Study of the Female Dominance Hierarchy of Mandrillus Leucophaeus

The Problem

In the summer of 2006, newly enrolled in the high school at the Columbus Zoo and Aquarium, I embarked on the biggest task I had ever done: completing a masters-level thesis. Mandrillus leucophaeus, commonly known as the drill, is an endangered monkey that lives in an extremely small range in northern-western Africa. The Columbus Zoo and Aquarium is one of only four zoos in the United States that house these animals. They were cited as the monkey most in need of conservation. The abstract of the thesis is below. -------------- This study investigated the possibility of a female dominance hierarchy in the troop of Mandrillus leucophaeus, more commonly known as drills, at the Columbus Zoo and Aquarium, sponsored by the Columbus Zoo and Aquarium School. Due to the rarity of this dwindling species, both in the wild and in zoos, their group and social interactions, especially between females, have not been exclusively studied. Therefore, this study aims to provide a building block to support the hypothesis that the drills, like other old-world monkeys, also have a dominance hierarchy system existing between the female inhabitants of a specified troop. There were three subjects in this study: Nora, an eleven year old female; Eschu, a six year old female and Bioko, a fifteen year old female. The project commenced by a week of observation meant to classify everyday, non-specialized behaviors of the drills from behaviors exclusively displayed that resulted in dominant and submissive behaviors, thus classified as hierarchical behaviors. Once these behaviors were differentiated and placed in appropriate ethograms, observations for the actual study began. Dominant behaviors that were specified included chasing, head bobbing, chest rubbing and purposefully walking. A chase is classified as when the aggressor, instead of pausing after dominance is exerted, follows the fleeing recipient, usually at a steady pace. Head bobbing is classified as occurring when the aggressor bobs her head (quick, sharp nods) towards the recipient. Chest rubbing, a unique behavior that is not exerted over another subordinate, is classified as a female rubbing her chest against an object or substrate in order to present dominance over that area. This is due to the glands in their chest that excrete a scent on the area it is rubbed against. Purposefully walking is a gait done by the aggressor female with stronger than usual strides directed at the submissive female. It is a very contained gait. Submissive behaviors that were specified included fleeing, eye averting, displaying rear, acknowledging and looking back. A flee is defined as a behavior where the recipient moves away from the initiator/dominant female, usually with haste, in any direction. Eye averting occurs when the recipient averts her eyes to the ground or in front of her, then back to the aggressor, then to the ground again. The displaying of rear occurs when another female exerts dominance and the recipient presents her rear to the aggressor. Often occurs with eye averting. Acknowledgment is defined as the lack of dominance display of the aggressor, yet the acknowledgment on the part of the recipient. That is, the recipient recognizes the other female’s dominance and acknowledges it by any of the other submissive behaviors. In this behavior, the dominant female does not make an aggressive movement toward the submissive female, but is still considered dominant. Looking back occurs when the submissive female flees or has just finished fleeing. She will look behind her to see where the aggressor female is located. Observations were collected for 100 minutes per day and collected on a specially formed data sheet. The data sheet contained columns for the initiator to be acknowledged (that is, the dominant aggressor female that initiated the dominance display), the recipient of the dominance display (the submissive female), a detailed explanation of what occurred using the hierarchical terms and a column for the time started and duration of the behavior. The first statistical analysis involved comparing the mean times dominant of all three females against each other, which resulted in the conclusive result that a significant difference existed between the mean times dominant of Nora and Bioko, and that Nora’s mean time dominant was significantly greater than Bioko’s mean time dominant. The second statistical analysis compared the mean times submissive of all three females against each other, which also resulted in the conclusive result that a significant difference existed between the mean times dominant of Nora and Bioko, and that Bioko’s mean time submissive was significantly greater than Nora’s mean time submissive. This test was run because all dominance did not necessarily involve a receiver, and thus submission was not proportional to dominance. The third and final statistical analysis compared the mean times dominant and submissive of each individual female. Nora’s mean time dominant and mean time submissive were compared and found, conclusively, that a significant difference existed and that Nora’s mean time dominant was significantly greater than her mean time submissive. Eschu’s mean time dominant and mean time submissive did not provide a conclusive result and a significant difference was not found to exist between her mean time dominant and mean time submissive, thus concluding that the values were close to the same. Finally, Bioko’s mean time dominant and mean time submissive provided a conclusive result and a significant difference was found to exist, indicating that her mean time submissive was significantly greater than her mean time dominant. By analyzing the results, a firm and conclusive female hierarchy was found to exist between the females in the drill troop at the Columbus Zoo and Aquarium. Further analyzing the results, Nora was found to be the alpha female, most dominant and least submissive, followed by Eschu whose time dominant and time submissive were nearly equal and finally by Bioko who was found to be the omega female, least dominant and most submissive.

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