Page 36 - Summer 2021

INAUDIBLE NOISE POLLUTION
swamp out any courtship pulses from male planthoppers, reducing the female’s ability to perceive, locate, or assess the caller. In the height of the summer, when mating season is in full swing, vehicles may pass her leaf every few seconds, reducing her many mating opportunities to just a few. When she finally finds a mate, she may be less discerning in her choice, impacting generations to come.
Noise Impacts Behavior and Physiology
Masking, where an acoustic signal of interest (natural environment stimulus) and pollutant sound occur simulta- neously in time and space and with similar characteristics, is, perhaps, the most often observed impact of anthro- pogenic sound to vertebrates and invertebrates alike. Of the many invertebrates that rely on sound to communi- cate during courtship and mating, few have encountered masking sound under scientific observation and experi- mentation. For those that have, such as crickets, katydids, grasshoppers, fruit flies, stink bugs, and leafhoppers, anthropogenic sound tends to drown out any information encoded in calls, reducing the receiver’s ability to identify, locate, and assess potential mates (Raboin and Elias, 2019).
For Ormea ochracea, a tiny parasitoid fly, the presence of masking sound means that they are unable to find a cricket host in which to deposit their larvae. Females of this spe- cies have exquisitely sensitive ears tuned to the frequencies of calling crickets that allows them to locate individuals with accuracy. On arriving at a calling cricket, O. ochracea deposits her larvae so that they might feed on the cricket as they grow from larva to pupa. When anthropogenic sound overlaps with cricket calling, O. ochracea’s accuracy in finding a host plummets, potentially limiting her ability to successfully raise offspring (Lee and Mason, 2017).
Burying beetles are scavenging invertebrates that scour the landscape in search of dead and decomposing ani- mals. On locating one, paired burying beetles cooperate to excavate a grave beneath the carcass, eventually low- ering it to a depth at which it can be easily covered with soil (see youtu.be/Ua1wC59phpk). They remain with the carcass underground for weeks to lay eggs and work side by side in caring for the young. Beetles facilitate carcass burial, courtship, and care for the young with their part- ner via stridulatory vibrations. However, low-frequency anthropogenic sound disrupts cooperation, possibly throughmasking,andresultsinfeweroffspringforpairs (Phillips et al., 2020).
Sensory pollutants can also mislead animals into invoking inappropriate responses when the animal wrongly attributes the sensory pollutant as being a natural stimulus. Anthro- pogenic sound is misleading when it is similar in acoustic characteristics to natural stimuli but occurs differently in time or space. Misleading sensory pollutants often lead to mal- adaptive behaviors or ecological traps that result in increased mortality, predation, or decreased reproductive success of individuals. Worm charming can be thought of as a mislead- ing stimulus because it mimics the acoustic frequencies of earthworm predators. The worms, having evolved behavioral responses to these frequencies, respond predictably and yet still find themselves in the hands of a predator, humans.
But anthropogenic sound need not overlap in location, characteristics, or even sensory modality to cause harm. Distraction occurs when a pollutant stimulus occupies the limited attention of an individual, thereby involuntarily diverting their attention from some other stimulus or task.
Through distraction, anthropogenic sound can penetrate deep (higher level) cognitive processes of animals, such as risk assessment, navigation, memory retrieval, and learn- ing. Hermit crabs retreat inside their shells when predators approach. They rely, in part, on sound to identify when external conditions merit withdrawal, but in the presence of sound from boats, distracted hermit crabs allow preda- tors to get closer before retreating, exposing themselves to increased risk of predation (Chan et al., 2010).
Finally, anthropogenic sound does not only impact the ways that invertebrates send, receive, and process information (by masking, misleading, or distracting), it can also impact basic physiological and developmental processes. Some sounds have long been considered stressors for humans. Similarly, road sound raised the heart rates of monarch caterpillars (Davis et al., 2018), and boat sound elicited an immune response in lobsters consistent with a stress response (Celi et al., 2015). When exposed to chronic anthropogenic sound during development, crickets took longer to reach maturity and had shorter life spans (Gurule-Small and Tinghitella, 2019). Interestingly, diverse stress responses of a small group of sediment-dwelling marine invertebrates in the presence of anthropogenic sound altered the behavior and physiology in a such a way that impacted overall ecosystem nutrient cycling (Solan et al., 2016).
Among the many lingering questions surrounding invertebrates and anthropogenic sound, some of the
36 Acoustics Today • Summer 2021