Nile Rat

Arvicanthis niloticus (Lesson, 1842)

Other Names: African Grass Rat, Kasu Rat, Nile Grass Rat, Unstriped or African Unstriped Grass Mouse or Rat, Unstriped Nile Rat, Arvicanthus Bush Rat (former name).

INTRODUCTION Uncommon in captivity. The exact number of species belonging to the genus Arvicanthis (ranging somewhere between one and seven) is still being debated. Most authorities agree that there are probably five distinct species, all hailing from tropical Africa, of which Arvicanthis niloticus is by far the commonest in captivity and realistically the only one you are likely to encounter. The differences between the species are very slight, however, and the number will probably be reassessed in the light of future research. They are easy to keep and breed, but are more timid than domestic mice or rats. They are clean animals and very entertaining to watch as they go about their normal daily business. Nile Rats are not easy to handle, being agile and quick when they need to be if perhaps not quite as limber as Domestic Rats (Rattus norvegicus) and capable of some impressive leaps.

MEASUREMENTS Head/body length 110-200 mm. Tail length 100-150 mm. Weight 50-120 g. Nile Rats seem to be slow to achieve their maximum size.

APPEARANCE The stout, almost 'podgy', body is olive brown or tawny to dark grey-brown in colour, heavily ticked with black and yellowish (strong agouti) hairs along the sides. This ticking may form almost straight lines in places. The fur appears grizzled due to dark tips at the ends of the hairs. The hairs are coarse and stiff, giving the coat a slightly harsh and somewhat spiny texture. The underparts are slightly paler than the upper parts but there is no sharp line of demarcation as you see in many other rodent species. Most, but by no means all, specimens show a clear dorsal stripe running along the spine. The intensity of this stripe varies from individual to individual, with some animals lacking it altogether. The bicoloured tail unusually for rodents, it is slightly shorter than the body is functionally naked apart from a covering of very fine, short and stiff hairs. 

The narrow head, in configuration more like a mouse than a rat, features close-set eyes and rounded, usually reddish, ears. The incisor teeth are not grooved as in some other rodents. This is a useful feature for distinguishing Arvicanthis from the very similar-looking African Groove-toothed Rat (Mylomys dybowskyi). 

The legs are short. The fifth digit ('finger') on each front foot, although considerably reduced, cannot be considered vestigial because it remains fully functional and bears a claw. The second, third and fourth toes on each hind foot are rather long, whereas the first and fifth toes are much shorter 

Females have six mammae.

DISTRIBUTION This genus is extremely widespread across most of Africa. The forma typica species occurs in the south-western Arabian Peninsula, Nile Delta of Egypt, Senegal and southern Mauritania to western Ethiopia and south to eastern Zambia. Some authorities believe that this species enjoys an even broader range, as very similar-looking murines are to be found in other African countries besides the ones mentioned above. It depends whether you consider these to be completely different species, or merely subspecies, of Arvicanthis niloticus. The greatest concentrations of Arvicanthis are in East Africa as well as along the Nile River, but it is unclear whether these are always A. niloticus or, in some cases, are other, closely related species. Clearly much taxonomic work remains to be done.

NATURAL HABITAT Nile Rats are most common in savannah grasslands, but are also reported from forests and scrub. In the northern part of the species' range, it can survive even in a desert climate, provided the area receives some form of regular irrigation, either natural or manmade. A closely related species, The Abyssinian Grass Rat (Arvicanthis abyssinicus) has been found at elevations of 3,700 metres.

SOCIAL STRUCTURE Highly social as a rule, numbers vary considerably from area to area from scattered, small colonies of a handful of animals to situations where there are literally tens of thousands of animals. Like lemmings, the wild population is prone to periodic explosions and sudden collapses. In some years, populations increase rapidly to plague proportions of as many as 250 animals per hectare but 'crash' equally dramatically when the food supply fails. In the most serious outbreaks, the animals are so numerous as to be literally underfoot. It is relatively rare for numbers to reach this magnitude, however, and colonies of around 65 to 100 animals per hectare are more usual.

Most of the time they live in much smaller units comprising several adult males and females. Although members of the colony live harmoniously together and fights are practically unheard of, both males and females can be aggressive towards unrelated animals of the same sex.       

In captivity they can be kept in almost any combination from pairs and single-sex groups to substantial colonies of both sexes. As in the wild, fighting is rare and I personally have never known specimens to attack each other, even when an animal was unwell and obviously vulnerable. It is said that bucks tend to fight if they have been living with a doe for any length of time and are then put into bachelor groups.

TEMPERAMENT Although non- aggressive and certainly will not 'go' for the keeper, they are less than easy to handle (see 'Handling' below) and may bite if restrained.

