Discussion
The study has revealed that the study area supports a viable population of geladas, though isolated from other sub populations in south and north wollo administrative zones. However, the area supports fewer sub population of gelada (mean population size 229) as compared to other ranges of gelada (Beehneret al. 2008, 4620 individuals SMNP; Adem 2009, 529 individuals ; Ayalew 2009, 914 individuals; Abu 2011, 338 individuals; Moges 2015, 1502 individuals, Abie and Bekele, 2017, 1608 individuals and Goshme and Yihune 2018, 435 individuals). This could be attributed to the fact that the area is the smallest area as compared to its other ranges and the habitat is characterized by ravine topography, which opens in to farm lands in all directions and more drier and with less habitat quality as compared to other ranges in comparison. Furthermore, threats such as habitat destruction in search of the mineral opal and deforestation, retaliatory kill against crop damage, which are intense in the present area (DWANRDO, 2016) as compared to its other ranges, could reduce the population size. However, the population size is larger than the recent report from estern escarpments of Tigray (Hawzien and Ganta-Afeshum districts) 105 individuals (Girmay and Dati, 2020). This could be due the fact that the range of species in Tigrai are more drier and with poorer habitat quality.
Significantly higher population of gelada was recorded during the dry season that the wet season unlike the populations of the species over its some other ranges. For example, there was no significant seasonal variations in populations of gelada in Gich, SMNP (Woldegeorgis, 2015), Debrelibanos, central Ethiopia (Abie and Bekele, 2017), Wof washa, central Ethiopia (Goshme and Yihune, 2018), eastern escarpments of Tigray (Girmay and Dati, 2020). This is mainly attributed to the reproductive and crop raiding behavior of the species and the weather condition at the particular study area. Normally, geladas give birth after wet seasons, during early dry season (Ejigu and Bekele, 2014). This could increase the number of youngs in the following dry season ultimately increasing the total population during the dry season. The most plausible explanation comes from the fact that there is intense agricultural encroachment in the surrounding habitat of the species in the present study area and wet season is a crop growing season, geladas were observed to widely involve in crop raiding behaviour during the wet season. Gelada is identified as one of a crop raider primate over most of its ranges (Yihune et al., 2009; Moges, 2015). The diet of geladas is predominantly composed of grasses and rhizomes (Moges, 2015). However, cereal crops may be taken as an alternative food source where agriculture encroaches onto the habitat of the gelada (Yihune et al., 2009). Due to crop raiding behavior during the wet season, farmers chase away geladas from their habitats bordering farmlands in to nearby cliffs using guard animal like dogs and stone at geladas to protect crop damage, which could reduce the sighting of the animals during wet season. During the dry season, crops are harvested and this enables geladas to roam freely among all habitat types. Similarly, Kifle et al. (2013), recorded fewer individuals of geladas during the wet season than dry season in Wonchit valley and discussed that this could be attributed to the fact that wet season is a cultivation season and hence, farmers have to chase away geladas far away from cropland and most of their open habitat types. Furthermore, the foggy weather conditions during wet season as compared to a bright sunny day during the dry season could reduce the sighting of the animals during the wet season. The study area receives 75- 80% rainfall during the wet season (ENMA, 2017). This high amount of rainfall and fog could affect the probability of detection of individuals’ gelada in the study area. Hunter (2001), reported that excessive rainfall causes stress among geladas population and geladas to overcome this heavy rainfall they aggregate themselves in their cliff.
In addition, human disturbances like firewood collection, opal mining, grass cutting and livestock encroachments were higher during the wet season than the dry season in the study area. When farmlands were covered by crops during wet season, livestock moved into the habitats of geladas for the sake of better foraging opportunities. This introduces disturbances and competition for forage sources that could reduce the probability of detection of gelada individuals during wet season. During the wet season, human disturbance and livestock encroachments are higher than the dry season over most highlands of Ethiopia (Hassain, Asghar, Frid & Nurberdief, 2008; Girma, Bekele & Graham, 2012). Furthermore, other disturbances such as firewood collection, grass cutting and opal extraction (Rondeau et al., 2010) are more frequent during the wet season than the dry season. During the wet seasons, farmers are relatively free from their routine farm activities and have time to get engage in activities such as firewood collection, grass cutting and opal extraction, which is peculiar only to the range of gelada under study. Moreover, during the wet season there are no incomes from crops and hence, they have to engage in any other economic activities to meet household economic demand (DWANRDO, 2016). All these increase disturbances during the wet season that reduce the sighting of the animals during the wet season.
