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Not counting the tail, ship rats are slightly shorter than the average Norway rat, and much lighter in build. Their bodies are arched and muscular, like a 6" strip of hard rubber with feet at either end: but they lack the high, powerful rump and full, pear-shaped body of the average Norway rat.
Instead, their torso is much the same depth all the way along: and though their hind legs are quite long they have a crouching gait which keeps the back fairly level. This gives them an outline like a giant mouse - or a squirrel with a shaved tail. This squirrel-like conformation is no accident, as they are an arboreal species, designed to scamper and climb - rather than to bound, burrow and swim like their Norway cousins. Consequently they are astonishingly agile - they can run straight up embossed wallpaper, or gallop across a cage-roof at full tilt upside-down - but they cannot make the prodigious leaps of the active Norway rat.
The tail, if intact (which it often isn't), is slightly longer than the body and head combined: proportionately much longer and more slender than in the Norway rat, not much fatter at the base than it is halfway along, and generally with a brittle, twig-like appearance [whereas the tail of a Norway rat is generally fleshy and slightly floppy, thick at the base and - except in very hot countries - shorter than the combined length of head and body]. The ship rat tail is quite prehensile: they can't actually hand from it, spider-monkey style, but many individuals can twist their tail round branches and use it to brace themselves and support at least part of their weight.
The extremities of the Norway rat are usually pink or medium brown, though the tail may be quite dark in some black individuals. In ship rats (except in albino or fawn-type diluted animals) the tail and testicles, and usually the ear-margins and the soles of the hands and feet as well, are nearly always charcoal grey: though some individuals in the population at the rat-temple in Deshnok appear to have medium grey-brown tails, and bucks with very pale undersides may have purple rather than blackish testicles. The nose-leather, however, is always bright pink. Some individuals also have pink on the underside of the tail for the first inch or so, and I know of one Canadian buck with a pink stripe running all the way along under the tail.
The coat is generally sleaker and shinier than the coat of a wild Norway rat, but it has extremely long, harsh guard-hairs, which catch the light like porcupine quills. These long hairs mean that a ship rat's apparent size and outline change very much according to whether the coat is flat or puffed up.
The ears are usually much bigger than those of a Norway rat: fine and flimsy, with visible horizontal creases. When they are feeling doubtful about something, some ship rats actually fold their ears back along these creases, like little rain-hats.
It is said in text-books that the back of the ear in Norway rats is fairly hairy, whereas the back of a ship rat's ear is naked, apart from some fluff where it joins the skull. It's actually more complicated than that. Ship rats do have hairs on the backs of their ears, whilst the ears of Norway rats appear naked at first sight (unless the fur is in a colour which contrasts strongly with the skin). In fact, they probably have about the same density of hairs. But the hairs on the back of a ship rat's ears are extremely short - only about as long as the thickness of a human fingernail - and very fine, and would probably be invisible without magnification: although they can be seen in blow-ups of very high-resolution photographs. The hairs on the back of a Norway rat's ear are about an eighth of an inch long and, whilst seldom thick enough to make it obviously furry, are enough to give it a streaky look around the edge of the ear, like something painted with a coarse-bristled brush; as distinct from the velvety, airbrushed look of a ship rat's ear. [Similarly, it is said that human beings - Europeans, anyway - have roughly the same number and distribution of hairs as a chimpanzee: but so much shorter and finer that we appear near-naked rather than shaggy. Well, most of us do...]
The overall shape of the head is very different from that of the Norway rat. The average Norway rat has a short head which is broad and triangular when viewed from the top, but not very deep. The mouth is set well back, the cheek muscles are large and powerful, and the highest point of the skull is just behind the eyes.
In the average ship rat, by contrast, the skull is narrow and deep, like an axe-blade, and very long. The highest point is directly above the eyes, and the mouth comes quite far forward. In some animals the fur on the top of the head tends to stick up like a crew-cut, and where this meets the back-slanting fur on the muzzle and cheeks the change of direction forms a slight ridge along the top of the nose.
The muzzle is long and more oval than triangular, being much broader across the whiskers than where it joins the main box of the skull: seen from the top the head makes a sort of figure-8, widest across the cheeks and whiskers and with a narrower "waist" in between. The skull is hardly wider than the maximum width of the muzzle, though they fit plenty of brain-power in there - as well as a remarkably long and muscular tongue. The bulging muzzle causes the mouth to curve in a permanent smile. Some ship rats have a narrower-than-average muzzle, and in this case their heads, seen from above, look as if they've been in a pencil-sharpener.