BEHAVIOUR Unusually, Nile Rats are mostly diurnal. They are most active in the early morning before the sun is overhead, and again in the late afternoon, but they also seem to have periods of limited activity during the evening and night-time. There is a lot of controversy in this area, with a few authorities claiming that some field studies suggest that the species is crepuscular or even nocturnal in its habits. The confusion probably arises because there are a number of related species which are very similar in appearance. Accurate identification in the field is notoriously difficult and probably several different species have inadvertently been included in these studies, thus invalidating the results. There is also a possibility that, with a species as successful and wide-ranging as this, populations may behave differently according to their locale and habitat.        

Adult home ranges overlap considerably. Territories are more extensive during the rainy season, contracting during the dry season. Territory size varies depending on habitat, availability of food or population numbers. In the rainy season, adult males enjoy a home range of about 2,750 square metres. Females and young animals have slightly smaller ranges of about 950 square metres. In the dry season, ranges average 1,400 square metres for adult males and 600 square metres for females and youngsters. In periods of high density, males stray up to 37 metres from the home nest (females up to 38 metres), and up to 86 metres in times of low density (females a mere 47 metres). Both sexes possess a strong homing instinct and captured specimens have successfully returned to their home territory when released within a quarter of a mile of the trapping site.    

Nile Rats are capable of digging burrows in soft earth, but usually they take up home in crevices, tangles of long grass, beneath rocks or fallen trees, or inside termite mounds. They construct long runways through the grass, often leading under shrubs and rocks, radiating out from their nest sites. These smooth dirt trails, resembling well-trodden miniature hiking paths, usually follow the natural formations of the ground. The rats are careful to keep the pathways free from obstructions such as small stones which could hinder a quick getaway if pursued by a predator, and even go to some lengths to keep the surrounding grass clipped out of the way.

COMMUNICATION Mostly silent, but squeaks and grunts are heard from time to time.

DIET Nile Rats are primarily herbivorous, consuming large quantities of seeds, leaves, grass shoots, and (to the dismay of local farmers) cultivated crops. In times when other fare is unavailable, they will add insects and fruits to their diet. In captivity they should be offered a basic seed diet, supplemented with a variety of fresh fruit and vegetables (apple, carrot and cabbage are particularly relished) as well as garden weeds (such as dandelion leaves), grass clippings, lab blocks (good for the teeth), and boiled rice. As a treat, bacon rind or live food in the form of meal worms or crickets can be given occasionally. A non-coccidiostat rabbit mixture adds variety to the diet. Hay provides essential roughage, as well as being used for bedding. Wood gnaws are necessary to prevent the teeth becoming overgrown. Nile Rats do not drink much, particularly if they receive fruit or vegetables regularly, but fresh, clean drinking water must be available at all times.

SEXING Mature males are easily distinguished by the presence of a scrotal sac.

ACCOMMODATION They are sizeable rodents that require a fair amount of space and, although they have been accommodated quite successfully in large laboratory cages, I prefer a 48" glass aquarium with a cage extension built on top. The minimum length of the tank for a single pair is about 60 cm, but, of course, the bigger the better. A wire lid is essential.

The enclosure should be filled with hard-wood branches for climbing and gnawing (apple wood is ideal for this) and a selection of cardboard tubes, ceramic pipes, metal ladders, and a nest box filled with soft meadow hay. I recommend changing the furnishings or rearranging them every time the rats are cleaned out to give them something new to investigate. Hay or straw can be spread in quantity around the cage.

Nile Rats derive considerable enjoyment from an exercise wheel. Such a wheel should be as wide as possible and, most importantly, should not have open rungs which could trap legs or tails. The only 'solid' wheels I have come across have been made of plastic, and as plastic would not last more than a week or two (if as long as that) in a cage containing Nile Rats, I suggest weaving cardboard between the rungs of a spoked metal wheel to render it safer. Of course, the cardboard will need to be renewed frequently, but this minor inconvenience is preferable to the injuries that can potentially result from leaving the wheel unaltered.

I always provide my animals with a warm and cosy sleeping box, such as a budgerigar nest box with a specially enlarged entrance hole, but it has been opined by some other keepers that a sleeping box should be given only to established animals, because it does sometimes appear that if one is given to a newly obtained pair, taming may prove difficult. 

Nile Rats are very clean animals and will not usually foul their own nest. The animals can, in fact, become quite agitated if the bedding is renewed too often. Unless the cage is very small or overcrowded (which must never be allowed to happen), cleaning need not be carried out much more frequently than about once every two weeks. The bedding in the sleeping quarters can be changed every month.

