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I have spent over 40 years studying the behavior of our closest living relatives, the apes. In this paper, I review my research on the spacing, mating, and vocal behavior of gibbons and orangutans (Pongo pygmaeus) and the vocal and social behavior of chimpanzees (Pan troglodytes). I devote special attention to results derived from a 25‐year‐long study of a remarkable and extraordinarily large group of chimpanzees that has recently fissioned at Ngogo in Kibale National Park, Uganda. I conclude with some advice for the next generation of field primatologists. Research Highlights: I review the results of my field research on gibbons, orangutans, and chimpanzees.My findings provide insights into the spacing, mating, and vocal behavior of gibbons and orangutans and the vocal and social behavior of chimpanzees. [ABSTRACT FROM AUTHOR]

Primate social groups frequently contain multiple males. Male group size has been hypothesized to result from male mating competition, but the selective factors responsible for the evolution of multimale groups are unclear. Short breeding seasons create situations that are not conducive for single males to monopolize mating access to females, and may therefore favor the formation of large male groups. Alternately, since the costs of mate defense increase with the spatial clumping of females, female group size may be a primary determinant of the number of males in a primate group. We used comparative methods designed to control for the potentially confounding effects of hidden third variables associated with phylogeny to test the breeding season and female group size hypotheses for the evolution of multimale groups. Our results revealed no association between breeding season duration and the number of males in groups. In contrast, we provide support for the female group size hypothesis by demonstrating a strong pattern of correlated evolution between female and male group size. [ABSTRACT FROM AUTHOR]

Acoustic analyses and field playback experiments were conducted to investigate interspecific song differences and discrimination in male gibbons. Songs were recorded and analyzed from males of three populations: agile gibbons (Hylobates agilis) in West Kalimantan and South Sumatra and Müller's gibbons (Hylobates muelleri) in East Kalimantan. Male songs from each of the three populations were played back to West Kalimantan agile gibbons. Acoustic analyses indicated that the songs of the two Species differed in spectral features and in the temporal patterning of syllables. Agile gibbon songs from the West Kalimantan and South Sumatran populations varied in song durations and in the maximum frequencies of one song element. Field playbacks showed that West Katimantan agile gibbons respond in a similar manner to local songs and to conspecific songs from South Sumatra. In contrast, the animals responded differentially to conspecific and heterospecific Müller's gibbon songs. These results indicate that gibbons discriminate specific differences in songs, and support the division of agile and Müller's gibbons into distinct species. [ABSTRACT FROM AUTHOR]

The article introduces the 2006 special issue of "Primates." Articles published in the periodical consists of selected papers presented to the international symposium "African Great Apes: Evolution, Diversity and Conservation" help in Kyoto, Japan, March 3-5, 2004. Topics selected for publication includes behavioral diversity and origins of human culture, sympatry and coexistence of African great apes and natural environments and evolution of these apes.