Object permanence in orangutans, gorillas, and black-and-white ruffed lemurs
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This study examined object permanence in Sumatran orangutans (Pongo abelii), Western lowland gorillas (Gorilla gorilla gorilla), and black-and-white-ruffed lemurs (Varecia variegata) at Zoo Atlanta. A literature review reveals two main issues with object permanence research in non-human primates. One of the issues is that it is difficult to make valid comparisons between different species because very few studies have been conducted using appropriate controls. Thus, one of the goals of this study was to conduct control trials for all tasks in the traditional object permanence test battery, in order to reliably assess and compare performance in the species under study. The second issue is concerned with the finding that all of the non-human primate species tested so far have failed one of the more difficult tasks in the test battery, namely the non-adjacent double invisible displacement task. It has been hypothesized that this performance limitation is a result of the manner in which the task is presented. Thus, the second goal of this study was to modify the existing methodology and present the task to gorillas and orangutans in locomotive space to see if performance improves. This is the first study to present this task to non-human primate species in locomotive space. This study found that orangutans were the only species to reliably pass most tasks in the traditional object permanence test battery. Black-and-white ruffed lemurs failed most visible and invisible displacement tasks. Owing to the small sample size of gorillas in this study, further research is required before any firm conclusions can be made about the ability of this species to solve visible and invisible displacement tasks in the traditional object permanence test battery. Presenting the boxes in locomotive space to gorillas and orangutans did not improve performance on the non-adjacent double invisible displacement task. Further research is required to resolve the question of whether this performance limitation is a result of the manner in which the task was presented.