Equivalence classification by California sea lions using class-specific reinforcers.

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.

Masking in three pinnipeds: underwater, low-frequency critical ratios.

Behavioral techniques were used to determine underwater masked hearing thresholds for a northern elephant seal (Mirounga angustirostris), a harbor seal (Phoca vitulina), and a California sea lion (Zalophus californianus). Octave-band white noise maskers were centered at five test frequencies ranging from 200 to 2500 Hz; a slightly wider noise band was used for testing at 100 Hz. Critical ratios were calculated at one masking noise level for each test frequency. Above 200 Hz, critical ratios increased with frequency. This pattern is similar to that observed in most animals tested, and indicates that these pinnipeds lack specializations for detecting low-frequency tonal sounds in noise. However, the individual pinnipeds in this study, particularly the northern elephant seal, detected signals at relatively low signal-to-noise ratios. These results provide a means of estimating zones of auditory masking for pinnipeds exposed to anthropogenic noise sources.

Why pinnipeds don't echolocate.

Odontocete cetaceans have evolved a highly advanced system of active biosonar. It has been hypothesized that other groups of marine animals, such as the pinnipeds, possess analogous sound production, reception, and processing mechanisms that allow for underwater orientation using active echolocation. Despite sporadic investigation over the past 30 years, the accumulated evidence in favor of the pinniped echolocation hypothesis is unconvincing. We argue that an advanced echolocation system is unlikely to have evolved in pinnipeds primarily because of constraints imposed by the obligate amphibious functioning of the pinniped auditory system. As a result of these constraints, pinnipeds have not developed highly acute, aquatic, high frequency sound production or reception systems required for underwater echolocation. Instead, it appears that pinnipeds have evolved enhanced visual, tactile, and passive listening skills. The evolutionary refinement of alternative sensory systems allows pinnipeds to effectively forage, navigate, and avoid predators under water despite the lack of active biosonar capabilities.

Underwater temporary threshold shift induced by octave-band noise in three species of pinniped.

Pure-tone sound detection thresholds were obtained in water for one harbor seal (Phoca vitulina), two California sea lions (Zalophus californianus), and one northern elephant seal (Mirounga angustirostris) before and immediately following exposure to octave-band noise. Additional thresholds were obtained following a 24-h recovery period. Test frequencies ranged from 100 Hz to 2000 Hz and octave-band exposure levels were approximately 60-75 dB SL (sensation level at center frequency). Each subject was trained to dive into a noise field and remain stationed underwater during a noise-exposure period that lasted a total of 20-22 min. Following exposure, three of the subjects showed threshold shifts averaging 4.8 dB (Phoca), 4.9 dB (Zalophus), and 4.6 dB (Mirounga). Recovery to baseline threshold levels was observed in test sessions conducted within 24 h of noise exposure. Control sessions in which the subjects completed a simulated noise exposure produced shifts that were significantly smaller than those observed following noise exposure. These results indicate that noise of moderate intensity and duration is sufficient to induce TTS under water in these pinniped species.

Low-frequency amphibious hearing in pinnipeds: methods, measurements, noise, and ecology.

Aerial low-frequency (100-6400 Hz) hearing thresholds were obtained for one California sea lion (Zalophus californianus), one harbor seal (Phoca vitulina), and one northern elephant seal (Mirounga angustirostris). Underwater thresholds over a similar frequency range (75-6300 or 6400 Hz) were obtained for these three animals in addition to another California sea lion. Such data are critical, not only for understanding mechanisms about amphibious hearing and relating them to pinniped ecology and evolution, but also for identifying species at risk to man-made noise in the marine environment. Under water, the elephant seal was most sensitive, followed by the harbor seal and the sea lions. In air, the harbor seal was most sensitive, followed by the older of the two sea lions and the elephant seal. The following trends emerged from comparisons of each subject's aerial and underwater thresholds: (a) the sea lion (although possessing some aquatic modifications) is adapted to hear best in air; (b) the harbor seal hears almost equally well in air and under water; and (c) the elephant seal's auditory system is adapted for underwater functioning at the expense of aerial hearing sensitivity. These differences became evident only when aerial and underwater thresholds were compared with respect to sound pressure rather than intensity. When such biologically relevant comparisons are made, differences in auditory sensitivity can be shown to relate directly to ecology and life history.

Artificial language comprehension and size transposition by a California sea lion (Zalophus californianus).

A variation of the conditional discrimination procedure defines relations between stimuli (for example, gestural signs and their referents), and it has been used to study language comprehension in California sea lions. The animals followed instructions given by a trainer's gestures designating properties of size, brightness, and location (adjectives), types of objects (nouns), and actions (verbs). The signs can be combined and recombined according to a conditional sequence or syntax. In this study, we sought to determine whether adjectives for size had an absolute meaning, that is, small and large, as well as a comparative meaning, that is, smaller and larger. A sea lion, Rocky, was given experience with signs designating standard small and large spheres in commands like LARGE BALL MOUTH. On transposition tests, the small ball was removed and the previously designated large ball was paired with an even larger one. The results showed that the adjectives had both an absolute and a relative meaning. Object choices and searching behavior revealed that the sea lion processed information about the relation of size as well as about the specific characteristics of the sizes of spheres that instantiated the relations.

Underwater audiogram of the California sea lion by the conditioned vocalization technique.

Conditioning techniques were developed demonstrating that pure tone frequencies under water can exert nearly perfect control over the underwater click vocalizations of the California sea lion (Zalophus californianus). Conditioned vocalizations proved to be a reliable way of obtaining underwater sound detection thresholds in Zalophus at 13 different frequencies, covering a frequency range of 250 to 64,000 Hz. The audiogram generated by these threshold measurements suggests that under water, the range of maximal sensitivity for Zalophus lies between one and 28 kHz with best sensitivity at 16 kHz. Between 28 and 36 kHz there is a loss in sensitivity of 60 dB/octave. However, with relatively intense acoustic signals (> 38 dB re 1 mub underwater), Zalophus will respond to frequencies at least as high as 192 kHz. These results are compared with the underwater hearing of other marine mammals.