There is, however, a certain amount of overlap: some ship rats have a comparatively broad skull, and some Norway rats have a needle nose. The most diagnostic features of a ship rat are the flimsy, naked ears, the skinny dark tail and the droppings, which are small, black and almost dry even when fresh.
The bite of a ship rat is much less severe than that of a Norway rat. This is partly because they don't have as much cheek-muscle, and partly because they are smaller. Also, the teeth are smaller even in proportion - delicate little pincers, rather than the industrial-strength wire-cutters of the Norway rat.
Ship rats come in a wider range of colours than most wild animals - or at least, unlike most wild animals they come in four common colours (plus several rare ones), rather than one common colour and a scattering of rarities. Their principal colours are black, steel (black-backed and grey-sided), white-bellied agouti and grey-bellied agouti, plus a lot of variations in shade.
The black and agouti animals used to be thought of as separate sub-species, so for historical reasons they have peculiar Latin names: black ship rats, which were thought of as the basic form, being called "rattus" and the white-bellied and grey-bellied agoutis respectively "frugivorous" and "alexandrinus". However, evidence from breeding captive colonies shows that all the colours are fully interfertile. All wild colonies seem to include at least some steel animals: and since steels have to have one black and one agouti gene at the same locus, that means that in all wild populations the black and agouti genes are present, and black and agouti animals are cross-breeding freely. The supposedly distinctly-coloured "sub-species" are just colonies in which a given colour is particularly common. Colonies of the "Asian Black Rat", a.k.a. Rattus rattus rattus, are mostly black but include a fair proportion of agoutis and other colours; colonies of the supposedly agouti "Alexandrine Rat" Rattus rattus alexandrinus include a minority of steel specimens and so on. Populations living in colder regions tend to have a higher percentage of black and steel individuals, while those in hotter regions are mostly agouti.
This unusual degree of variation seems to come about because steels are more vigorous than the other colours: selection pressure will therefore favour steels, which keeps both the black and agouti genes in play. Also, ship rats are not strongly selected for camouflage, as they are animals which rely more on speed than on stealth. It is worth noting that some northern European populations of the red squirrel, which has a very similar lifestyle, likewise include a high percentage of black individuals
In any case one would not expect either Norway or ship rats to split into separate species or sub-species in the way other animals do, because their habit of hitching a ride with humans means that their populations are constantly mixing, with little opportunity for genetic isolation and separate development. They do to some extent isolate themselves territorially, each family/line holding its own patch: but there is always likely to be more influx from outside than there is for most mammals, who cannot simply hop on a plane or train or a liner. Outside zoos, the nearest thing to genetically isolated populations are probably the two island colonies (Lundy and the Shiant Islands) in Britain, a few island populations elsewhere, and the 20,000-strong, centuries-old super-pack at the temple at Deshnok in north-west India. A small amount of racial variation has developed over the years - Lundy rats have unusually long under-fur, whilst many of the rats at Deshnok have paler tails than the ship rat norm - but they are a long way from forming separate species. These are mere racial or "breed" characteristics: the Lundy animals certainly are completely cross-fertile with other strains.
There are however some ship rat populations in Asia in which mutation has caused a change in karyotype (number and shape of chromosomes). These populations will eventually form separate species, if they are not so already: as they will at best have fertility problems and a high incidence of birth abnormalities when cross-breeding with other strains. Also, the Norway rat is believed to have evolved from an Asian population of Rattus rattus very recently - within the last 10,000 years. DNA tests show that most ship rats in North America are of Asian stock. There appears to be a group of animals (read, I've seen photographs of two bucks) around Vancouver, Canada who are definitely ship rats, in that they have naked-backed ears, skinny twig-like tails and small dry droppings, but who are very Norway rat-like in overall shape, colouring and to some extent behaviour; and I do wonder if these could be a remnant of the intermediate group which gave rise to the Norway rat.
Black individuals are much commoner among ship rats than in R. norvegicus populations, which may have something to do with the fact that the black colour is due to a different gene. The basic black ship rat was originally thought of as the type sub-species, and given the name Rattus rattus rattus. Black ship rats are therefore commonly referred to as "rattus" even though it is now known that this is just a colour gene. The underside in these animals is usually a lighter shade than the back - an even, unticked dark blue-grey.
