During a survey of the geographical distribution and abundance of hamadryas baboons (Papio hamadryas hamadryas) in central Eritrea, we collected detailed demographic data on six bands at four sites in different ecogeographical zones. The proportions of age-sex classes within the six bands differed only with respect to juveniles. The general social organization of the Eritrean hamadryas baboons is similar to that reported for Ethiopia and Saudi Arabia. Eritrean hamadryas baboons live in a nested fission-fusion system, with one-male units as the basic social entity. Although the baboons were not provisioned, as they are in many places in Saudi Arabia, and habitat quality in Eritrea is lower than that in Ethiopia, sex-ratios and group composition corresponded more to those found in the Saudi Arabian population. Sex-ratios within the study population, in bands and also in one-male units were significantly more female-biased than in Ethiopian ones, and one-male units tended to be larger. Data from Eritrea suggest that these differences are due to a combination of a heavily fluctuating rainfall pattern and differential maturation of the sexes.
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Zinner, D., Peláez, F. & Torkler, F. Group Composition and Adult Sex-ratio of Hamadryas Baboons (Papio hamadryas hamadryas) in Central Eritrea. International Journal of Primatology 22, 415–430 (2001). https://doi.org/10.1023/A:1010755628658