Page 25 - 2018Fall

used for decades with success across several psychophysi- cal methods (e.g., Fay, 1995). Killer whales (Orcinus orca) and bottlenose porpoises (Tursiops truncatus), like bats, are known to use echolocation for navigation and prey capture and are more sensitive at high frequencies than many other animals (Johnson, 1967; Hall and Johnson, 1971). California sea lions (Zalophus californianus) and cormorants (Phalacro- corax carbo) are sensitive to both underwater and airborne sounds, although both seem to be better suited for under- water listening than airborne listening (Schusterman et al., 1972; Schusterman, 1974; Johansen et al., 2016).
Once baseline audiograms are obtained for animals, com- parisons can be made for different strains, species, sexes, and ages. In the cat, the shape of the audiogram changes remark- ably through development (Ehret and Romand, 1981). Be- havioral audiograms from very young animals are extremely difficult to obtain, making this a relatively underexplored field of study.
Auditory threshold sensitivities from aged animals also show differences across species. Aged mice of the C57BL/6J strain show a progressive hearing loss from high to low fre- quencies, as in humans, whereas mice of the CBA/CaJ strain showed no significant hearing loss within the same time pe- riod (Prosen et al., 2003). CBA/CaJ mice do lose their high- frequency hearing; it just occurs at a later point in their life span (Kobrina and Dent, 2016). Strain differences are thus an important consideration not just for hearing but also for changes in auditory acuity across the life span.
The ability to regenerate sensory hair cells of the inner ear (the cells that convert sound energy into electric signals that go to the brain) has made birds an interesting model for stud- ies of hearing and hearing loss over the years. In one study by Langemann and colleagues (1999), European starlings well past their typical life spans (8-13 years old) showed hearing comparable to that found in young starlings (6-12 months old). This demonstrates that the regenerating sensory epi- thelium of birds, typically attributed to repairing damaged hair cells, protects birds from the typical hearing loss such as that seen in mammals like the rhesus macaque (Macaca mulatta; Bennett et al., 1983), false killer whale (Pseudorca crassidens; Kloepper et al., 2010), and human (reviewed in Gates and Mills, 2005). Advances have been made in recent years to extend the nonmammalian hair cell regeneration capacity to mammals in attempts to eliminate presbycusis (reviewed in Rubel et al., 2013; Lewis et al., 2016).
The audiogram is an important first step in cataloging the acoustic world of an animal. However, the detection of simple stimuli in completely quiet environments tells us little about the everyday life of most animal species. Animal bioacoustics researchers are also interested in knowing how animals can (or cannot) discriminate between sounds, categorize sounds, and localize sounds in their environments. A wide variety of animals have been tested on these tasks using animal psycho- physics, and they complement the audiogram in rounding out what is known about the acoustic world of many species.
Discrimination of Complex Signals
Animals that use acoustic signals for communication often must judge one signal against another for evaluating potential mates or to determine if a nearby animal is familiar or unfamil- iar. In birds, females are known to prefer males with larger song repertoires over those with smaller song repertoires, songs produced at a fast rate over slower songs, songs with broader bandwidths than with narrower bandwidths, and songs with very little variation from rendition to rendition over songs with more variation (reviewed in Searcy and Yasukawa, 1996).
Species-specific acoustic signal preferences like these are seen across the animal kingdom. Presumably, then, animals must be able to discriminate between different acoustic sig- nals to aid in the mate choice process. Psychophysical stud- ies measuring the discrimination of natural vocalizations by birds have shown that they can discriminate among calls from their own species better than calls from another spe- cies (Dooling et al., 1992), that some birds are capable of dis- criminating extremely small differences in signals differing in temporal fine structure (reviewed in Dooling, 2004), that males learn to discriminate between familiar songs at a fast- er rate than they learn to discriminate between unfamiliar songs, and that call discrimination can differ across seasons (Cynx and Nottebohm, 1992). Normal social development has also been shown to be important for vocalization dis- crimination by birds (Sturdy et al., 2001).
Psychophysical studies on the perception of vocalizations have not been limited to birds. Four species of macaque mon- keys (Macaca sp.) discriminate between vocalizations using different acoustic cues depending on their experience with the vocalizations, paralleling humans on speech perception tasks (Zoloth et al., 1979). Human speech perception has been measured in several animal species. Animals such as chinchil- las (Chinchilla lanigera) have been found to have similar cat- egorical boundaries as humans (reviewed in Kuhl, 1981).
Fall 2017 | Acoustics Today | 23