Decibels, Denizens, and Dens C. T. Tromborg and Richard G. Coss Department of Psychology, University of California, Davis As a greater proportion of the world's remaining wildlife is relegated to increasingly restricted remnants of undisturbed natural habitats and the artificial conditions of captivity, efforts should be made to provide captive animals with behaviorally and ecologically relevant environments (Carpenter, 1983; Conway, 1974; Markowitz, 1982). Artificial habitats have traditionally been improved by increasing their physical size and substrate complexity (Erwin, 1986; Thomas, 1986). This form of augmentation has frequently involved the introduction of elements characteristic of natural environments into zoo exhibits to increase their ecological significance to captive animals (Hutchings, Hancocks, and Crockett, 1984). Occasionally, interactive technologies have been introduced to provide animals with opportunities to influence their surroundings while engaging in species-typical behavior (Markowitz, 1982). Until recently, however, virtually none of these procedures have focused on the acoustic dimension of artificial environments (Tromborg, 1993). This is surprising in view of the importance of sound in the natural histories of many organisms (Ehret, 1980; 1990). For example, a considerable amount of research has been conducted on the structure and function of vocal signals and the influence of habitat acoustics on their morphology (Waser and Brown, 1986; Wiley and Richards, 1978). Other researchers have investigated the importance of audio-vocal behavior in the ontogeny of antipredator and social behaviors (Cleveland and Snowdon, 1982; Cheney and Seyfarth, 1985; Knudsen, 1984: Snowdon, 1986; Tromborg and Coss, in preparation). Still others have focused their research on the influence of unnatural noise as it relates to the genesis of acoustically mediated pathologies (see Gamble, 1982). However, research focusing on the effects of the acoustic environments characteristic of zoos and aquaria on captive animals has been rare. Studies focusing on the application of sound as an element of environmental enrichment procedures have been even less frequently conducted. This is surprising in view of the observation that most zoos and aquaria are located in or near relatively noisy urban centers. Recent research at three zoological parks in Northern California revealed that sound pressure levels (SPLs) near exhibits ranged from 68 dB to 78 dB SPL. Although these values do not attain the theoretical maximum of 85 dB SPL proposed as acceptable maxima