Multimammate Mice

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(Praomys (Mastomys) natalensis)

* Occasionally one may find it listed in older reference sources by the archaic scientific name of Rattus natalensis or sometimes even Mus natalensis, both of which are outdated.

Other Names

Natal Multimammate Rat, Common African Rat, Soft-furred or African Soft-furred Rat.

Introduction

One of the commonest African rodents, this is an ideal species for the tyro breeder, being undemanding, easy to keep and (a definite plus for those keeping it in a bedroom) odour-free. Multimammate Rats are so-called because the females possess a superfluity of teats, more than any other rodent. (Multimammate = Latin, meaning literally "many-breasted".) It possesses long, soft, silky fur, hence its alternative name of Soft-furred Rat. There are about seven species (authorities disagree on the exact number), but Praomys coucha and Praomys natalensis are almost identical and, where the two ranges overlap in southern Africa, are impossible to tell apart, and usually the only way to be certain is to count the chromosomes.

The Multimammate Rat appears to be a connecting link between the true mice and rats, though it shows slightly more affinity to the rats than it does to the mice. Since no one can be quite sure about its relationship to other members of the Muridae, it has been alternatively classified at various times within the genus Mus (the true mice), Rattus (the true rats) – although it shares features of both genera, latest research shows that it is not directly related to either – and Mastomys. Indeed some sources, including the venerable Walker's Mammals of the World, still discuss the species under Mastomys, which is why this is included in parenthesis within the scientific name given at the top of this page, since the two names are currently interchangeable. It is a popular laboratory animal but only in recent years has it started to emerge on the pet market. It lacks a gall bladder and females – yes, females – possess a well-developed prostate gland. American readers should be aware that in some western states the import, transport and possession of all species of Praomys is strictly prohibited. The semi-dry terrain of certain states such as Nevada is very similar to Southern Africa and the authorities are paranoid that, in the event of their escape, there is a danger of their becoming naturalised. This would be disastrous, as not only is the Multimammate Rat a serious crop pest, but there is a possibility of feral animals forming a link between wild reservoirs of plague and man.

Size

Head/body length 15 cm; tail length 11 cm; adult weight 60g.

History in Captivity

The original animals were captured in Natal province, South Africa, and introduced into the laboratory by one Dr Davis in 1939. Only recently has it become established as a pet animal. Given its accommodating nature, ease of maintenance and readiness to breed, it is mystifying that this species is not seen more often in pet shops.

Colours, Patterns & Varieties

My personal favourite is the normal agouti coloration, although most if not all of those animals currently kept in the U.K. are of various colour mutations. The names given to some of these reflect the locale in which the foundation parent stock was captured; thus we have mutations with colourful names like Bargawanath Dilute (discovered in the original laboratory capture of 1939 in Bergawanath, near Johannesburg), and Iscor Dilution (obtained from animals trapped at Iscor, near Pretoria). This latter coloration is interesting. It is very similar to the Bargawanath and any differences that do exist are so subtle that the two varieties may, in fact, be the same! In addition to these two there is also an Albino strain, a Golden Yellow (sometimes erroneously called Lutino), and a variety called the Umbrous.

Distribution

Widespread and locally very common throughout Africa south of the Sahara, excluding West Africa. It is believed the species was once confined to southern Africa but spread northwards as a human commensal. In some other areas it is declining because of competition with the larger, more aggressive Black or Ship Rat Rattus rattus, which was inadvertently introduced into Africa by the early explorers. Its apparent absence from West Africa is perplexing. Most probably the prevailing habitat of dense rain forest is not to its liking, although the forest is now very fragmented and only in parts of Central Africa is it still continuous. Or maybe it could not compete with the indigenous fauna already found there. But possibly its absence from this area is misleading. Few field studies have been conducted on the smaller mammals of West Africa and it is conceivable that further research might ultimately show that the species does have a toehold there.

Natural Habitat

The Multimammate Rat occupies a wide range of habitat, absent only from desert, semi-desert and mountains, but most often seen in and around human habitation. Originally restricted mostly to open savannah and, to a lesser extent, marshes but, like the House Mouse, Brown and Black Rats, it has now moved into villages and is found in barns, outbuildings and lofts, as well as in both cultivated and abandoned fields. In the main it avoids large metropolises because it cannot compete with the Black Rat which is usually very numerous in towns and cities.

