Assessment of individual differences in the preferred proximity to a human feeder by partitioned raisin test, with two species of macaque monkeys

Abstract

All animals must acquire food and mates by approaching them despite possibilities of accompanying risks and thus are frequently encountered with approach-avoidance conflicts in daily lives. Behavioral individual differences in such situations may be considered as one of the most biologically fundamental personality trains. “Partitioned raisin test” was devised to assess this trait with macaque monkeys. It involved throwing raisins into groups of monkeys and observing the preferred distance of each from the human feeder, a source of possible harm. The test was administered to 4 groups of Japanese monkeys (30 total) and 3 groups of rhesus monkeys (19 total), all l-yr-old and matched in history. Individual differences in the preferred proximity to the feeder, as expressed by the Proximity Index (PI), were found in both species.PI was not correlated with a measure of dominance over the raisins. Individual differences inPI were also not due to territorial effects unrelated to the location of the feeder.PI was stable in five of the six monkeys re-tested after one year of interval in a newly organized group, where there supposedly had been a change in their social structure. Partitioned raisin test was shown to be capable of depicting individual differences related to differential approach bias in an approach-avoidance conflict situation. Although possible confounding effects by social factors need to be delineated in the following studies, the method may provide a handy and widely applicable way for the assessment of this trait with monkeys.