The ability to group dissimilar stimuli into categories on the basis of common stimulus relations (stimulus equivalence) or common functional relations (functional equivalence) has been convincingly demonstrated in verbally competent subjects. However, there are investigations with verbally limited humans and with nonhuman animals that suggest that the formation and use of classification schemes based on equivalence does not depend on linguistic skills. The present investigation documented the ability of two California sea lions to classify stimuli into functional classes using a simple discrimination reversal procedure. Following the formation of functional classes in this context, the second experiment showed transfer of the relations that emerged between class members to a matching-to-sample procedure. The third experiment demonstrated that the functional classes could be expanded through traditionally defined equivalence relations. In these three experiments, appropriate within-class responding produced class-specific food reinforcers. Experiment 3 addressed the role of these reinforcers in equivalence classification and showed that the class-specific reinforcers were sufficient to relate new stimuli to the functional classes. These findings show that sea lions can form equivalence classes in simple and conditional discrimination procedures, and that class-specific reinforcers can become equivalence class members.

Behavioral psychophysical techniques were used to evaluate the residual effects of underwater noise on the hearing sensitivity of three pinnipeds: a California sea lion (Zalophus californianus), a harbor seal (Phoca vitulina), and a northern elephant seal (Mirounga angustirostris). Temporary threshold shift (TTS), defined as the difference between auditory thresholds obtained before and after noise exposure, was assessed. The subjects were exposed to octave-band noise centered at 2500 Hz at two sound pressure levels: 80 and 95 dB SL (re: auditory threshold at 2500 Hz). Noise exposure durations were 22, 25, and 50 min. Threshold shifts were assessed at 2500 and 3530 Hz. Mean threshold shifts ranged from 2.9-12.2 dB. Full recovery of auditory sensitivity occurred within 24 h of noise exposure. Control sequences, comprising sham noise exposures, did not result in significant mean threshold shifts for any subject. Threshold shift magnitudes increased with increasing noise sound exposure level (SEL) for two of the three subjects. The results underscore the importance of including sound exposure metrics (incorporating sound pressure level and exposure duration) in order to fully assess the effects of noise on marine mammal hearing.

Experiments have shown that human and nonhuman subjects are capable of performing new arbitrary stimulus-stimulus relations without error. When subjects that are experienced with matchingto-sample procedures are presented with a novel sample, a novel comparison, and a familiar comparison, most respond by correctly selecting the novel comparison in the presence of the new sample. This exclusion paradigm was expanded with two California sea lions that had previously formed two 10-member equivalence classes in a matching-to-sample procedure. Rather than being presented with a novel sample on a given trial, the sea lions were presented with a randomly selected familiar member of one class as the sample. One of the comparisons was a randomly selected familiar member of the alternative class, and the other was a novel stimulus. When required to choose which comparison matched the sample, the subjects reliably rejected the familiar comparison, and instead selected the unfamiliar one. Next, the sea lions were presented with transfer problems that could not be solved by exclusion; they immediately grouped the new stimuli into the appropriate classes. These findings show that exclusion procedures can rapidly generate new stimulus relations that can be used to expand stimulus classes.Key words: exclusion, fast mapping, equivalence, symmetry, differential outcomes, class-specific reinforcement, California sea lionsIn the context of human language learning (semantics), appropriate responding can be facilitated by presenting new problems in the context of familiar alternatives. For example, if a child is asked ''Which one is the pafe?'' he or she may examine an array of familiar, already named objects and then select the novel item. This phenomenon, called fast mapping, linguistic inference, or the disambiguation effect in the field of psycholinguistics, is central to the development of language. In behavior analysis and animal cognition, this type of errorless performance is known as exResearch was supported by ONR Grant N00014-99-1064 to R. J. Schusterman. A DoD AASERT Fellowship and a GAANN Fellowship provided funding for C. R. Kastak. A portion of this manuscript was presented as an invited talk to the American Psychological Association in San Francisco, August 2001. We thank Tom Zentall for constructive comments during the preparation of this article. We greatly appreciate the encouragement given to us by David Kastak throughout the experiment, and we are particularly thankful to him for his insights during the preparation of the final manuscript. We also thank the dedicated research team at the Pinniped Cognition and Sensory Systems Laboratory, especially Kirsten Jensen and Shannon Spillman.Correspondence and reprint requests may be addressed to either author at Long Marine Laboratory, University of California at Santa Cruz, 100 Shaffer Road, Santa Cruz, California 95060 (e-mail: coll@cats.ucsc.edu or rjschust@cats.ucsc.edu).clusion or emergent matching (Wilkinson, Dube, & McIlvane, 1998). Dixon (1977) coined the term e...

Interest in the hearing capabilities of California sea lions (Zalophus californianus) was first stimulated by the echolocation hypothesis and more recently by rising concern about coastal noise pollution. During a series of audiometric tests, we measured the absolute hearing sensitivity of two sea lions and two of their human investigators. Aerial hearing curves for each subject were obtained with a go/no-go procedure and standard psychophysics. Additionally, underwater hearing curves were obtained for the sea lions using the same procedures. Underwater, the older sea lion (22–25 years of age) showed hearing losses relative to the younger sea lion (13–16 years) that ranged from 10 dB at lower frequencies to 50 dB near the upper frequency limit. The older sea lions’ hearing losses in air were consistent with those measured underwater. The older human (69 years) tested also showed losses relative to the younger human (22 years). These differences ranged from 15 dB at lower frequencies up to 35 dB at the highest frequency tested. The results obtained in this study document age-related hearing losses in sea lions and humans. The findings are consistent with data on presbycusis in other mammalian species, showing that maximum hearing loss occurs at the highest frequencies.

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