Natural Predators

Being so numerous and relatively conspicuous in its natural haunts, the species finds itself prey to a wide variety of carnivores, especially owls, but also civets, genets, eagles and snakes.

Temperament

Whilst this animal is not what I would call aggressive, males in particular are inclined to bite, even when unprovoked. Sometimes I have been bitten simply when replacing the food or water bowls. The bite is never sufficient to break the skin but it is quite painful. Consequently I desist from handling the species except when necessary, although it could be argued that only by regular handling do animals become tame. But it is not my intention here to dissuade anyone from keeping this species; I might just have been unlucky with my present animals, and other individuals from different strains might be less ready to nip.

Social Structure

Gregarious. A lone Multimammate Rat is bored and depressed. Once, when I was forced to keep a single specimen on its own as it was sick and needed time to convalesce, it remained in one corner of the isolation cage, eating very little, and was obviously unhappy with its lot. Although Multimammate Rats prefer to live in colonies, rather than as pairs – and in the wild several females will live harmoniously together in one burrow – they do form monogamous pairs. But in a colony situation they breed, if anything, a little too freely and, unless you can find sufficient homes for the youngsters, the enclosure will soon become overcrowded. At one point I had as many as 78 animals in a (very) large cage, but even at this high density there did not appear to be any aggression within the group. Eventually I was forced to separate the sexes to prevent any more births until the population had declined to a more manageable level. Both males and females will live in single sex groups, although female only groups appear slightly more stable.

Chris Henwood noted that occasionally individuals will destroy entire litters of babies, as well as cannibalising dead cage companions. He stresses that attacks on cage mates are rare, as is the nibbling of ears and tails, but, when slight damage such as this is inflicted, the victim often seems to make no attempt to escape its persecutor.

Let us deal with these observations systematically. I have found that, in a group situation, pups born in the open are invariably killed but, once nesting boxes are provided, the rearing rate is very good, even when the cage is slightly overcrowded. It seems that neonatal mortality can be attributed to inexperienced mothers, as this phenomenon seems to be commonest with a doe's first litter, or to a lack of privacy. As for the dead bodies of cage companions being eaten, there is nothing unusual about this. Most rodents are opportunist omnivores and not averse to eating meat when the chance arises. This makes good practical sense since many species hail from arid regions where food of any kind is in short supply and the corpse of another rodent is a welcome addition to an otherwise spartan diet. Chris Henwood agrees with me that aggression towards fellow cage mates is abnormal and probably a result of in-breeding, possibly combined with stress. Only once did I experience a bout of aggressive activity within my group. I did not witness any attacks, but several animals were found with terrible injuries. Most of them, despite treatment, died of their injuries or from secondary infection; those I believed were beyond help were euthanased immediately. The period of aggression was brief and I suspect that only one rogue animal was the culprit. I never did discover which one was responsible, and the attacks ceased as suddenly as they had started (perhaps he had died) and at present the colony is harmonious once more.

Multimammate Rats are surprisingly tolerant of other species and, being less territorial than the Domestic or Fancy Rat Rattus norvegicus, will usually accept strangers into their midst, something which is almost impossible with some rodents.

Behaviour

Nocturnal. Although terrestrial, it is adept at climbing and can even ascend dangling strings. It also swims well but is less competent at digging, although it will gouge its own burrow in soft or cracked ground if it is unable to find a rock crevice or abandoned burrow of some other animal. In captivity it can be induced to excavate its own burrow by sprinkling water onto a tray of sand to simulate rainfall. Burrows which it digs itself are usually a network of galleries without a central chamber. Those used for nesting are generally more extensive than those used at other times or for other purposes.

Handling

To quickly transfer an animal from one cage to another (whilst its permanent home is being cleaned, for example) I find the easiest method is to pick the animal up by the base of the tail (at the point where it joins the body). Never grasp the tail at any other point or you risk damaging it, and never keep the animal suspended in mid-air for any longer than is necessary. Quickly transfer it to its new cage or else set it down on a flat surface or the back of your hand. Failure to do so and the animal will panic. It will then either 'spin' (in which extreme case the tail may break off completely) or it will attempt to climb up its own tail to bite the handler.

