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Animal Bioacoustics
body parts together in a process known as stridulation. Al- though insects such as grasshoppers are probably the most famous animals to do this, stridulation is documented in a wide variety of animals including catfish, seahorses, birds such as manakins and hummingbirds, and spiny lobsters. Other animals such as rattlesnakes vibrate appendages to make sounds, and still others force air through a small ori- fice to call. This last mechanism of sound production occurs through the larynx in humans and nonhuman primates and through an organ known as the syrinx in birds. The syrinx is a specialized version of the larynx that allows songbirds to breathe while they sing and results in the beautifully com- plex songs we hear outside our windows each spring (e.g., Marler and Slabbekoorn, 2004). The complexity of song pro- duced by songbirds is the subject of interest for many animal bioacousticians.
Speaking of songs, the next stage of the animal communi- cation process studied by animal bioacousticians involves the signals themselves. Whether produced via stridulation or forcing air through a syrinx, many signals that animals produce can be quite complex and meaningful to receivers. Animals can convey information about species, family, and sometimes even individual identity in their signals. They can let others around them know about some new positive (food) or negative (predatory) object in their environment. Animal bioacousticians have learned that females of some species of birds prefer highly stereotyped acoustic signals, whereas other females prefer males who are good improvis- ers. Some animals such as whales have the ability to change many spectrotemporal properties of their sounds, whereas others such as ants are limited. Finally, many species of ani- mals are born knowing their vocalizations, whereas others must be exposed to a tutor to produce species-appropriate calls and songs. Knowing what sounds animals make and understanding their associated behavior is crucial informa- tion for population monitoring by noninvasive passive lis- tening and, ultimately, for conservation management. Ani- mal bioacousticians have borrowed signal-processing tools from unrelated fields (e.g., computer science or geophysics) and developed algorithms for the detection, classification, localization, tracking, and density estimation of (vocal) ani- mals (e.g., Au and Hastings, 2008).
Another popular area of research by animal bioacousticians is the effect of the environment on animal communication (Wiley, 2015). A male frog may produce a beautifully com- plex set of calls intended to attract all female conspecifics in the vicinity to mate with him, but if the environment is too
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noisy, the females will not receive those signals and he will be out of luck. Noise in the environment can be weather related or produced by humans or other animals. Noise can mask animal communication, change behavior, induce stress, in- jure tissues, and thus disrupt critical life functions (e.g., Pop- per and Hawkins, 2012, 2016). Animals have adapted vari- ous ways to lessen the effects of noise on the communication process. Some whales near whale-watching boats produce longer calls than those in quieter environments. Many spe- cies of animals produce louder calls in noise than in quiet. Other animals produce higher frequency signals to avoid the low-frequency noise of cities. These changes in sound emis- sion are collectively known as the Lombard Effect that has been well documented in humans. Still, the broader impact of noise on animals and the limits of many species to adapt in noisy environments need much more attention.
Finally, the last stage of the communication process is the receiver, with many animal bioacousticians studying animal sound reception mechanisms, anatomy, and neurophysiol- ogy. Discovering what animals hear can be accomplished in the field or in the laboratory by performing playback or psy- choacoustic experiments. Bioacousticians know a lot about what animals detect, discriminate, localize, and categorize (e.g., Fay, 1988). Animals such as bats, cats, dolphins, and barn owls have excellent auditory acuity and are used as behavioral, anatomical, and physiological models for audi- tory processing. Insects are often able to localize sounds ac- curately despite having ears right next to each other. Some animals have coevolved with their predators to avoid being eaten, such as crickets hearing high-frequency bat calls and fish hearing high-frequency dolphin signals. Comparative hearing studies have advanced the field of animal bioacous- tics in both quiet and noisy environments for the purposes of understanding the evolution of auditory systems as well as for creating models of human hearing and disorders.
Some animals not only produce sound for communication but also for echolocation. Bats and dolphins have a biosonar system that lets them navigate and forage in dark places, at night or in murky deep waters (Griffin, 1958; Au, 1993). So- nar stands for SOund Navigation And Ranging and involves the emission of brief broadband clicks and the processing of echoes. The time delay between the outgoing and incoming click carries information about the range of the reflecting object, whereas the intensity and phase differences between the incoming clicks at the two ears yield information about the direction to the reflecting object. The biosonar system contains only one source and two receivers (the ears) as op-