Osun State (/ˈoʊʃuːn/; Yoruba: Ìpínlẹ̀ Ọ̀ṣun) is in southwestern Nigeria, bounded to the east by Ekiti and Ondo states, to the north by Kwara State, to the south by Ogun State and to the west by Oyo State. Named for the River Osun, a vital river which flows through the state, The state was formed from the southeast of Oyo State on 27 August 1991 and has its capital as the city of Osogbo. Geographically, the state is divided between the Nigerian lowland forests in most of the state and the drier Guinean forest–savanna mosaic in the north. The major geographical features are rivers including the state’s namesake, the River Osun which bisects the state’s interior before forming much of the state’s southwestern border with Oyo State and flowing south. Other important rivers are the Erinle and Oba rivers, both Osun tributaries which flow from the north before meeting the Osun along the southwestern border. Among the state’s nature are mona monkey, common kestrel, purple heron, and royal antelope populations along with some of Nigeria’s last remaining Nigeria-Cameroon chimpanzee and African forest elephant populations in the heavily threatened forests along the southern borders with Ondo and Ogun states.