A determination was made of the gestational interval over which the Subhuman Primate Pregnancy Test, a hemagglutination inhibition test for urinary chorionic gonadotropin, accurately indicated conception and the continuation of pregnancy in an orangutan (Pongo pygmaseus). The initial positive diagnostic test response occurred about 8 months before parturition and positive responses continued th...

Social interactions are the result of individuals' cooperative and competitive tendencies expressed over an extended period of time. Although social manipulation, i.e., using another individual to achieve one's own goals, is a crucial aspect of social interactions, there has been no comprehensive attempt to differentiate its various types and to map its cognitive and motivational determinants. ...

Several primate species form expectations based on others' outcomes, responding negatively when their outcomes differ from their partners'. The function and evolutionary pathway of this behavior are unknown, in part because all of the species which have been tested thus far share traits related to a gregarious lifestyle, intelligence, and cooperativeness. Our goal was to test whether inequity i...

Primates spend a significant proportion of their lives at sleeping sites, and the selection of a secure and stable nest tree can be crucial for the individual’s survival and fitness. This study examined nesting site preferences of Bornean Orangutans Pongo pygmaeus wurmbii (Tiedemann) in a degraded peat-swamp forest in the Sabangau catchment, Central Kalimantan, Indonesia. Orangutan nests were i...

Understanding the behavior of others in a wide variety of circumstances requires an understanding of their psychological states. Humans' nearest primate relatives, the great apes, understand many psychological states of others, for example, perceptions, goals, and desires. However, so far there is little evidence that they possess the key marker of advanced human social cognition: an understand...

Symmetric structures are of importance in relation to aesthetic preference. To investigate whether the preference for symmetric patterns is unique to humans, independent of their cultural background, we compared two human populations with distinct cultural backgrounds (Namibian hunter-gatherers and German town dwellers) with one species of non-human great apes (Orangutans) in their viewing beha...