The role of cross-fostering in sign language studies of chimpanzees

In cross-fostering, the young of one species are reared by adults of another. In our cross-fostering laboratory, two-way communication by means of American Sign Language (ASL) brought the rearing conditions for chimpanzees much closer to those of human children than was possible in earlier studies. At the same time, ASL provided a means by which chimpanzees could express their intelligence in ways that permit closer comparisons with human children. Cross-fostered chimpanzees can communicate with human observers whose only source of information is the American Sign Language (ASL) signs of the chimpanzees. In order to show that the chimpanzees could name natural language categories — that the sign DOG could refer to any dog, FLOWER to any flower, SHOE to any shoe — each test trial was a first trial in that tests slides were presented only once. Analysis of errors showed that two aspects of the signs, gestural form and conceptual category, governed the distribution of errors. Like human adults and human children who sign, the chimpanzees modulated their signs in meaningful ways. Observations in field notes, video tape records, and systematic experiments illustrate how these modulations were related to the verbal and nonverbal, context and how they made signs more visible, more versatile, and more informative.