Knowledge regarding dispersal patterns in great apes may help in
understanding the evolution of dispersal patterns and social grouping in early
hominoids, as well as in our own species. However, the social structure and dispersal
system of orang-utans (Pongo spp.) remains little understood despite past research.
We addressed this question by conducting genetic analyses on a wild orang-utan
(Pongo pygmaeus wurmbii) population from the Sabangau peat-swamp in Borneo.
We estimated pairwise relatedness among 16 adult individuals using 19 polymorphic
microsatellite markers. Mean relatedness among females was significantly higher
than in males, irrespective of the relatedness estimator used, following the pattern
predicted for male dispersal. Our results support field observations that average
dispersal distance for females is less than for males, suggesting that female orangutans
are philopatric, whereas males disperse. This contrasts with previous findings
from other sites where anthropogenic influences were present. Based on qualitative
mitochondrial DNA analyses, it appears that unflanged adult males show some
degree of site fidelity compared to flanged males. Thus, male orang-utans may
disperse permanently from their natal range once they are fully flanged. Male-biased
dispersal and female philopatry in orang-utans differ from those of extant African
apes and are more similar to many Old World monkey species. Hence we
hypothesize that this system may represent the ancestral state of early hominoids.