Population estimation and behavioral ecology of Omo River guereza (Colobus guereza guereza) in Simien Mountains National Park, Ethiopia, MSc Thesis, by Wendimhunegn Mekuria, Bahir-Dar University, January 2026 Ethiopia harbors high faunal and floral endemism due to its wide range of habitats. Simien Mountains National Park (SMNP), part of the Eastern Afro-Montane biodiversity hotspot, is an important habitat for the endemic subspecies guereza (Colobus guereza guereza). Previous studies indicate that this subspecies is experiencing population declines across its potential habitats, largely due to habitat fragmentation and loss. However, detailed ecological information necessary for effective conservation and management remains limited in SMNP. Therefore, this first comprehensive study, conducted from February to August 2025, aimed to generate baseline data on the population status, behavioral ecology, and feeding ecology of C. g. guereza to support future conservation actions and management planning. Population size was estimated using a total count method. Behavioral and feeding patterns were studied in two focal groups inhabiting Continuous Forest (CF) and Fragmented Forest (FF) using instantaneous scan sampling at 15-minute intervals from 06:00 to 18:00 hours. Food availability was assessed through phenological monitoring of 12 key food plant species within the home ranges of the two study groups. Vegetation species richness and structure were surveyed using six 20 m × 30 m quadrats (0.36 ha) in each habitat type. Data were analyzed using SPSS version 27. Chi-square goodness-of-fit tests were applied to examine age–sex composition and dietary proportions. Seasonal variation in feeding was evaluated using dietary niche breadth and niche overlap indices, while MANOVA was used to test the main and interactive effects of season and habitat type on activity patterns. A total of 671 guereza individuals (712 in the dry season and 630 in the wet season), forming 56.5 groups, were recorded across montane forest, woodland, bushland, and ericaceous habitats, with a mean group size of 11.88 individuals. Guereza were predominantly distributed in montane forests, with significant variation across altitudinal gradients. Adults comprised 52.76% of the population, followed by subadults (26.75%). Activity budgets were dominated by resting (48.39%), feeding (23.62%), and moving (13.93%), with grooming and other behaviors accounting for the remainder. Activity patterns were significantly influenced by the interaction between season and habitat type. A total of 32 food plant species belonging to 26 families were identified, with marked habitat-specific variation (CF = 11 species; FF = 31 species). Apodytes dimidiata and Schefflera abyssinica were the most consumed species, with the latter showing the highest selection ratio. Young leaves dominated the diet (50.89%), followed by fruits, mature leaves, flowers, and other items. Guereza in continuous forests exhibited selective feeding on preferred species, whereas those in fragmented habitats adopted a more generalized diet. Overall, this study establishes a critical ecological baseline for C. g. guereza in SMNP and underscores the species’ vulnerability to habitat fragmentation, highlighting the need for habitat-focused conservation measures and further research on conservation challenges and community attitudes to inform effective management strategies.