Document Type

Thesis

Date of Degree Completion

Fall 2025

Degree Name

Master of Science (MS)

Department

Primate Behavior

Committee Chair

Mary Lee Jensvold

Second Committee Member

Mary Radeke

Third Committee Member

J. B. Mulcahy

Abstract

Cross-fostered chimpanzees that acquired signs of American Sign Language (ASL) use sign modulations in signed interactions with humans. In this study, the researcher examined the effects of familiarity and use of ASL on four signing chimpanzees' modulations of signs. Three of the chimpanzees had been cross-fostered and acquired signs from human researchers. The fourth acquired signs from other chimpanzees. Modulations are modifications to the form of a sign and alter the sign’s meaning. They affect intensity and comprehension and vary across conversational contexts among human and chimpanzee signers. During 4-min trials, the chimpanzees were presented with a human participant who was in one of four conditions: familiar-signer, familiar-nonsigner, unfamiliar-signer, and unfamiliar-nonsigner. The researcher coded sign duration and counted sign reiterations from videos of trials. Loulis was the only chimpanzee to participate in trials of all four conditions and to have any significant differences between conditions. Loulis used significantly longer sign durations and more iterations of signs with nonsigners. Specifically, he used longer durations with familiar-nonsigners than with unfamiliar-signers and significantly more iterations of signs with familiar-nonsigners than with familiar-signers. This study contributes to the understanding of how chimpanzees engage in social interactions and adds to our perception of their sensitivity to others. Understanding these behaviors can impact how we understand, interact with, and care for both captive and wild chimpanzees.

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Available for download on Wednesday, January 08, 2031

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