Dominance Hierachies in Chimpanzees and Humans (fsem spring09)

Other behaviors humans use to communicate and maintain individual status in established dominance hierarchies include voice pitch and voice pitch modulation. In humans, a lower voice pitch is associated with more powerful and dominant individuals. This occurs because androgen, a chemical in human bodies, affects both physical aggressiveness and low voice pitch in humans (Puts et al 2005). The correlation between a low voice pitch and increased aggressiveness means that people evolved to associate lower voice pitch with increased physical dominance (Puts et al 2005). A lower voice pitch is also an honest signal of health and vigor, because high androgen levels require healthy development and maintenance to sustain low voices (Puts et al 2005). Therefore, lowering one’s voice would communicate or reaffirm one’s perceived physical dominance among humans, similar to a piloerection among chimpanzees (Puts et al 2005).

Voice pitch modulation is less of an indicator of physical dominance and more an indicator of dominant social influence.  When the voices of a group speaking are filtered away, a ‘hum’ or tone remains. At the beginning of a conversation, there are multiple, discordant tones because each individual begins on the tone they prefer to speak at. However, over the course of a conversation less dominant individuals will unconsciously change their tone until it matches the dominant group member’s tone (de Waal 2005).

Observing the behaviors that establish, communicate and maintain dominance in humans is still more difficult than in chimpanzees. However, psychologists have identified specific postures associated with dominance in humans through the use of the Laban Movement Analysis (Levy and Duke 2003).  Originally used by dancers to evaluate and mimic body movements, the Laban Movement Analysis (LMA) evaluates three qualities that psychologists have associated to dominant behavioral styles:  areal factor, centrifugal factor, and emphasis factor. Areal factor is defined as ‘motor expansiveness’, or how much movement a person naturally exhibits. Centrifugal factor focuses on whether a person is oriented inward or outward, as indicated by body posture, and the emphasis factor specifies the level of muscle tension or pressure (Levy and Duke 2003). According to the LMA, dominant individuals can be ‘spotted’ by their body posture. They often exhibit more motor expansiveness (areal factor), are oriented outward (centrifugal factor), and have lower muscle tension compared to those around them (Levy and Duke 2003). Another study established nonverbal behaviors associated with submissiveness, including auto manipulation, increased muscle tension, higher frequency of agreeing gestures, and higher frequency of smiling and laughing (Luxen 2005).

The different approaches adult chimpanzees use to establish, communicate and maintain their position in a dominance hierarchy is influenced by their ‘behavioral style’, or combination of personality, temperament and preference (Anetsis 2005). While doing research on stress and dominance in chimpanzees, Dr. Stephanie Anestis found that dominant chimpanzees consistently engage in certain behavioral styles. By using a collection of modified intelligence and personality tests, she determined that dominance was most likely to be found in chimpanzees whose behavioral types reflected intelligence; they utilized aggression to maintain status, and enough mellowness to reduce the impacts of stress. Chimpanzees that were more affinitive, playful, and friendly were usually submissive. Consequently they exhibited these submissive behaviors more frequently; a ‘bob-crouch’, where an individual crouches over and bobs head while moving, pant-grunting, and ‘hand-reach’, a noncontact movement similar to a child reaching for its mother (Coe and Levin 2006).

In both transitive and intransitive dominance structures, dominance is established, communicated and maintained through specific behaviors.  In chimpanzees, these behaviors and their functions can be easily observed and clearly identified.  Such behaviors include repetitive rocking, a piloerection, bipedalism, charging, stamping, branching, throwing objects, and a behavior called a ‘hunch over’, where the displaying chimpanzee straddles a subordinate chimpanzee with its arms (Coe and Levin 2006). The function of each behavior varies; bipedalism and piloerection make the displaying chimpanzee look physically larger (and therefore stronger). Branching, charging, and stamping directly indicate strength. A ‘hunch over’ display emphasizes the social influence of the displaying chimpanzee (Coe and Levin 2006).  The frequency of these behaviors and their effectiveness at conveying dominance vary from individual to individual. Although these specific behaviors and postures are always present, adult chimpanzees use different approaches to develop and progress their status in the dominance hierarchy (Anestis 2005).