Zanzibar is two islands 59 kilometres apart, and they were not born the same way

Almost everyone talks about Zanzibar in the singular, and that hides the interesting part. It is several islands, and the two big ones are so close that on a map they look like sisters: 59 kilometres of sea between them, the same climate, the same vegetation at first glance.

But they do not share a biography. One broke away from the mainland along a fracture and was left isolated on the far side of a deep channel. The other rests on a shallow shelf that the sea has flooded and emptied several times, as ocean levels rose and fell.

01 Two islands and an archipelago

Start with the inventory, because scale matters. Unguja is the largest and the one almost everyone simply calls Zanzibar: a long island of gentle relief, around 1,666 square kilometres, holding most of the archipelago's population and tourism.

Pemba is smaller in area — 988 square kilometres — but hillier and considerably more fertile. Apart from a strip of its east coast, it is highly productive land: as well as the cloves that made it famous, it grows rice, coconut, banana, cassava and red beans.

To those two add Mafia, south of Unguja, and a fourth coral island, Latham, which has no permanent population and serves as a breeding ground for seabirds. Around all of them lie dozens of islets, a few settled and most not.

02 The one that broke away

Pemba is surrounded by relatively deep water, and that is the key to everything. It is thought to have been part of the mainland and to have split off along a fracture, the Pemba rift, approximately ten million years ago.

Ten million years is an enormous stretch of time in biological terms, and a deep channel does not dry out when sea level falls. Even if the ocean drops tens of metres during a glaciation, that channel remains a channel. Pemba has been isolated ever since, without interruption.

That depth has another consequence, this time for anyone who gets into the water. Around Pemba there are walls that fall away sheer, coral in good condition and very abundant marine life, precisely because the shallow shelf ends quickly and the bottom drops.

03 The one the sea covered and uncovered

Unguja works the other way round. It is separated from the mainland by the Zanzibar Channel, and that channel is shallow. When sea level drops far enough — and it has dropped many times during glaciations — the bottom emerges and the island stops being an island.

Every time that happened, land animals could walk across. And every time the sea rose again, those on the wrong side were shut in. Unguja is not an oceanic island: it is a piece of the mainland that comes and goes.

That explains a contrast that puzzles anyone arriving expecting two identical islands. Unguja has notable land mammals — the Zanzibar servaline genet, the African palm civet, the red colobus — a cast that does not fit a small island in the middle of the Indian Ocean.

04 The monkey that got smaller

The best documented case is the Zanzibar red colobus, endemic to Unguja and to nowhere else in the world. Its population is believed to have been isolated on the island when sea level rose towards the end of the Pleistocene.

Molecular analysis places its split from its mainland sister species at around 600,000 years ago, considerably earlier than had been assumed. And its skull tells the story: it is smaller than that of its mainland relatives.

That fits what is known as Foster's rule, or the island rule: when a large animal is shut into a territory with limited resources, it tends to become smaller over time. Here it can be measured in centimetres of skull.

There is another detail that catches the eye as soon as you watch one move. The name colobus comes from Greek and means, roughly, cut short, after the reduction or complete absence of an opposable thumb that characterises the genus. Its very long tail is no use for gripping: only for balance.

05 The leopard that came back

The most striking proof that Unguja was once connected to the mainland is an animal that had been given up for gone. In June 2018 a camera caught a leopard on the island, twenty-five years after it had been considered extinct there.

A cat that size does not swim to an oceanic island. It was there because at some point you could walk here, and it stayed hidden that long because the island, densely populated as it is, still keeps enough forest.

06 What this changes when choosing an island

The practical consequence is simple and almost nobody puts it this way: if you are going for land life — forest, monkeys, endemic wildlife — the larger island is the one that has it, because it was joined to the mainland. If you are going for the water, the northern one offers walls and coral the other cannot match.

And there is one last thing worth carrying with you when you look at the map. That very pale turquoise ringing the larger island, the one in every photograph, is not decorative: it is the shallow shelf, the same floor that dried out several times and across which the animals that are endemic today walked over.