Published and unpublished data are used to assess the faunal (animal) values of the Eastern Arc Mountains in terms of the numbers of endemic species, and number of species shared with the adjacent lowland Coastal Forests and with the Tanganyika-Nyasa Mountain Forest Group. Emphasis is placed on vertebrates, although some data for invertebrate groups are also provided.

At least 74 vertebrate species are strictly endemic to the Eastern Arc Mountains, split as follows: birds 10 species, mammals 11 species, reptiles 23 species and amphibians 30 species. A further 40 species are near-endemics, but range slightly more widely than the strict definition of the Arc. Eastern Arc Mountain blocks that possess endemic vertebrates are the Taita Hills (two species), the East and West Usambaras (12 species), the Ngurus (one species), the Ulugurus (13 species) and eastern Udzungwas (13 species).

A minimum estimate of 265 invertebrate species confined to single Eastern Arc Mountain blocks was obtained, although insufficient collection and taxonomic work means that this figure is certainly an underestimate. Detailed work in the Uluguru Mountains provided an estimation of 169 invertebrate species endemic to that mountain alone.

Almost all endemic species are closed-forest specialists, although there are Eastern Arc endemic birds and butterflies confined to montane grasslands and heathlands. The most important locations for the conservation of biodiversity are the east-facing scarps directly influenced by the Indian Ocean in the largest highlands. The North and South Pare Mountains, Rubehos and Ukagurus seem genuinely poorer in endemics than other areas. The Eastern Arc Mountains possess species with both an ancient history and those of more recent evolution. Ancient affinities of the fauna are with West Africa, Madagascar and even SE Asia. An extremely long history of forest cover and environmental stability are the likely causes of these remarkable affinities.