[N.B. If your ship rat has any health problem not listed here, try the section on health-problems of the fancy rat.]

Ship rats are full-size at 6-8 months, but may continue to grow slowly throughout life, often with a growth spurt during the second year. They remain extremely playful until a year or more old.

Lifespan is generally 2 to 3 years, with a few individuals making it to 3½: though some ship rats descended from the inbred black strain maintained by the UK pest-control firm Rentokil, and by London Zoo, suffer from a greatly increased risk of stroke and heart-attack and normally die at around 15 months.

Ship rats do not tolerate cold well: they are at risk of becoming run-down and vulnerable to infection if the temperature is much below 65°F.

Aside from their sensitivity to cold ship rats are very hardy, and rarely suffer infections. Their very large noses make them vulnerable to mild bouts of sniffles, but these seem to be harmless. When they do have a cold the discharge is clear or white - not red as in Norway rats.

Rats and mice cannot vomit, so are very wary of being force-fed strange substances they can't get rid of: if they do need medicines these should be given by injection or disguised in something really strong-tasting like liver-paté, unless you are sure the stuff tastes OK. Because they cannot vomit they do not need to be starved before operations, and can and should be given water as soon as they come round, to prevent kidney-damage.

Ship rats are not nearly as prone to tumours as are Norway rats, but some older animals develop what seems to be a skin cancer on the forehead. This is untreatable.

Ship rats occasionally collapse, breathing shallowly and making an appallingly loud whooping, hooting noise, like a human baby with croup. This appears to be a Transient Ischaemic Attack (TIA) - a minor stroke which can come on in repeated spasms, and which may then go away without damage or may lead to a full stroke - which itself may kill the animal, or may just leave it with some degree of disability. As with Norway rats, some strains of ship rat are abnormally prone to heart-attacks and/or strokes, which they may or may not survive.

There is nothing you can do for an affected animal while it is actually having an attack, except keep it as quiet as possible: but if it does recover, a very small amount of aspirin every day may help prevent a recurrence. The easiest way to administer it is sprinkled in some liquid or semi-liquid food: it's no use putting it in water, as ship rats drink very little.

Elderly rats occasionally suffer strokes, and rats of all ages very occasionally develop an inner-ear infection which causes loss of balance resembling stroke. One way to distinguish them is that the acute phase of inner-ear infection only lasts about a day, after which the animal regains at least some of its bearings. It usually takes rather longer - say three or four days - to show noticeable improvement after a stroke. Also, in ship rats inner-ear infection can actually cause bleeding from the ear.

Inner-ear infection may be helped by a combination of antibiotic and steroids, but stroke is untreatable. Even without treatment, however, nearly all rats recover very well from either condition after a few weeks. Until they regain their balance, animals which are badly affected should be kept in a plain single-storey cage with nothing they can fall off or over.

During the acute phase they may lose the ability to chew and need to be fed liquid feed (Complan, Readybrek, baby-food etc.) from a dropper or needle-less syringe: this may also be the case with very old and/or dying rats.

Ship rats with the obsessive-compulsive disorder "looping" (see below) and who suffer stroke or inner-ear infection usually have to be put down, as they will still attempt to loop even when unable to stand, and become extremely distressed.

Like Norway rats, ship rats occasionally develop encephalitis, an infection of the central nervous system, especially in the humid weather of spring and autumn. This leads to progressive paralysis, usually accompanied by fits. The condition is always fatal, and progresses much faster than in Norway rats: a ship rat may die of this condition in two days, where an affected Norway rat might bumble on with fairly mild symptoms for weeks.

Rats need to gnaw or grind their teeth frequently in order to prevent their front teeth from over-growing. Some old rats, especially those who have had strokes, may permanently lose the power to chew properly: in this case they will need a soft diet (catfood, pasta, bread-and-milk, cooked oatmeal etc.) and will probably need their front teeth clipping about once a fortnight. Instructions on how to do this can be found in the dental-care section of the Norway rat part of this site.

If a rat is off its food or eating awkwardly, the first thing you should do is check the teeth for overgrowth.

All rats have rather delicate chests, so are not suitable pets for heavy smokers.

Very old ship rats, especially those of the "frugivorous" (white-bellied agouti) colour, tend to become emaciated and fragile-looking.

Ship rats from inbred strains may suffer from obsessive behaviour. In particular, animals derived from a strain maintained by the pest-control firm Rentokil in Britain often suffer from obsessive "looping". This is a condition in which the animal loops-the-loop, flipping from floor to wall to ceiling (upside-down) to wall to floor of its cage over and over, sometimes for hours.

It is common for healthy ship rats (and chipmunks) to loop: the abnormality here is in the obsession. If a rat stops between every couple of loops for a quick scratch, it's taking normal exercise: if it flips over and over without pause in a kind of trance it's a looper and shouldn't be bred from.

Being as flighty and variable as they are, ship rats can flit right out of life. They are not nearly as tenacious of life as are Norway rats - probably in part because they don't have such big fat-reserves to tide them over illness, and also because they don't have such an interest in food anyway, and so are more inclined just to stop eating. A sick ship rat may give up and decide to die despite all your efforts to persuade it otherwise, where a Norway rat would soldier on and recover.

All rats have some anatomical peculiarities which may baffle a vet who is only used to dogs and cats. They have a large, knobbly caecum which can be mistaken for an internal tumour. Males do not have even rudimentary nipples. Females have a sort of rudimentary penis called the genital papilla: they urinate through this, rather than through the vagina - so a prolapsed uterus can be snipped off from the outside and stitched at the vagina, without blocking the urethra.

There's another anatomical oddity which your vet should know about but many rat-lovers won't. Rats - and rodents and lagomorphs (rabbits etc.) generally - do not have lips running round the edge of their mouth in the way that most mammals do. Instead the upper lip folds in into a toothless gap called the diastema, behind the top front teeth.

If you open a rat's mouth you will find that there are folds of actual, hairy skin tucked inside the mouth behind the front teeth, covering the front part of the roof of the mouth. The function of these giant, in-folded lips is to create a strong skin barrier between the front teeth and the mouth proper - the tongue and throat - so that the animal can gnaw splintery, inedible and even slightly poisonous materials without the shreds falling onto the tongue and being swallowed.

The ship rat is of course the original "plague rat": but the plague was carried by rat fleas, not by the rat itself, and it killed the rat as well as humans. A healthy, cage-dwelling ship rat is not carrying any horrible diseases: nor is there any risk from wild ship rats unless they live in a country where bubonic plague is still very common. And even where it is common, plague is readily curable with antibiotics if caught early: so any continuing threat is due more to poor health-care than to the ship rat as such.

In any case we owe a huge debt to the "plague rat" who seems to be, bizarrely, the saviour of much of the human race. Many people think that The Black Death, the greatest pandemic of the Middle Ages, was not bubonic plague but anthrax: but even if The Black Death was bubonic plague, the tens of millions who died of plague throughout the Mediaeval period are as nothing to the numbers that would die if Aids were to take hold in the rest of the world as virulently as it has done in Africa.

Although they are not related (one being a virus and the other a bacillus), the latest research indicates that Aids and bubonic plague use very similar methods to hijack the body's immune system, and therefore resistance to one disease confers resistance to the other. Europeans and Asians - and their American and Australian descendants - are descended from populations that survived repeated major outbreaks of bubonic plague, and therefore have inherited considerable resistance to both plague and Aids. Africans, however, have never suffered a major bubonic plague pandemic - and are now having an Aids pandemic instead.