Demographic Structure of Zanzibar Red Colobus Populations in Unprotected Coral Rag and Mangrove Forests

More than half of the global population of the endangered Zanzibar red colobus (Procolobus kirkii) live outside the single major protected area on Zanzibar Island. We present data on the 2 largest, discrete subpopulations living in unprotected areas at extremes of the species' range. We compare the size and structure of 11 groups, specifically 6 core groups inhabiting interior, mature forest with 5 peripheral groups living in disturbed/degraded edge habitats. Groups living in southern mangrove forest--a species-poor but more productive and less seasonal habitat than coral rag thicket--had larger group sizes and more heterogeneous age structure, were more stable, and had higher rates of infant survival than did groups in northern coral rag. Group size ranged from 5.5 ± 1.6 SD (the smallest reported for this species) in edge coral rag to 31.2 + 1.9 SD in core mangroves. Edge groups were significantly smaller than core groups in northern coral rag while in the south, where all groups had access to mangroves, we found no significant difference in mean group size between edge and core areas. Groups using mangroves exhibited frequent social play, an indicator of habitat quality, and had a higher ratio of births per female per year. We suggest that mangroves are an important refuge and possibly source habitat for Zanzibar red colobus. We urge the conservation of mangrove and remaining coral rag in the unprotected areas described here in an effort to sustain this endemic species throughout its range.