Group composition and adult sex-ratio of hamadryas baboons (Papio hamadryas hamadryas) in central Eritrea

During a survey of the geographical distribution and abundance of hamadryas baboons (Papio hamadryas hamadryas) in central Eritrea, we collected detai led demographic data on six bands at four sites in different ecogeographica l zones. The proportions of age-sex classes within the six bands differed o nly with respect to juveniles The general social organization of the Eritre an hamadryas baboons is similar to that reported for Ethiopia and Saudi Ara bia. Eritrean hamadryas baboons live in a nested fission-fusion system, wit h one-male units as the basic social entity. Although the baboons were nonp rovisioned, as they are in many places in Saudi Arabia, and habitat quality in Eritrea is lower than that in Ethiopia, sex-ratios and group compositio n corresponded more to those found in the Saudi Arabian population. Sex-rat ios within the study population, in bands and also in one-male units were s ignificantly, more female-biased than in Ethiopian ones and one-male units fended to be larger. Data from Eritrea suggest that these differences are d ue to a combination of a heavily fluctuating rainfall pattern and different ial maturation of the sexes.