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ABSTRACT. A 2.4-year-old male chimpanzee in the Mahale Mountains, Tanzania became separated from his mother probably because of contagion of the flu-like sickness. He was adopted alternately by two unrelated immigrant females for at least six days before he was reunited with his mother. This episode is described and analyzed in relation to the babysitter relationships among female chimpanzees. The adaptive significance of the babysitting behavior of nulliparous females is discussed.

ABSTRACT. K Group, originally one of the two major study groups of chimpanzees since 1965 in the Mahale Mountains National Park, western Tanzania, was almost extinct by 1983: at most seven individuals remained in the group at the beginning of 1983. K Group continued to exist for more than four years, but in 1987 a male was left alone at the age of 15 after all the other chimpanzees of the group emigrated or disappeared. Since then he has been observed sporadically for more than five years only within the former range of K Group, without having any contact with the many resident chimpanzees of the neighboring M Group, the other major study group. The present observations reconfirm the strong philopatric tendency of adult male chimpanzees.