes and fruit flies buzz their wings to transmit similar infor- mation through particle motion (Albert and Göpfert, 2015; Menda et al., 2019). The mechanisms, modalities, and con- text by which invertebrates use sound for reproduction may be specific to each species but, when taken broadly, it is clear that sound is a primary vehicle for sexual selection and evo- lution across invertebrates.
What reproducing is to the fitness of solitary inverte- brates, collaboration via communication is to the fitness of social invertebrates. Bees, termites, and ants commu- nicate information within communities and sometimes across species, in part, with sound. When an intruder enters or threatens an ant or termite colony, observing members beat their heads against the substrate, produc- ing percussive vibrations that alert the rest of the colony of an imminent attack (Hill, 2009). Ants and bees convey instructions about where and when to find food in vibra- tions, as in the remarkable waggle dance of honeybees (Michelsen, 2003). In an indication of the importance of acoustic communication, even some caterpillars that live
Figure 2. Male Habronattus pugillis jumping spiders (order Araneae, family Salticidae) attract mates by producing courtship songs made up of substrate-borne sound paired
with visual displays. Photograph courtesy of Marshal Hedin.
34 Acoustics Today • Summer 2021