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Animal Psychoacoustics
Figure 4. Barn owl (Tyto alba) in an operant setup in the laboratory of Dr. Georg Klump at the Technical University of Munich. The owl sitting on the rear perch is breaking an infra- red beam (left). When it discriminates a change in the repeat- ing background sound, it is required to fly to the front perch (right), breaking another infrared beam for a food reinforce- ment. Photo courtesy of Willi Maile, used with permission.
Psychoacoustics studies have revealed interesting trends in animal hearing, with some studies showing a remarkably conserved evolution of auditory processing across verte- brates and others showing unique adaptations for animals to thrive in specific environments. Samples of some inter- esting findings obtained using animal psychoacoustics are presented in the sections below. The studies presented span the various psychoacoustic techniques used by researchers interested in knowing more about the auditory world of ani- mals. These are not meant to be exhaustive; they are simply interesting to the author and hopefully to the reader. Where possible, the original or most complete measures have been presented instead of the most recent findings in order to highlight the pioneering investigators of this field.
Audiograms
Arguably, the most common measure of hearing in animals is the audiogram. Animals are placed in soundproof cham- bers lined with echo-reducing foam and presented with sounds of various frequencies and intensities from a cali- brated loudspeaker. After hundreds or thousands of trials to control for variation in responding and to confirm that the animal has given its most accurate results, the threshold, of- ten the lowest intensity an animal can hear 50% of the time at a given frequency, is mapped across several frequencies, and the audiogram is generated (Figure 5).
Across the animal kingdom, audiograms show great variabil- ity that can take the form of differences in how well animals hear overall (sensitivity), the frequency at which they hear best, the highest and lowest frequency they can hear, and how many frequencies they can hear. In air, larger animals like elephants (Elephas maximus) tend to have better low-
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frequency hearing than smaller animals such as mice have (Ehret, 1974; Heffner and Heffner, 1982). Moreover, preda- tors such as cats and barn owls tend to hear better than prey animals such as European starlings (Sturnus vulgaris; Neff and Hind, 1955; Konishi, 1973; Dooling et al., 1986). Euro- pean starlings and Cumberland turtles (Pseudemys scripta) hear a narrower range of frequencies than mammals.
The frequency of best sensitivity in birds like the budgerigar, the European starling, and several other songbirds is cor- related with the frequencies contained in the birds’ vocaliza- tions (Dooling, 2004). The same is true of mice (Ehret, 1989) and bats (Bohn et al., 2006), although for both groups, the tuning seems to be more correlated with infant than with adult calls so that mothers could more easily hear their own offspring calling. Bats such as the big brown bat (Eptesicus fuscus) and greater horseshoe bat (Rhinolophus ferrumequi- num) also have very good high-frequency hearing, linked to the detection and localization of their echolocation signals used for navigation and prey capture (e.g., Koay et al., 1997).
Underwater audiograms for different animals show simi- lar variations. Goldfish (Carassius auratus) are amenable to the respiratory conditioning technique, which has been
Figure 5. Sample behavioral audiograms from barn owls (blue line; Konishi, 1973), humans (dashed black line; Sivian and White, 1933), elephants (Elephas maximus, gray line; Heffner and Heffner, 1982), turtles (Pseudemys scripta, green line; Patterson, 1966), European starlings (Sturnus vulgaris, cyan line; Dooling et al., 1986), mice (purple line; Ehret, 1974), horseshoe bats (Rhinolophus ferrumequinum, black line; Long and Schnitzler, 1975), cats (Felis catus, red line; Neff and Hind, 1955) and chimpanzees (Pan troglodytes, yel- low line; Elder, 1935).