INTRODUCTION

The close phylogenetic relationship between the African apes and humans has been known to Western science for well over a century. In comparing the differences between the African apes and humans, Thomas Henry Huxley (1863, p. 123) commented:

Since the last Wenner-Gren conference on the great apes, the chasm to which Huxley referred has narrowed considerably. Ongoing research in captivity and in the field has progressively reduced a previously long list of traits that could be employed to differentiate the African apes from humans, and with this research, it has become increasingly clear that humankind's uniqueness may depend on a single characteristic, namely our ability to use speech and language.

While the proposition that speech is special is arguably true, it is important to note that this claim is based on very few data regarding the vocal behavior of our closest living relatives in the wild. Pioneering field work by Schaller (1963), Goodall (1968a), Marler (1969, 1976), Fossey (1972), Marler & Hobbett (1975) and Marler & Tenaza (1977) gave us an early, but admittedly rudimentary glimpse into the vocal repertoires of the African apes.