Sociological and demographic characteristics of a recently found arsi gelada population in Ethiopia

A new population of gelada baboons isolated from other known gelada populat ions living on the opposite side of the Rift Valley was found in 1989 in th e Arsi region of Ethiopia (MORI & BELAY, 1990). Subsequent observations wer e conducted on the new gelada population in three study periods of 16 to 55 days over three consecutive years, 1994-1996. When we compare this populat ion with other well studied northern populations, densities were lower, ban d size was smaller and the ratio of juveniles to adult females was lower. B ased on OHSAWA and DUNBAR's (1984) criteria these results suggest that this population exploits the harshest environment among known gelada population s. This may be due to the severe effect of the dry season at low altitude i n the study site. Band size was small in the study site. Association rates of units were low, and each unit behaved more independently than those of n orthern populations. Age-sex composition of units changed drastically betwe en consecutive study periods, indicating unstable unit structure. Moreover, there seems to be a tendency in this population whereby several adult male s easily invade and are incorporated into a unit but the unit is expected t o thereafter divide. The phenomenon can be explained by two factors: (1)Eac h unit fed rather independently in this area, where units fed intensively o n both plateau and the slope of the cliff; and (2) Males may be incorporate d in a unit for defense against leopards. As the units were very small in s ize and unstable as compared with the Semien gelada units, the traditional concept of a stable gelada unit formed by blood related females, does not s eem to fit this population.