Document Type

Thesis

Date of Degree Completion

Spring 2026

Degree Name

Master of Science (MS)

Department

Primate Behavior

Committee Chair

Mary Lee Jensvold

Second Committee Member

Jessica Mayhew

Third Committee Member

Mary Radeke

Abstract

Chimpanzees are highly social primates, using vocal and gestural forms of communication in various contexts. Considering their genetic, biological, and behavioral similarity to humans, their language capacities are of interest. In the 1960s, an infant chimpanzee, Washoe, was raised like a Deaf human child and acquired signs of American Sign Language. In the 1970s, Tatu, Dar, and Moja also acquired signs from human caregivers. Loulis acquired signs from Washoe and other signing chimpanzees. These chimpanzees acquired signs in patterns that resembled human children, including modulation of signs. Caregivers at Central Washington University used sign logs to document all aspects of signed interactions by the chimpanzees. Researchers also determined the dominance hierarchy of the social group, and it was as follows: Washoe and Loulis were high-ranking, Dar and Tatu were middle-ranking, and Moja was low-ranking (Sanz et al., 1996; Hayashida et al., 2002). The current study explored the dominance hierarchy, contexts, and modulation use in chimpanzee-to-chimpanzee interactions recorded in sign logs. Emphatic modulations, followed by directional modulations, occurred the most across signers, ranks, and contexts. Washoe produced the most glosses, modulated glosses, and modulations, while Moja produced the least. The most modulations were produced in reassurance contexts, and the least were produced in discipline contexts. HURRY and HUG/LOVE were frequently used and modulated glosses. Loulis received the most glosses and modulations in dyads, while Moja received the least. These findings contribute to our understanding of how chimpanzees use signs in interactions with each other.

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