Zanzibar Island, properly called Unguja, is a low coral island in the Indian Ocean off mainland Tanzania, separated from the Tanzanian coast by the Zanzibar Channel, roughly 30 to 37 km wide. It is about 85 km long and up to 30 to 39 km wide, covers about 1,666 km² (some sources give 1,464 km²), and rises no higher than about 120 m. Fringing coral reefs, mangrove creeks and a dry limestone scrub called coral rag surround a forested core. Unguja is the largest island of the Zanzibar Archipelago, part of semi-autonomous Zanzibar within Tanzania, and had 896,721 people at the 2012 census, most around Zanzibar City and Stone Town. Its geography is easiest to read through its animals, because they record when the island was cut off from Africa.
Why Unguja’s Animals Look African and Pemba’s Do Not
During the last ice age, sea level was so low that the Zanzibar Channel, today 120 km long and only a few dozen metres deep in the centre, was about 120 m shallower, and Unguja was joined to the mainland. When the sea rose at the end of the Pleistocene, it trapped populations of mainland mammals on the new island. Their descendants evolved in isolation, which is why Unguja has its own red colobus, its own genet and, possibly still, its own leopard.
Pemba, roughly 50 to 60 km north of Unguja, tells a different story. It is surrounded by deep water and is thought to have split from the mainland along the Pemba rift some 10 million years ago, far earlier. It has its own endemics, from a giant fruit bat to a scops owl, found nowhere else. The two islands are a natural experiment in how long isolation shapes wildlife.
Jozani Forest and the Zanzibar Red Colobus
The island’s flagship animal lives in the south-centre, in Jozani–Chwaka Bay National Park, the only national park in Zanzibar, managed under IUCN category II and part of UNESCO’s Man and the Biosphere Programme. The Zanzibar red colobus (Piliocolobus kirkii) is endemic to Unguja. It is also called Kirk’s red colobus after Sir John Kirk, the British Resident who brought it to scientists’ attention, and it was adopted as Zanzibar’s flagship conservation species in the mid-1990s. Molecular studies suggest it split from its closest relative, the Udzungwa red colobus of mainland Tanzania, about 600,000 years ago.
Estimates put the surviving population at about 1,600 to 3,000 animals, and it is listed as Endangered. Jozani provides about 25 km² of protected habitat, yet roughly half of all colobus live outside protected zones. Groups living in the shambas (farms) next to the park reach higher densities than those inside it, which makes farmland as important to the species’ future as the forest itself.
Jozani is more than one monkey. The reserve combines groundwater forest, mangroves, wetlands and coral reef, and surveys have counted 168 bird species, 30 of global or regional importance, and 291 plant species, 21 of them endangered. Sykes monkeys, bushbabies, more than 50 kinds of butterfly and the nocturnal Zanzibar tree hyrax share the forest. The core and buffer zones are uninhabited, but about 16,423 people live in the transition zone, farming, fishing, keeping bees and running tourism.
Coral Rag Thicket: Aders’s Duiker and the Servaline Genet
East of Jozani, most of Unguja’s south and east is coral rag: raised fossil reef limestone with thin soil and almost no surface water. It looks unpromising, but it holds two of the island’s rarest mammals. Aders’s duiker, a small forest antelope about 30 cm tall at the shoulder, survives only on Zanzibar and in coastal Kenya, and on Zanzibar it is restricted to tall thicket forest growing on waterless coral rag. Individuals in the east of the island weigh around 12 kg, those in the south only about 7.5 kg. A 2022 study proposed moving it into a genus of its own, Leucocephalophus.
The Zanzibar servaline genet was unknown to science until 1995, when wildlife consultant Tony Archer obtained a dried skin and skull in the village of Kitogani in south-central Unguja; the subspecies Genetta servalina archeri is named after him. Live animals were first photographed in January 2003, when camera traps in Jozani–Chwaka Bay caught servalines at four sites, two in groundwater forest and two in dry scrub to the north-east. Its closest relative lives in the Eastern Arc Mountains on the mainland, the same pattern of ice-age isolation that produced the colobus.
