The capacity of nonhuman primates to actively modify the acoustic structure of existing sounds or vocalizations in their repertoire appears limited. Several studies have reported population or community differences in the acoustical structure of nonhuman primate long distance calls and have suggested vocal learning as a mechanism for explaining such variation. In addition, recent studies on great apes have indicated that there are repertoire differences between populations. Some populations have sounds in their repertoire that others have not. These differences have also been suggested to be the result of vocal learning. On yet another level great apes can, after extensive human training, also learn some species atypical vocalizations. Here we show a new aspect of great ape vocal learning by providing data that an orangutan has spontaneously (without any training) acquired a human whistle and can modulate the duration and number of whistles to copy a human model. This might indicate that the learning capacities of great apes in the auditory domain might be more flexible than hitherto assumed.
There is considerable evidence indicating that chimpanzees can delay gratification for extended time intervals, particularly in the accumulation task in which food items accumulate within a participant's reach until the participant begins to consume them. However, there is limited evidence that other ape species might also exhibit this capacity, despite there being a number of similar studies indicating that nonape species (e.g., monkeys and birds) can delay gratification, but not for nearly as long as chimpanzees. To help define the taxonomic distribution of delay of gratification behavior in the order Primates, we tested 6 orangutans in the current experiments and compared their performance with comparable data from a previous study with capuchin monkeys. We varied delay length and visibility of the items that were still available for accumulation to determine the impact of these factors on performance. Species differences on the accumulation task emerged when comparing the current data to data from a previous study. Orangutans outperformed capuchin monkeys, suggesting that ape species may generally show better delay of gratification and delay maintenance abilities than monkeys. However, more studies are necessary to rule out alternative hypotheses on the distribution of delay maintenance abilities across primate species.
The ability to monitor and control one's own cognitive states, metacognition, is crucial for effective learning and problem solving. Although the literature on animal metacognition has grown considerably during last 15 years, there have been few studies examining whether great apes share such introspective abilities with humans. Here, we tested whether four gorillas could meet two criteria of animal metacognition, the increase in escape responses as a function of task difficulty and the chosen-forced performance advantage. During testing, the subjects participated in a series of object choice memory tests in which a preferable reward (two grapes) was placed under one of two or three blue cups. The apes were required to correctly select the baited blue cup in this primary test. Importantly, the subjects also had an escape response (a yellow cup), where they could obtain a secure but smaller reward (one grape) without taking the memory test. Although the gorillas received a relatively small number of trials and thus experienced little training, three gorillas significantly declined the memory tests more often in difficult trials (e.g., when the location of the preferred reward conflicted with side bias) than in easy trials (e.g., when there was no such conflict). Moreover, even when objective cues were eliminated that corresponded to task difficulty, one of the successful gorillas showed evidence suggestive of improved memory performance with the help of escape response by selectively avoiding trials in which he would be likely to err before the memory test actually proceeded. Together, these findings demonstrate that at least some gorillas may be able to make optimal choices on the basis of their own memory trace strength about the location of the preferred reward.
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