HANDLING This is not always easy, despite the fact that usually they are non-aggressive. If you do need to pick one up, either because it seems unwell or you need to move it to another cage, you should place one hand around the shoulders with your thumb under the lower jaw. Every movement should be made fairly slowly. If you are nervous, the rat will sense this. As the rat has rather poor eyesight, you must let it take its time to get accustomed to your hand, but do not allow the Nile Rat to sniff your fingers or it may decide to take an experimental nip. Unless the rat is frightened, this initial bite will be made quite half-heartedly and is usually not too painful, but it can be a shock if you are not expecting it and it is only too easy to instinctively wrench your hand away, thereby panicking the animal. Instead, tighten your hand into a fist and let it sniff your knuckles, as it will not be able to get such effective purchase on this part of your hand.

Wrapping your hand around its shoulders, push the jawbone forward with your thumb and forefinger. In this position the rat will be incapable of biting. Do not put pressure on the throat. Written down like this, the manoeuvre sounds horribly complicated but it is really quite straightforward with practice, although, like everything, it may take several attempts to master it.

Alternatively, wear stout gauntlets, although I personally do not like them for handling animals as they reduce sensitivity in the fingers.

Owing to their speed and ability to jump, Nile Rats can be difficult to recapture if they get loose.

REPRODUCTION Some sources report that, in the wild, reproduction is generally restricted to the wet season when several litters may be born in quick succession. Others contradict these observations by insisting that breeding does in fact occur during the first half of the dry season when vegetation is at its most verdant and there is little chance of the burrows flooding. Clearly there is a serious discrepancy here. Probably the breeding season varies from area to area. In captivity it breeds throughout the year.

Females are polyoestrous and spontaneous ovulators. As the birth grows imminent, a nest of fine grasses is built in a burrow or on the surface. In captivity, hay is the favoured nesting material. Females typically bear about 4-6 young (range: 1-12) which are born after a gestation period of 21-24 days, although it can be as short as 18 days. Young are born blind. The birth weight is about 3-6 grams depending on the size of the litter. Offspring from smaller litters tend to weigh slightly more than those from larger litters. The female experiences a post-partum oestrus and is ready for mating again within one to two days of giving birth.

Unlike Domestic Rats (Rattus norvegicus), Nile Rat babies are born in a semi-precocious state, being fully furred. Eyes open within only a few days of birth in some books it is (erroneously) reported that eyes do not open until 14-16 days at which point the young animals are able to scurry about and are even surprisingly nimble. They continue to suckle up to about 21 days. They are completely independent at six weeks and sexually mature at 3-4 months. The buck will not intentionally harm the babies and can safely to left in the cage (although you may decide to remove him temporarily to prevent the female conceiving again too soon after the birth). Does prove themselves exemplary mothers and usually all the pups are reared. If the female dies suddenly, or deserts the young for any reason, it is possible to hand-rear them. For very young pups, a replacement rat milk food should be given by syringe every two hours throughout the day and night, gradually reducing the frequency of the feeds as the young develop.

NATURAL PREDATORS Most mammalian, avian and reptilian carnivores, particularly eagles, jackals, leopards, and mongooses.

CONSERVATION STATUS Not threatened or endangered at present due to its versatility and rapid reproductive rate. It is, however, killed in large numbers for a variety of reasons: as food for local people, because it is considered a major agricultural pest in some parts of Africa due to its penchant for destroying crops (cereals, cotton, groundnuts, sugar cane, etc.), and because wild individuals are implicated as a vector for bubonic plague (Yersinia pestis). In regions where food is plentiful, populations can expand rapidly from relatively small numbers to reach pest proportions within a matter of only a few weeks. This is a major problem. In Kenya, for example, villagers rely on subsistence crops such as wheat and maize. Nile Rats eat the seedlings and the wheat grain, but controlling the rat population is expensive, involving poisons which are often indiscriminate and less than humane, so control by such means is best avoided unless absolutely necessary. Wild Nile Rat populations must be assessed early in the season to estimate the density and to gauge whether some sort of control might be necessary because, by the time crops are damaged, it is already too late.

LIFE SPAN 5-6 years have been reported on numerous occasions and the captive record stands at 6 years 8 months.

HISTORY IN THE U.K. It is popularly thought that the species was first introduced into British collections by zoologist Clinton Keeling in 1961 when he obtained a small group from a Dr. Jacobi of Amsterdam. At that time they were known by the cumbersome epithet of Arvicanthus Bush Rats. They bred well at Mr Keeling's Ashover Zoological Garden, Derbyshire, England. From the original nucleus of only 5-7 animals, a thriving captive population was soon established. Then disaster struck. For some unexplained reason (possibly in-breeding due to the old curse of the original gene pool having been too small), male births predominated and so eventually, of course, the species died out in captivity in Britain. The Zoological Society of London (London Zoo) imported a second group in the early 1970s and it is from those that most of the present British population derives.


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