Evelyn B. Hanggi

Color Vision in Horses (Equus caballus): Deficiencies Identified Using a Pseudoisochromatic Plate Test

In the past, equine color vision was tested with stimuli composed either of painted cards or photographic slides or through physiological testing using electroretinogram flicker photometry. Some studies produced similar results, but others did not, demonstrating that there was not yet a definitive answer regarding color vision in horses (Equus caballus). In this study, a pseudoisochromatic plate test--which is highly effective in testing color vision both in small children and in adult humans--was used for the first time on a nonhuman animal. Stimuli consisted of different colored dotted circles set against backgrounds of varying dots. The coloration of the circles corresponded to the visual capabilities of different types of color deficiencies (anomalous trichromacy and dichromacy). Four horses were tested on a 2-choice discrimination task. All horses successfully reached criterion for gray circles and demonstration circles. None of the horses were able to discriminate the protan-deutan plate or the individual protan or deutan plates. However, all were able to discriminate the tritan plate. The results suggest that horses are dichromats with color vision capabilities similar to those of humans with red-green color deficiencies.

Acoustic Signalling in Mother-Pup Reunions, Interspecies Bonding, and Affiliation by Kinship in California Sea Lions (Zalophus Californianus)

Pinnipeds are among the most precocial mammals (Bowen, 1991) and they are also among the most vocal. Their acoustic behavior occurs in both the atmosphere and the hydrosphere, and it is clear from both field and laboratory observations that natural selection has favored the vocal-auditory communication channel in these amphibious mammals (Schusterman, 1978). California sea lions (Zalophus californianus), in particular are one of the most vocal of all the pinnipeds, and their acoustic signals have been hypothesized primarily to repel or attract individuals by identifying the caller as to species, sex, age, location in space, and as an individual (Schusterman, 1978; Miller, 1991>).

Stimulus discrimination by horses under scotopic conditions

Scotopic vision in horses (Equus caballus) was investigated using behavioral measurements for the first time. Four horses were tested for the ability to make simple visual discriminations of geometric figures (circles and triangles) under various brightness levels within an enclosed building. Measurements of brightness ranging from 10.37 to 24.12 magnitudes per square arcsecond (mag/arcsec(2); in candelas per square meter-7.70 to 2.43E-05cd/m(2)) were taken using a Sky Quality Meter. These values approximated outdoor conditions ranging from twilight in open country to a dark moonless night in dense forest. The horses were able to solve the discrimination problems in all brightness settings up to 23.77mag/arcsec(2) (3.35E-05cd/m(2)). Moreover, they easily navigated their way around obstacles located within the testing area in extremely dim light (>23.50mag/arcsec(2); 4.30E-05cd/m(2)), which were in conditions too dark for the human experimenters to see. These findings support physiological data that reveal a rod-dominated visual system as well as observations of equine activity at night.

Remembering in California sea lions: Using priming cues to facilitate language-like performance

Two sea lions(Zalophus californianus) were trained to respond to relational commands fey bringing the last designated object (transport item, or TI) to the earlier designated object (goal item, or GI). Most errors on this task involved forgetting of the GI due to interference from memory for the TI. We reasoned that instructing the animals to act directly on the same object several times in single-object commands immediately before a relational command that included the same object as the GI should make the GI associative episode less prone to interference by the TI associative episode. Results confirmed that prior cuing or prompting in this manner did enable the sea lions to perform significantly better on relational commands when no cuing was given.

Lateral vision in horses: a behavioral investigation.

This study investigated lateral vision in horses (Equus caballus) for the first time from a behavioral point of view. Three horses were tested using a novel experimental design to determine the range of their lateral and caudolateral vision with respect to stimulus detection and discrimination. Real-life stimuli were presented along a curvilinear wall in one of four different positions (A, B, C, D) and one of two height locations (Top, Bottom) on both sides of the horse. To test for stimulus detection, the correct stimulus was paired against a control; for stimulus discrimination, the correct stimulus was paired against another object. To indicate that the correct stimulus was detected or discriminated, the horses pushed one of two paddles. All horses scored significantly above chance on stimulus detection trials regardless of stimulus position or location. They also accurately discriminated between stimuli when objects appeared in positions A, B, and C for the top or bottom locations; however, they failed to discriminate these stimuli at position D. This study supports physiological descriptions of the equine eye and provides new behavioral data showing that horses can detect the appearance of objects within an almost fully encompassing circle and are able to identify objects within most but not all of their panoramic field of view.

Underwater Acoustic Displays by Male Harbor Seals (Phoca Vitulina):Initial Results

Underwater vocalizations have been studied for a number of the phocid pinnipeds. Weddell seals (Leptonychotes weddelli) produce a wide variety of vocalizations that include chirps, trills, whistles and chugs (Schevill and Watkins, 1965; Thomas and Kuechle, 1982). These vocalizations have been classified by Thomas and Kuechle (1982) into 12 underwater calls with 34 call types. They used descriptive terminology to identify these vocalizations, i.e., what-chunk, guttural glug, mew, jaw claps, and found that male Weddell seals have a larger, more complex repertoire than females, which they appear to use in underwater territorial displays during the breeding season. Weddell seals are an excellent example of the correlation between a highly polygamous breeding system and large vocal repertoire size (Evans and Bastian, 1969). Another polygamous phocid, the harp seal (Phoca groenlandica), maintains underwater territories (Mohl, Terhune, and Ronald, 1975) and is also very vocal, with 15 vocalizations in its repertoire.