For the same historical reasons, the ticked brown ship rat which in any other animal would be called "agouti" is known as "frugivorous" or "alexandrinus". "Frugivorous" animals have a white stomach and throat, while "alexandrinus" rats are grey or tawny underneath.
The top coat of frugivorous Rattus rattus which have been kept in Britain in recent years varies from a cold, greyish pale brown to bright ginger, but other shades of agouti are known to occur, including some much darker animals. Frugivorous ship rats tend to be gentler and more docile than darker colours, and of lighter build, and are more likely to become very thin in old age.
The rattus and frugivorous (i.e. black and agouti) genes are co-dominant: in combination they produce a third colour known as "steel". This gives a three-tone animal with a silvered black back, a slightly ticked pale-grey underside and brownish-grey sides. As with the frugivorous colour the precise shade varies: some steels have a whitish-grey underside, some more of a snuff-colour. Some animals also have a dilution factor which turns all the black hairs chocolate brown.
Steels are not really very steel-like in appearance: more like cast-iron. They were called "steels" by analogy with the colour already called steel in rabbits, which is genetically identical but a lot paler. Steel ship rats tend to be almost black as kittens, but some have white roaning spreading up the flanks from the outset. They become lighter as they get older, mainly due to an increase in this roaning/silvering: some end up so heavily silvered that they are almost pure white. Conversely a few are, and remain, very dark, making it hard to be sure whether you have a dark steel or a rusty black: but even dark steels should have a ticked oatmeal-grey belly, whereas the belly in black individuals is black or an even blue-grey. Steels are generally more physically vigorous than other colours - more muscular and active.
Historical accounts refer to an "Alexandrine" rat, a.k.a. Rattus rattus alexandrinus, described as brown with a grey belly. There are no "alexandrinus" in captivity in the UK, and it is not clear whether there are any in the wild or whether reports of R. r. alexandrinus from Lundy refer to diluted, chocolate steels - but true alexandrinus animals are certainly quite common in North American populations. The majority of the Indian R. rattus at the temple of Deshnok are also grey-bellied agoutis: although in their case some of them do look as if they are probably chocolate steels. It is normal for any ticked animal to be darkest along the centre of the back, but most steels - and some of the rats at Deshnok - have an obvious broad band along the sides which is somewhat darker than the belly but very much lighter than the back.
I have seen photographs of two tame Californian Rattus rattus alexandrinus does, and a buck from West Canada. One doe has a chocolate-agouti top-coat and a warm, brownish-grey underside and legs: a very pretty and extremely mouse-like colouration, resembling a ticked version of a chocolate-and-tan show mouse. The other has more red in her top-coat and a bright orange demarcation-line between her top and belly coats: this orange line is often seen in house/fancy mice but not in Norway rats. The buck - the same animal who has the pink stripe under his tail - has a grey-brown back and a ticked, pale oatmeal-coloured underside, very similar in colour to a standard wild Norway rat, but with a white V on his chest resembling the markings seen in some species of wood-mouse. Photographs of all three animals can be seen in the rAt Gallery. Those of the Deshnok alexandrinus that I have seen in close-up are either a very dull grey-brown or a bright ginger-agouti, with the belly-fur being a colder, greyer colour than the top-coat: in both cases the belly is not very much lighter in shade than the back.
Like the frugivorous colour, alexandrinus rats tend to be more amiable than black or steel individuals. It is likely that alexandrinus with black gives steel or something very like it, the same as frugivorous with black. The ship rats in the 10-20,000-strong colony at the temple at Deshnok seem to be mainly alexandrinus, but there are a very small number of black ones, some who look like true black-backed, grey-sided steels and quite a lot who appear to be diluted, chocolate steels (as well as a few in more exotic colours such as dove-grey - which may be black plus a dilution factor - and silver-fawn), although I have not noted any frugivorous individuals in photographs of the temple rats.
Black-eyed cream, light grey and albino and/or pink-eyed pale fawn ship rats are known to occur, but as far as I know there are none in captivity at present. Grizzled coats (a 50:50 mix of dark and light hairs) and cinnamons are also reported - though the latter may just be the already-described light, bright, gingerish variants of the frugivorous and alexandrinus coats.
Ship rats were actually shown by the embryonic British Rat Fancy prior to the First World War. It is reported that the colours being displayed included green rats; but because of the early date we have no colour images of these animals and don't know to what extent they really were green; and this semi-domesticated strain was lost during the war.
