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Studies reveal that chimpanzees adjust their rate of food calling according to both ecological and social factors (Hauser et al., 1993; Brosnan and de Waal, in prep.) We hypothesized that capuchins, like chimpanzees, modify their food calling depending on these factors. Group housed capuchin monkeys at Yerkes Primate Center were tested to see how their food calling was affected by changes in food quantity and audience conditions. Food quantity varied between high (ten apple slices) and low (one apple slice) food conditions, while audience varied among no audience, one audience member, and group audience. We anticipated that capuchins would 1) call more frequently in the high food condition than in the low food condition and 2) less frequently when no audience was present than when there was an audience. Overall, rank produced the strongest effect. Low ranking individuals called more than high ranking individuals for all conditions. Lower ranking individuals altered their calling based on the audience condition, with food calling frequencies increasing with audience size. Furthermore, this effect was intensified by the presence of high quantities of food. High ranking individuals showed no such effects. These trends indicate that low ranking individuals alter their food calling behavior based on social and ecological factors, suggesting that rank strongly influences capuchin’s sensitivity to changes in their surroundings
Many nonhuman primate species adjust their vocalization rates according to changes in environmental and social conditions (Hauser et al., 1993, van Krunkelsven et al., 1996, Hauser & Marler, 1992) With the presence of an audience, chimpanzees enhanced their calls when presented with large, shareable quantities of food and suppressed their calls when presented with small quantities of food (Brosnan et al., in prep). Capuchins, like chimpanzees, engage in food sharing and cooperative hunting and thus are also likely to adjust their calls to announce the presence of food. Capuchins are at greater risk of predation than chimpanzees, which may cause capuchins to suppress their calls when separated from the group. Capuchins also have rigid hierarchical systems suggesting that rank may influence the frequency of their food calls. We hypothesized that capuchins would adjust their vocalization rates according to the quantity of food provided and the size of the audience present during testing.
Four subjects and one designated audience member were used throughout the experiment: Ozzie, Winnie, Nancy, Lulu and Ike.
Subjects were placed in a test chamber and observed under each of the following conditions:
- No Audience (NA): The subject and the group were unable to see or hear each other.
- One Audience (OA): The subject occupied the test chamber with one other individual. Both monkeys had visual, auditory, and limited tactile access with one another.
- Group Audience (GA): The entire group was present for the test allowing both the subject and the group members to see and hear one another, though tactile contact was prevented.
Each condition included two test types:
- Low Food (LF): The subject was presented with a small quantity of food, which in this case was one slice or approximately 1/20th of an apple.
- High Food (HF): The subject was presented with a large, sharable quantity of food, which in this case was ten slices or approximately ½ of an apple.
The order in which the different conditions were tested as well as the order in which the subjects were tested was randomized to minimize predictability. Each test was video recorded and observed for both food-related and social behaviors. The frequency of vocalizations (food calls per minute) were calculated within the first minute of discovery and over the full length of each test.
All subjects called more for high food than for low food. Rank greatly affected the subject’s sensitivity to the presence of an audience. Low ranking individuals called more overall conditions than did high ranking individuals and exhibited more variation in their calling behavior between high food and low food and among audience conditions. Only low ranking individuals altered their calling behavior for different audience conditions. The only conditions under which high ranking individuals called more than low ranking individuals was during high food, no audience conditions.
Higher quantities of food induce more excitement in capuchins causing them to call more for large amounts of food than for small amounts of food. Greater levels of excitement and anxiety in the low ranking individuals could have caused the difference in calling rates between high and low ranking individuals. Typically, lower ranking individuals are denied first access to food resources, particularly when surrounded by higher ranking individuals. This suggests that lower ranking capuchins increase their calls to avoid either punishment or competition from higher ranking individuals. The vocalization rates for high ranking individuals were not affected by audience size because they were not under threat of punishment or competition from any other group members. The presence of high food intensified the disparity between the vocalization rates of high ranking and low ranking individuals. This suggests that lower ranking individuals become more anxious when presented with large, easily detected quantities of food than they do when presented with small quantities of food that can be eaten without fear of punishment. The only instance in which low and high ranking individuals called at approximately the same rate was during the no audience, high food tests. This trend suggests that lower ranking individuals actively suppressed their calls in high food, no audience conditions in order to eat as much food as possible without calling attention to their discovery and incurring punishment from high ranking individuals.
We thank the SURE program, Hughes Science Initiatives, and Yerkes Primate Research Center for providing us with this opportunity. We also thank Laura Mullen and Lauren Job for their dedicated help in collecting data and the Yerkes care staff for maintaining the monkey colony.
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