If it needs to be examined for any length of time or has to undergo veterinary treatment, it is best to gently 'scruff' the animal in such a way that it cannot turn and bite. This is done by grasping the loose skin at the back of the neck. If this is performed correctly, the animal will lie quite still and accept what is happening to it. Clear signs that it is suffering some discomfort is when it kicks or struggles. The correct procedure for scruffing an animal is quite difficult to get right and initially you may need to be shown how to do it by somebody more experienced.

Sexing

Sexing presents few problems as mature males have a large scrotal sac. Bucks are also larger.

Accommodation

Multimammate Rats can be kept in a large glass tank or hamster cage. I prefer the former as it allows me to provide a deep substrate, which would not be possible in a barred cage, and less mess finds its way into the room. Wooden cages are unsuitable as these animals are quite destructive and they will soon engineer their escape from any cage made of wood. Tanks should have a well-fitting, well-ventilated lid.

Multimammate Rats are substantial animals that require a lot of space. The cage or tank should be at least 60 cm (24") long, but preferably even larger. The size of the cage seems to have a direct bearing on breeding success, as the male needs to be able to pursue and catch the female before mating with her, and a small cage obviously would not provide sufficient space for this important ritual. Without this essential "foreplay" the female does not always conceive. If your goal is to breed the species, it is worth remembering that the space between the bars of a typical hamster cage, whilst being adequate to retain the adults, might be too wide and would allow the juveniles to escape.

Litter & Bedding

Provide as deep a litter base as possible. Traditional wood flakes are still very popular as a floor-covering, but can be dusty and there is evidence the sawdust irritates the eyes and mucus membranes. For this reason very fine sawdust (which, ridiculously, is still sold in some pet shops) should be avoided altogether. Pine wood, especially red cedar, is especially bad due to the aromatic nature of the wood. Constant inhalation of the phenol oils present in this type of wood can cause long-term liver damage and ultimately damage to the animals' immune system. The relatively high incidence of cancerous tumours in some rodents may be partially due to these oils.

Some rodent-keepers remain staunch advocates of wood-based cat litters (in the form of compressed sawdust pellets). Personally I don’t like this material. I find that most rodents will chew it into a very fine and unpleasant dust. In doing so, they inevitably ingest a fair quantity of it and, being so desiccated, it expands when it encounters the moist environment of the gut and can cause impaction.

A new type of floor covering which I have yet to try since I am unable to buy it in the large quantities I would need, is a relatively new type of cat litter made from recycled newspapers and sold under brand names such as Biocatlet™. This is clean, easy to use, and does not seem to be detrimental to the animals' health like some of the more traditional substrates.

Suitable bedding materials include strips of tissue paper, straw or soft meadow hay.

Accessories

Multimammate Rats are active animals and cages or tanks should be well furnished to reflect this. Provide an array of plastic pipes, as well as a latticework of branches and twigs to climb. Nest boxes, as mentioned above, are very important. I use wooden boxes, replacing them periodically when they become too dirty or gnawed. I am aware that many keepers have something of a "downer" on anything made of wood, citing the fact that wood absorbs smells and urine and can never be cleaned really thoroughly. Houses made of plastic, we are told, are much more hygienic. I prefer wood for precisely the same reason that the new vanguard dislikes it, because it does absorb the animals' natural smells. Rodents are olfactory beasts. Sight is not so important as the sense of smell, and a nest box has to smell of its own previous presence within it or the animal can get quite distressed.

I no longer use wheels for long-tailed rodents, even though some do undoubtedly enjoy the experience. The tail of a Multimammate Rat is very delicate and easily damaged and there is a risk that the animal will get its tail trapped in the spokes with disastrous consequences. The old-fashioned, metal wheels are the worst culprits, but the new designs in plastic are not much better. If you do provide a wheel, choose a solid rather than an open-spoked one, or weave a long strip of card between the spokes to render it safer.

