Cross-fostered chimpanzees modulate signs of American Sign Language

Evolutionary and developmental (Evo-Devo) biologists study the interaction between genetic endowment and developmental environment (Lewontin, 2001; Robert, 2004). Cross-fostering is a powerful tool for studying Evo-Devo. Chimpanzees lived under conditions very similar to the conditions of human children with human foster families who used American Sign Language (ASL) exclusively in their presence. In this environment, cross-fostered chimpanzees acquired and used signs as human children do. Intensive analyses of extensive video records of casual conversation show that Tatu at 46–48 months directionally modulated action signs to indicate actor and instrument as human signers do. Tatu directionally modulated action signs in responses to Wh-questions such as “Who?” but directional modulations failed to appear in responses to What Demonstrative questions such as “What that?” These results confirm and extend previous results for Dar at 37–48 months. Further analyses show that Tatu also quantitatively modulated all types of signs to indicate intensity as human signers do.