An ethological study of pygmy chimpanzees in Wamba, Zaïre: A comparison with chimpanzees

Abstract

An ethological study was conducted on a provisioned pygmy chimpanzee group in Wamba, Zaire and was compared with the author's previous study on common chimpanzees, in Mahale Mountains with special reference to the evolution of behavior systems. The relationships between behavior patterns were investigated by analyses of the intra- or inter-individual sequential behaviors. Analysis of behavior pathways showed that “bipedal” was not found (i.e., observed frequency lower than expected value) in the course of the charging display in pygmy chimps, though it was an important element of the charging display in common chimps. Additionally, “bipedal” was found to be an initating behavior pattern in sexual behavior or dominant-subordinate behavior in pygmy chimps. These differences were related to the decrease of the role of charging display and to the increase of the roles of sexual and dominant-subordinate behaviors in pygmy chimps. Sexual behavior, “female genito-genital rubbing,” “mutual present” and frequent mounting and presenting between males were suggested to have evolved together as a system. These characteristics of behaviors correspond to the fact that pygmy chimps form more stable and larger temporary groups than common chimps.