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Species Name: Zanzibar Giant Rat (Cricetomys sp. nov.)
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The Zanzibar Giant Rat (Cricetomys gambianus) is the only large rodent found in the Zanzibar archipelago. It is found only in the Zanzibar and Pemba islands and the adjacent Tumbatu island, where it lives in coconut plantations, forest, scrub, and grassland. The species is a nocturnal and secretive animal that is relatively little known. It is a member of the rodent family Muridae which is the largest and most successful family of mammals and comprises 25% of the 4000 known species of mammals. The giant rat’s closest relatives are found to the west on the African mainland. The genus Cricetomys is restricted to the forest and savannah zones of sub-Saharan Africa and evolutionary biology suggests that the members of the genus are descendants of rats that invaded Africa from Asia and later adapted to exploit life in the African environment.
This rat is a true giant in comparison with other species found on the island. Fossil remains of an even larger rat have been found suggesting that gigantism is a characteristic of this species. It is similar in appearance to the black rat, but has a somewhat broader head and more stoutly built body. The tail is much shorter than that of the black rat. The upper parts of the body are a shade of brown, mixed with a few black hairs. The under parts of the body are a dirty white and the tail is brown in colour. Adults attain a weight of up to 500g and have a body length of 200-250mm. This species is nocturnal and nests in burrows. During the day it can sometimes be seen emerging from or returning to a burrow.
This rat is an inhabitant of forest and scrub areas. It has been recorded at various locations in the west and central parts of Unguja and also on the smaller island of Tumbatu. This species has a very restricted geographic range and has been assessed as Endangered using IUCN Red Listing criteria. The principle threats to the survival of this rat are habitat loss and predation by introduced species. Anecdotal evidence suggests that there have been declines in the population of this rat since the 1980s. A detailed study of its ecology and an assessment of population numbers are needed in order to develop a conservation management plan. Given its rarity, this species is a difficult one to observe and study in the wild.
The giant rat has only been identified in the forests of the West Coast. The forests offer little water. The rat does not have to drink as it gains enough water from the invertebrates that it eats. This is a useful adaptation, as the forests are well above the high water table and the streams only flow for a few months of the year. The long period over which these streams fall dry is partly responsible for the rat’s predator, Barn Owl Tyto Alba, becoming a top predator. The rat is the owl’s main food source and the forests the best hunting grounds as the invertebrates have so much readily available water.
The rats at high altitudes and cooler climates have thick fur compared to the rats living at sea level. It is thought that this may be due to insular dwarfism, a biological phenomenon which describes the size reduction of large animals over a number of generations when their population’s range is limited to an islet or island. The phenomenon also affects the elephants. It may be due to the low metabolic rate of larger animals and the scarcity of food on the small islets forcing the animals to grow to a smaller size.
To increase chances of seeing the Zanzibar giant rat on the trip, spend the night at Ngezi forest. As the rat is nocturnal, this is the time when it is most active. During the day, the rat spends most of its time in a burrow, so you will have little luck if you only visit during the day. On the trail through the forest, you will find rat burrows just off the track. The rat has long been regarded as a pest because it eats cultivated crops, particularly coconuts, sweet potatoes, and bananas. So if you walk a bit further afield in the forest where there is human habitation, you may find a farmer who traps the rat, and he may be willing to part with a live one. This may seem like a chance to “take the rat out of the wild,” but remember that where the farmer lives, the rat is doing what it was doing before the farming was there. Then again, a smiling man with a rat in a cage telling you it is a good tourist attraction presented the image that I now have as the header of this page. The best place to see one without hunting or getting a captive one for yourself is probably Pemba. There are many who see it as a pest.