Thesis

Hand preference, fear responses, and exploration in Geoffroy's Marmosets (Callithrix geoffroyi)

Handedness is the preferred use of one hand over the other. The consistent preferred use of one hand is characteristic of many human and nonhuman primates and may reflect hemispheric specialization. Hopkins and Bennett (1994) and Cameron and Rogers (1999) found that right-handed chimpanzees and marmosets were more willing to explore novel situations than left-handed individuals, leading to the tentative conclusion that hemispheric dominance for use of hands may predict certain behavioral tendencies. This study examined the relationships between handedness, exploration, and fear responses in two groups (N = 18) of Geoffroy's marmosets ( Cal/ithrix geoffroyi) at the Center for Conservation and Research for Endangered Species at the San Diego Wild Animal Park. Handedness was assessed by observations of multiple (2: 50) instances of reaching for food and bringing it to the mouth. Based on the Handedness Index, there were 7 right-handed, 5 left-handed, and 6 ambidextrous individuals. In the first of two experiments, both conducted prior to the handedness assessment, the marmosets were exposed to 13 novel objects and 16 novel foods, an ecologically valid measure of response to novelty that has not been included in prior studies of handedness and behavior. The data shows that right-handed marmosets explored novel objects and novel foods significantly more and significantly faster than left-handed marmosets. All correlations and means were in the predicted direction. To date, the implied relationship between left-handedness and fearfulness has not been directly measured. In Experiment 2 the taped calls of three raptors were played to induce fear in the marmosets. Right-handed marmosets recovered from their "freeze" reaction significantly quicker than left-handed marmosets. Together with the results of Experiment I, my data support the notion that at least some lateralized traits (e.g. fearfulness) might be more characteristic of nonhuman primate individuals whose handedness is lateralized in the same hemisphere as that trait. KEYWORDS: Callithrix, laterality, handedness, hemispheric specialization, exploration, fear responses

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