Gorilla Hierarchy

I. Introduction

For my subject matter I observed gorillas at the San Francisco Zoo. Gorillas are social primates and live in groups averaging 7-17 individuals (Maple & Hoff, 1982, pg. 49). They have a polygynous social model with one dominant male, many females, infants, and possibly a few subordinate males. Physically there is much sexual dimorphism between males and females, with females generally weighing 150-200 pounds and males 350-400 pounds (Erwin, Maple, Mitchell, 1979, pg. 245). Based on this information I hypothesize that gorillas will maintain this dominance hierarchy even in captivity.

II. Materials and Methods

The gorilla exhibit at the San Francisco Zoo was comprised of one male silverback named Mkbuwa, two adult females, Pogo and Bawnay, and one juvenile female Nneka (daughter of Bawnay). The gorillas were viable from nearly every side of their large round outdoor enclosure which was attached to a small building (presumably their night quarters).

For my data collection method I scanned behavior ten times at five-minute intervals. I informally observed the gorillas for approximately 25 minutes before beginning my scans to get acquainted with them and their general behavioral variations. I began my scans at approximately 3pm, which is a period of activity and movement between the afternoon resting period and early evening sleeping period (Maple & Hoff, 1982, pg. 40).

During my observation I focused on recording specific types of aggressive and friendly behavior as well as other behaviors in general in order to determine if the single male dominant structure could be observed. As well as having a general Ð''aggressive' category I also broke it down into more specific types: aggressive call, aggressive charge and aggressive strutÐ'--which I created after witnessing Mkubwa engage in them. I also included a category for actions which looked like the gorilla was trying to get into the enclosure (noted as I).

There are many variables that may have affected my data. The one I think is most significant is the presence of so many humans. In the wild, the silverback protects his group and young. It is likely that Mkbuwa is acting aggressively not just because of his nature or the condition of captivity, but simply by facing so many strange human beings staring in at him and his group. Intense staring is perceived even by humans to be a form of aggression. According to Maple and Hoff, aggressive displays are most often directed towards humans (1982, pg. 52).

Another problem I had was knowing what behaviors to record and which not to record. I was not sure if I should effectively take a photo of the group every five minutes and write what they were doing during that time, or if upon the passage of five minutes I should observe the group for one minute and record any Ð''significant' behaviors that occurred during that time. I instinctively took this second approach, although upon examining my data and percent of time spent resting, I believe my data report lower rates of resting than I actually observed.

III. Results

One significant aspect of my results is that the three females' behaviors were relatively the same in comparison to the silverbacks. Also, it is significant that 50% of his behavior recorded was aggressive in nature. With the remainder of his time he spent 20% of it moving around (non aggressively), and 10% each in friendly behavior, trying to get into the enclosure and resting. I did not observe him eating. Of the 50% aggressive behavior, 30% was observed as aggressive struts, 10% in an aggressive charge (toward another gorilla), and 10% making an aggressive call (with his hand cupped over his mouth).

For Pogo, I observed her moving around 40% of the time, engaging in friendly behavior 30% of the time, resting 20% of the time, and trying to get into the enclosure 10% of the time. I did not observe Pogo eating or engaging in aggressive behavior. Of the 40% of time Pogo spent moving around, 30% of it appeared to be in avoidance of Mkubwa, while only 10% was for general transportation. It is also interesting to note of the 30% of time Pogo spent engaging in friendly behavior, none of it was in interaction with other gorillas. She spent 10% of the time interacting in a playful manner with herself (playing with a suspended crate) and 20% engaging in playful behavior with humans (at one point she stuck her tongue out and Ð''wagged' it at me).

For Nneka, She spent 30% of the time engaging in friendly behaviors, 30% of the time moving, 20% of the time resting, 10% of the time trying to get into the enclosure, and 10% of the time eating. She was the only gorilla I observed eating. All 30% of her friendly behaviors were directed at gorillas. Two were with her mother, and one was directed at Mkubwa (at which point he aggressively charged). She had no interactions with Pogo or humans that I could tell.

For Bawnay she spent 30% of the time moving, 30% of the time resting, 20% of the time engaged in friendly behaviors, and 20% of her time trying to get into the enclosure. I did not witness her eating or acting aggressively. She only seemed to interact with Nneka.

IV. Discussion

I observed several aggressive behaviors from the male silverback Mkubwa as well as clearly avoidant behavior from one female Pogo. In gorilla groups, females sometimes avoid males, but males do not avoid females (Maple & Hoff, 1982, pg. 52). Additionally males put on aggressive displays but females never do toward the male.

The group's juvenile female, Nneka tried to engage in friendly behavior with Mkubwa but he aggressively charged at her. Nneka's behavior is consistent with young gorillas in the wild that need the protection of the powerful male (Taylor, 2003, pg. 321). Maple and Hoff found that as infants increase in age they also increased their time spent around the silverback (1983, pg. 51). One other piece of information I found on this topic is that male gorillas in captivity prefer