Sloths with greenish fur have algae growing in their coats, while the green kitten which was born in 1995 was tested to see if it had excessively high levels of copper. The green vervet monkey, Cercopithecus aethiops, just seems to be a grey-brown which gives a greenish impression, the way that blue fancy rats are grey, but a grey that gives a blue impression. Some squirrel monkeys also have a grey-green agouti coat, as does Dasyprocta azarae, the Green Agouti (an "agouti" in this context being a large South American Caviomorph rodent - if the Caviomorphs are rodents, which is currently in dispute - vaguely related to the guinea-pig: other animals with ticked coats are called "agoutis" because their coat resembles the coat of this sort of agouti): whilst the Green Acouchi, Myoprocta acouchy a.k.a. Myoprocta pratti, another South American Caviomorph, is a pretty definite olive-green. Ruby Cook's dark, greyish baby frugivorous buck Gooby certainly has a greenish look in photographs, and this may well be the colour that was shown before WWI.
The differences between agouti/frugivorous, black/rattus, steel and black-eyed cream ship rats are due to different alleles at the E (=Extension of black) locus. This is a gene which controls the width of the black band in the hairs of animals with a ticked coat. There are no known mutations at this locus in the Norway rat, but ship rats have at least two.
Agouti/frugivorous animals have the normal extension of black gene E, combined with either another E or the recessive exclusion of black gene e. Black/rattus animals are homozygous for the dominant extension of black gene ED, which causes the black band to spread and cover the entire hair, giving a dense blue-black coat. This is different in origin from the slightly brownish black coat which occurs in Norway rats, where the recessive "self" gene, aa on the A (=Agouti) locus eliminates ticking altogether.
In some small mammals ED is fully dominant to E, so that an animal with an EDE gene-pair comes out black: but in ship rats (and rabbits) the two genes are co-dominant, and such an animal comes out as a steel - with completely extended dense black hairs on the back, and agouti hairs with a wide black band on the sides and stomach.
Black-eyed cream ship rats are homozygous for the exclusion of black gene e, which removes black from the hairs without affecting the skin: so the coat is yellowish cream while the tail, ears, feet and genitals are the normal Rattus rattus charcoal grey. These attractive animals were found living wild in a grain-store in Britain and were bred at Edinburgh University during the 1920s (or thereabouts): but the strain unfortunately died out, leaving behind only black and white photographs which show the dark skin contrasting with the pale fur.
For anyone who wants to take it further, the main reference for ship rat genetics is Anthony G Searle's Comparative Genetics of Coat Colour in Mammals, pub. Logos Press, London & Academic Press, New York (1968). However, note that when this book was written the genetics of the steel ship rat were not known, and Searle assumes them to be super-dark alexandrinus.
According to Searle, frugivorous or white-bellied agouti ship rats have an allele Aw at the Agouti locus, dominant to normal grey-bellied agouti (alexandrinus) A, and there is a recessive non-agouti aa form of black coat, as well as the ED black.
In mice the Aw gene gives rise to either a white (called "fox") or a yellow (called "tan") belly, according to the presence of other modifiers, and this white or tan belly is visible with numerous different colours of top-coat. So why don't black or steel ship rats carrying the Aw gene have white bellies? It occurs to me that they may be white-bellied, genetically. The common ED form of black in ship rats is genetically different from black in most domestic animals - it's an extension of the black band in the ticked coat rather than - as it is in e.g. Norway rats - an absence of ticking. So maybe black or "rattus" ship rats with their sometimes black and sometimes blue-grey stomachs, and steels with their sometimes tan and sometimes light oatmeal stomachs, are actually showing the differing influences of the A (grey-bellied) or Aw (white-bellied) genes, but with a black band introduced to muddy and darken the colour.
White spots on head, chest and belly, and white fingers and toes, can be found with all colours. However the white bellies of frugivorous animals, and the white V on the chest of the Canadian alexandrinus buck described above, do not look like the result of a spotting gene. They are too precisely placed - in the same way that some breeds of dogs have exactly-placed ginger eyebrows and feet, but don't turn up with random patches and dribbles of ginger elsewhere.