Cleaning Out

How often one should clean out the animals depends on the size of the cage and the number of occupants. Once or twice a week is normally sufficient.

Feeding

Omnivorous. In the wild, seeds and fruits comprise the bulk of its diet, but insects are also taken, as are human leftovers. Multimammate Rats are partial to vegetables and grain crops and can become a serious pest in agricultural areas. In East Africa, for example, it gorges itself on maize from April to May, and then transfers its attention to sorghum in mid-summer before moving onto cotton crops in early autumn. In captivity a good rodent mixture forms the basis of its diet, to which can be added slices of apple, banana, carrot, pear, etc. Cabbage and broccoli leaves are especially relished.

Health

All illnesses are potentially serious, so at the first sign of trouble you should consult your veterinary surgeon without delay. Any sign of diarrhoea must be investigated immediately as dehydration occurs rapidly. These animals are very susceptible to various cancers. Some, such as squamous cell carcinomas, are associated with the papilloma virus; others are almost certainly hereditary in origin. Stomach cancer in particular is very common. Pet animals, being descended from those unfortunates bred for biomedical research, are genetically predisposed to tumours. Usually cancers affect older animals but it is not uncommon for younger animals to be affected too. All my original animals eventually succumbed to what I can only describe as suppurating "warts" all over their bodies. These tumours, which grew with frightening rapidity, were diagnosed by a vet, who performed a biopsy on one of the animals, as being caused by a virus. I accepted that prognosis until I learned that specimens I had sold to another rodentologist over one year before had developed similar "warts" at exactly the same time, even though the two groups had had no contact with each other for that length of time. This suggests that a virus was unlikely to have been the causal agent. It is probable that the cause in this case was hereditary. There was nothing I could do for them and it was with regret that I had all the animals euthanased, except for one pair which at that time did not appear to be affected, although in due course both these animals also developed the same ugly "wart-like" growths. I have since obtained a new colony from a different source. They are all young animals but it will be interesting to see if they too fall victim to these tumours as they get older.

They are prone to carcinoid tumours and are the most susceptible rodent to osteoarthritis.

Reproduction

One of the most fecund of African mammals, the Multimammate Rat breeds prolifically when conditions are favourable in its natural habitat, occasionally reaching plague proportions when hundreds or even thousands of individuals may be seen at a time. It breeds throughout the year, but breeding peaks towards the end of the rainy season and the beginning of the dry season when vegetation is at its most luxuriant and food most easily attainable. With a species that is this prolific, captive populations can soon rise to an unmanageable level if homes cannot be found promptly for the offspring.

Females have 8-12 pairs of teats, rather than the usual 5 or 6 pairs possessed by other small rodents, but as many as 16-18 pairs are not unknown. This is, at first, surprising because litters are hardly ever that big. The average litter consists of about 8 babies. The explanation is possibly because females practise the 'aunty' system, in which species the nursing instinct is so strong that they will temporarily 'adopt' babies which are not their own. Spiny Mice also indulge in this curious, altruistic behaviour. However, unlike Multimammate Rats, Spiny Mice are not equipped with extra teats, but then there is no need because Spiny Mice produce fewer babies.

The oestrus cycle is 7-8 days. Female Multimammate Rats produce two or more litters per season. In common with many other rodents, they experience a postpartum oestrus and males remain with the females after the babies are born, often standing guard at the entrance to the burrow.

Gestation

Usually 23 days, but it can be as little as 21 or as long as 26 days.

Young

6-12 (usually about 8). Singletons are rare and, in these cases, it is not always certain whether just a single baby was born or whether the others had been destroyed at an early age. Litters can be much larger, although this is comparatively rare. The record is for 22 foetuses found in a single female. Birth weight is about 1.8g. At birth the young are blind and covered with sparse hairs. Eyes open at 14-16 days. They are weaned at 21-24 days, but remain in the nest for several more days even though the female may have another litter within 25 days of the first birth. Sexual maturity is attained at 3½ months.

Life Span

Males live up to 3 years; females to about 2 years old, but occasionally older animals have been recorded. The wide age discrepancy between the sexes is probably the result of stress imposed on the female during breeding activities.

By Russell Tofts