The Zanzibar Leopard, 1932 to the 2018 Camera Trap
The island’s largest land carnivore has the murkiest record. Reginald Innes Pocock described the Zanzibar leopard as a subspecies, Panthera pardus adersi, in 1932. It was smaller than mainland leopards, with rosettes broken into spots. Genetic work in 1996 folded it back into the African leopard, though some authors still use Pocock’s name. In the 20th century, conflict with villagers and a belief that sorcerers kept leopards as familiars led to deliberate campaigns to wipe it out, and a planned conservation programme in the mid-1990s was dropped when researchers judged the population could not survive.
Only six skins reached museums, including the type specimen in the Natural History Museum, London, and a faded mount in the Zanzibar Museum. The last confirmed sighting of a living animal was in the early 1980s, although people working the coral rag of the south and east kept reporting leopards into the early 2000s. In June 2018 a camera trap set for the Animal Planet series Extinct or Alive filmed a leopard on Unguja; some experts remain sceptical because the exact locality is unknown and the animal could be an introduced mainland leopard, so the population is still best described as extirpated or nearly so.
Chwaka Bay, Menai Bay and the Kizimkazi Dolphins
Unguja’s coasts are where most visitors meet its wildlife. Chwaka Bay on the east coast forms the marine side of the national park, its mangroves acting as nurseries for reef fish. In the south-west, the Menai Bay Conservation Area, established in August 1997, covers 470 km² of reef, seagrass and mangrove and is Zanzibar’s largest marine protected area, rated IUCN category VI. It is run jointly by the Fisheries Department and 19 villages, which set up committees to stop blast fishing and illegal circular nets.
The best-known result is at Kizimkazi, a fishing village on the southern coast. Villagers who once hunted dolphins for food now take visitors out to watch them; the conservation effort has drawn about 200 bottlenose and humpback dolphins to the village’s shores, and about 20,000 tourists visited the area as early as 1998.
Mnemba’s 600 Reef Fish and Nesting Green Turtles
Off the north-east coast, opposite Muyuni Beach, Mnemba Island is a triangle of sand about 500 m across, ringed by an oval reef roughly 7 by 4 km that is protected as the Mnemba Island Marine Conservation Area. Its waters hold about 600 species of reef fish, three species of dolphin and visiting whale sharks. Green sea turtles nest on its beaches between February and September, and a monitoring and protection project has run since 1996.
Chumbe Coral Park and Changuu’s Tortoises in the Zanzibar Channel
On the sheltered west side, in the Zanzibar Channel, Chumbe Island became the island’s conservation pioneer. Its fringing reef was closed to fishing, boating and diving in 1992, and in 1994 the island and its waters were declared Chumbe Island Coral Park, Tanzania’s first marine park, managed by a non-profit company. The reef had survived in good condition because the island lay within a military zone. Closer to Stone Town, Changuu (Prison Island), 5.6 km offshore, keeps a herd of endangered Aldabra giant tortoises, originally a gift from the British governor of the Seychelles: a reminder that not every animal on Zanzibar’s islets is native.
Pemba’s 29,000 Flying Foxes and the Ngezi Forest
Pemba, 988 km² and hillier and more fertile than Unguja, shows what 10 million years of isolation produces. The Pemba flying fox (Pteropus voeltzkowi), with a wingspan of 1.6 m, lives nowhere else. Shotgun hunting and forest clearance cut it to only a few hundred animals in the early 1990s. Pemba’s Forestry Department then ran an education campaign, set up village wildlife clubs to guard roosts and banned shotgun hunting over most of the island. By 2007 there were about 20,000 bats, and by 2011 nearly 29,000. A single roost can hold up to 850 bats, and some roosts are in traditional graveyards that taboo keeps people away from.
The Pemba scops owl, a 15 cm owl with a 45 cm wingspan, is likewise endemic and listed as Vulnerable. It now depends largely on two remnants of native forest: Ngezi, a 14.4 km² reserve in the north-west created in 1959, and Msitu Mkuu, only 3 km². Ngezi also shelters an endemic vervet monkey subspecies and introduced red colobus. Off Pemba’s west coast, the 1 km² Misali Island, whose name comes from the Swahili for prayer mat, protects more than 300 fish species, 40 coral species, nesting green and hawksbill turtles and coconut crabs.
Read together, Unguja and Pemba show how island size, depth of water and time since isolation set the wildlife of the Swahili coast. Their reefs grew in the same way described in our explainer on how coral reefs form, and their rhythm of long and short rains is covered in the guide to the climate of Tanzania; for the animal the mainland chose as its emblem, compare the national animal of Tanzania.