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The behavior of the orangutan is the least well known of all the apes. There have been fewer studies of orangutans than of the other nonhu­ man apes, either in captive environments or in the wild. To complicate matters, orangutans behave very differently in different environ­ ments. When forced to live in captivity with other orangutans, they show social play and a wide range of other behaviors that are rarely observed in wild orangutans (Horr, 1977; Maple 6c Zucker, 1978). In addition, ex-captive orangutans may display well-developed social behavior (Rijksen, 1978), even though wild orangutans lead a relatively solitary existence, albeit not as solitary as once thought (Galdikas, 1984; Mitani, Grether, Rodman 6c Priatna, 1991). Contrary to earlier opinions (e.g., Kohler, 1926), in the laboratory orangutans display the most well-developed abilities of all of the apes to manipulate objects as tools and to solve problems. Orangutans undergoing re­ habilitation in reserves in Indonesia and Malay­ sia display more use of tools than do wild or­ angutans, and the latter use tools in their natural environment far less frequently than wild chimpanzees (Galdikas, 1982). Orangu­ tans are the most arboreal of the apes, and in that environment they may use their cognitive capacities to negotiate locomotion through the trees and to learn spatiotemporal relationships about the location of food and other resources (Bard, 1990). Orangutans show a remarkable ability to adapt to different environments, and their cognitive abilities and sociality vary ac­ cordingly (Mitani et al., 1991). Not surpris­ ingly, therefore, orangutans present the com­ parative psychologist with one of the greatest challenges, and that challenge is imperative, since the species is diminishing in number along

with the destruction of its native habitat (Kap­ lan 6c Rogers, 1994).