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Hostetter, A. B., Russell, J. L., Freeman, H., & Hopkins, W. D. (2007). Now you see me, now you don't: evidence that chimpanzees understand the role of the eyes in attention. Anim. Cogn., 10(1), 55–62.
Abstract: Chimpanzees appear to understand something about the attentional states of others; in the present experiment, we investigated whether they understand that the attentional state of a human is based on eye gaze. In all, 116 adult chimpanzees were offered food by an experimenter who engaged in one of the four experimental manipulations: eyes closed, eyes open, hand over eyes, and hand over mouth. The communicative behavior of the chimpanzees was observed. More visible behaviors were produced when the experimenter's eyes were visible than when the experimenter's eyes were not visible. More vocalizations were produced when the experimenter's eyes were closed than when they were open, but there were no differences in other attention getting behaviors. There was no effect of age or rearing history. The results suggest that chimpanzees use the presence of the eyes as a cue that their visual gestures will be effective.
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Hopper, L. M., Price, S. A., Freeman, H. D., Lambeth, S. P., Schapiro, S. J., & Kendal, R. L. (2013). Influence of personality, age, sex, and estrous state on chimpanzee problem-solving success. Anim. Cogn., 17(4), 835–847.
Abstract: Despite the importance of individual problem solvers for group- and individual-level fitness, the correlates of individual problem-solving success are still an open topic of investigation. In addition to demographic factors, such as age or sex, certain personality dimensions have also been revealed as reliable correlates of problem-solving by animals. Such correlates, however, have been little-studied in chimpanzees. To empirically test the influence of age, sex, estrous state, and different personality factors on chimpanzee problem-solving, we individually tested 36 captive chimpanzees with two novel foraging puzzles. We included both female (N = 24) and male (N = 12) adult chimpanzees (aged 14–47 years) in our sample. We also controlled for the females’ estrous state—a potential influence on cognitive reasoning—by testing cycling females both when their sexual swelling was maximally tumescent (associated with the luteinizing hormone surge of a female’s estrous cycle) and again when it was detumescent. Although we found no correlation between the chimpanzees’ success with either puzzle and their age or sex, the chimpanzees’ personality ratings did correlate with responses to the novel foraging puzzles. Specifically, male chimpanzees that were rated highly on the factors Methodical, Openness (to experience), and Dominance spent longer interacting with the puzzles. There was also a positive relationship between the latency of females to begin interacting with the two tasks and their rating on the factor Reactivity/Undependability. No other significant correlations were found, but we report tentative evidence for increased problem-solving success by the females when they had detumescent estrous swellings.
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