A small population of wild chimpanzees was studied at Bossou, Guinea, for six months from November 1976 to May 1977. All the chimpanzees except dependent infants were identified without artificial feeding and were observed from within 20 m almost every day. The population size of 21 chimpanzees was little changed from 1967. Although the sex ratio (male/female) of immatures (infants, juveniles, and adolescents) was 0.833 in January 1977, the adult sex ratio was 0.429. More than half of the males must therefore have disappeared. No desertion of males from the Bossou group was confirmed during the study period but two emigrated males from other groups did come to join the Bossou group for a short period.
Four out of six mothers had two or three children. From the age discrepancy of brothers/sisters, the mean birth interval from a mother was estimated to be about four years. An elder sister of two infants who had an age-gap of only three years, or perhaps less, disappeared without receiving sufficient care from her mother.
Mothers who had infants aged about 1 year or more showed swelling of their sexual skin and were confirmed to mate with males.