Symbol Comprehension and Learning: A "Vocabulary" Test of Three Chimpanzees (Pan Troglodytes)

Language comprehension in the great apes has been investigated through a variety of paradigms. This experiment employed a match-to-sample computer task to investigate the current language comprehension of three chimpanzees (Pan troglodytes) raised in different language environments but with a similar symbol system (lexigrams). Each of these animals still uses the lexigram keyboard system on a daily basis but none of the animals are the focus of ongoing ape language research programs. Six testing conditions were employed utilizing photographs, lexigrams and spoken English. The results indicated that all apes retained knowledge of at least some of the symbols that they had previously learned, and they each learned differing numbers of new lexigrams that were never taught to them. This indicates that rearing history is important not only in initial symbol acquisition in apes, but also in extended recall of the symbols, particularly when those symbols are used infrequently later in life. Differences in the current ages of these apes requires that such a conclusion be made tentatively, and suggests the need to continue the examination of symbol vocabulary size at various times throughout the life of each ape.