Understanding of sex and age composition of wild animal populations is important for evaluating the viability of the species as these variables reflect the structure and the dynamics of wild animal population (Wilson et al., 1996). The male to female gelada sex ratio in the study area revealed female biased ratio (the adult male to female ratio is 1:1.96). The result is consistent with other studies (e.g. Woldegeoriges (2015) (the adult male to female ratio is 1:3) at Gich area of the Simen Mountain National Park, Abie and Bekele 2017 (the adult male to female ratio is 1:3.57) at Deberelibanos highlands, central Ethiopia and Moges, 2019 (the adult male to female ratio is 1:4.1 in Arsi). The female biased ratio is a characteristic of naturally growing population under normal circumstances (Beehner et al., 2008). Secondly, the reduced number of male geladas could be attributed to retaliatory killings induced by the species involvement in intense crop raiding behavior and presence of predators. Kifle et al. (2013), reported that the numbers of males in Wonchit valley were small due to similar crop raiding behavior and retaliatory killing. In addition, the natural predator caracal(Files carcal ) has been observed to be frequent over most of the habitats of gelada. Caracal could potentially prey on adult and sub adult males that usually behaviorally isolate them from the main groups of gelada and roam freely. Similarly, Roberts and Dunbar (1991) reported that the male, especially bachelors and sub adult male geladas are always vulnerable to predators. However, the reduced number of females as compared to above studies carried out elsewhere over the ranges of geladas could indicate predicts reduced population growth and signal for immediate conservation attention.
Unequal mean ratio of juveniles to other age class in the study area was observed. The possible reason of the small proportion of juveniles in the study area could be less availability of food resource (due to anthropogenic activities) that limits reproduction in the study area as discussed above. Dunbar (1987), discussed that food resource availability negatively correlates with the birth rate of gelada population. Furthermore, the presence of natural predators such as common jackal in the study area could reduce the juvenile population. Common jackals were seen frequently moving between habitat patches looking for prays. Juveniles at this stage are vulnerable towards predators. Furthermore, unproductive female geladas may exist in the population that could reduce the population of juveniles. Dunbar (2002), revealed that the existence of unproductive female gelada attributes to the existence of the small number of juveniles in gelada populations.
The mean group size (11.5) of gelada at Kotu forest was higher than Chenek, Simien Mountains National Park (10.5) (Ejigu and Bekele, 2014), Wof Washa Forest (Goshme and Yihune, 2018), Borena Saynit National Park (Kifle, 2018), and Guassa Community Conservation Area (8.9) (Mamo and Wube, 2019). However, fewer average group size than Gich, SMNP (15.4) (Woldegeorgis, 2015) and Arsi (14.9) (Moges, 2019). According to Ohsawa and Dunbar (1984), mean group size in geladas is mainly dependent on the social factors that are related to group congregation. Variations in group size are the results of cost and benefits of group congregation mainly associated food and predators availability (Majolo, Vizioli and Schino, 2008; Ejigu and Bekele, 2014). Generally, larger groups benefit from defending predators and smaller groups benefit from less competion for food. The relatively higher mean group size of geladas in the present study area is probably attributed to the risk of predation from carnivores such as caracal that were observed to frequently prey on geladas during the study period. On the other hand the higher group size and band size during dry season than wet season could be due to crop raiding behaviour during dry season. During dry season crop are harvested and could congregate to feed on crops and crops residue, specially during early dry season (Kifle et al., 2013). During the dry season predation risk could be minimized due to absence of foggy weather, presence of open habitat that increase vigilance. It has been reported that group size increases during dy season as there will not be partial restrictions (Beehner et al., 2008). However, the findings of the study contradict with findings of other similar studies elsewhere over the ranges of geladas that stated that the higher mean group size occurred during wet season than dry season (e.g. Abie and Bekele 2017; Goshme and Yihune 2018 ). This could be explained due to variations in food availability, climatic conditions and level of threats among the study areas and the above cited literatures.
The distribution pattern of Gelada in the study area was not uniform, varied depending on the food and water availability. The highest number of Gelada was recorded in the open grassland habitat while the least was in plantation forest habitat. It is known that the diet of geladas predominantly comprises grass species (Moges, 2019). As indicated in the study area description grass species highly preferred by the geladas such as Cynodon aethiopicus, Andropogone abyssinicus, Pennisetum clandestinum (Moges, 2015) are commonly abundant in the grassland habitat, while they are very scarce in plantation forest. Furthermore, the grassland habitat also offers permeant source of water for the geladas. The result is in line with other studies such as Dunbar (1992), Abi and Bekele (2017) and Goshme and Yihune (2018). The grassland habitat preference is attributed to the better availability food and water sources and increased vigilance effect. The open nature of the habitat enables gelada to take a measure to avoid predators by visual communication. Studies over different ranges of geladas concluded that open grassland is the most preferred habitat of geladas. This is attributed to the fact that grasses and rhizomes comprise the predominant diet of geladas (e.g. Dunbar 1987, 92; Moges, 2015)