White spotting genes, even the very regular kind which produce Dutch rabbits and hooded Norway rats, add patches of white on top of whatever coat-colour may be present, and are independent of any shading which accompanies that colour: so for example one may have a chocolate mouse shading to tan underneath, with "broken-marked" patches of white scattered about on both the chocolate and tan areas. Conversely the white belly of frugivorous ship rats, like the white belly of "marten" rabbits or "fox" mice, looks like a normal two-tone animal (dark back, pale belly) in which colour has been selectively removed from the underside, leaving a white area which exactly follows the normal division between back and belly coats. This type of white belly is not known to occur in fancy rats (though Searle believes it to exist in the wild Norway rat), except possibly in the case of the progressive-roaning type of "husky" markings.
Another obvious difference between ship and Norway rats is the smell. You do occasionally get a Norway rat with BO - a sweaty smell like unwashed socks - but it's an abnormality: on the whole even full-grown buck Rattus norvegicus just have a faint, quite pleasant smell rather like digestive biscuits; and I've even heard of one doe who smelt sweet and flowery, like white clover. Ship rats, on the other hand, have a bitter scent like wet wood-ash. Many individuals smell pretty strong: although those who live on their own are less likely to, as they don't have such a strong urge to scent-mark everything and their scent-glands are likely to be less active. [Since the two species are more closely-related than they look, I do know one Norway doe who has a faint but definite bitter, ship ratty smell.]
The other immediately noticeable differences are behavioural. Ship rats are far more flighty and frenetic than domesticated Norway rats, but extremely bold. They tend to rely on speed and agility more than stealth; and in fact selectively breed themselves for ever-increasing speed, since on-heat does like to make a group of bucks chase them at full pelt and then mate only with the fastest. Like ferrets and chinchillas, other than when eating they seem to have only two speeds: completely comatose, or ricocheting off the walls. Like chipmunks, many ship rats living in cages like to loop-the-loop, flipping upside-down onto the cage roof and then right-side-up again, repeatedly and at great speed.
They tear across the middle of the room (and your feet) instead of floating around the edges as nearly all Norway rats do, and make little attempt to conceal their whereabouts. A Norway rat who doesn't want to be caught squeezes into an improbably narrow crack and keeps quiet until you give up and go to bed: ship rats prefer to dance round you just out of range.
Ship rats move like smoke, like wind-blown leaves, like giant spiders drifting sidseways across the floor; like expensive special-effects fading out over here and reappearing instantaneously over there without going through the intervening space.
You can trace them by their breathing alone. This isn't a symptom of illness but a racial characteristic: when they are in exploratory mode they make a snuffling noise, almost as loud as a hedgehog's. And as they charge about under the furniture their long tails rattle audibly around the corners.
They are also usually incredibly noisy vocally - especially the does. Norway rats make few vocalizations within the range of human hearing - basically just hissing and squealing. Ship rats by contrast also squawk, bark, chatter, warble, quack like ducks and swear foully in what sounds like an excitable Eastern European language. They also tend to have a distressing fondness for things that rattle and twang. An American lady called Kerri Cawthon says of them "We had quite a few living in the jasmine bushes where I grew up. At night it sounded like a mad game of WWF Wrestling or a hocky game out in the back yard".
Ship rats will live in burrows if that's all that's available, but prefer treetops and similar high vantage-points. In America they are known as "roof rats" because of their habit of nesting high up in the rafters (in vast untidy heaps of twigs), whereas Norway rats are more likely to be found lurking in the basement.
They are generally much less interested in food than Norway rats are - and yet less easily put off it. Most Norway rats will not eat if they are nervous - but a ship rat will eat within 30 seconds of being caught from the wild. A ship rat will sit stolidly eating while screaming horrible threats at a rival out of the corner of its mouth. If you offer them something they don't fancy, however, they don't just ignore it - they snatch it from your fingers and then throw it over their shoulder and glare at you.
Young Norway rats roll and wrestle and pull each other's tails; young ship rats race around side-by-side slapping and pulling at each other and pushing each other off things. Norway rats usually spar by standing on their hind-legs and waltzing nose to nose; ship rats usually spar by standing on three legs, side-by-side, and kicking at each other (though Norway rats do sometimes kick-box, and ship rats sometimes waltz). Norway rats sleep piled up in a convivial heap on top of their bedding; ship rats sleep side by side, underneath their bedding. Norway rats curl up like a snail-shell, sitting on their haunches with their head tucked under their chest, or flop about at random; I have seen one of the Deshnok rats do the snail-shell trick, but ship rats more commonly curl on their side, nose to rump, like dogs.