DISCRIMINATION LEARNING IN LANGURS AND RHESUS MONKEYS

Our previous studies have demonstrated that langurs (Presbytis entellus) differ from rhesus monkeys (Macaca mulatta) in some forms and frequencies of social behavior (Manocha, 1966) and also in reactions to novel situations and objects (Singh & Manocha, 1966). The present study concerns possible differences in cognitive ability. Method.-Ss were Indian Langurs (Presbytis entellus), 2 males and 2 females with a mean estimated age of 7 yr. and 4 male and 2 female rhesus monkeys (Macaca mzrlatfa) whose mean estimated age was 8 yr. Ss were caught near urban areas. They were housed individually in metal cages measuring 3 ft. by 3 ft. by 3 ft. Previously they had been tested for reactions to novel situations. Discrimination learning tests were given in a modified Wisconsin General Test Apparatus (WGTA) as described in detail elsewhere (Singh, 1966). It consisted of a table measuring 395 ft. by 5 ft. with a superstructure supporting the opaque and one-way vision screen. A moveable test tray containing two foodwells was mounted on the table. Instead of the usual restraining cage, 3%-ft.-high side walls projected in front by 3 ft.; these also covered the sides of the living cage when the apparatus was set in place. Each S was trained to displace a gray wooden block placed on one of the foodwells and take a raisin until S responded within 5 sec. of the presentation of the test tray. Each trial for visual pattern discrimination began with the opaque screen down; the foodwell was baited with one raisin and the visual panerns, mounted on rhe wooden blocks, were placed at right angles over the two foodwells. The tray was then pushed ro S's side, the one-way vision screen lowered, and the opaque screen raised. Once S re