In three adjacent troops of olive baboons (Papio anubis), all males emigrated from their natal troop. There is evidence that the costs of inbreeding depression may exceed those of transferring to another troop. Migrations which were unlikely to be due to the avoidance of inbreeding were made by males with above average reproductive activity into troops with more oestrous females than the prior troop. During inter-troop encounters, immigrant ('transferred') males acted in ways that reduced the contact of females of their own troop with outsiders and lowered the probability of newcomers joining the troop. In contrast, 'natal' males showed interest in members of the other troop. Male dominance was strongly related to age, and in transferred males reproductive activity was related to dominance. In natal males dominance increased with age but reproductive activity decreased. Instead of engaging in aggressive competition for access to oestrous females, natalmales mated surreptitiously. Females showed a preference for transferred males over natal males, and for males who could not have been their father over males who could have been.
Bibliographical noteFunding Information:
Field work was supported by grants from the Ford Foundation and the W.T. Grant Foundation. I am grateful to the authorities of the Tanzania National Parks for permission to work in Gombe National Park and to Dr Jane Goodall for providing facilities in the Gombe Stream Research Centre and access to data collected after the termination of my field study. The continuity of the long-term records on the baboons is primarily due to the efforts of Dr Goodall and Baron Hugo van Lawick as well as those of Professor David Hamburg and Professor Robert Hinde; and Dr Leanne Nash and Dr Nick Owens both took care to provide subsequent researchers with information concerning the histories of individual baboons, as did Dr Tim Ransom before them. I thank Apollinaire Sindimwo for teaching me the identities of the baboons, Dr Paul Harvey and Dr Helena Kraemer for their generous help and advice on methods of data analysis, Professor Richard Andrew, Dr Brian Bertram, Dr Dorothy Cheney, Dr G. M. Clark, Anthony Collins, Dr Richard Dawkins, Dr Jeff Kurland, Pro-