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Bucks being very big and does very active, a rat of either sex needs a lot of space - a minimum of 3 square feet (e.g. 2' by 18"), with at least part of the cage being at least 10" high, to enable it to stand up on its hind legs. But several rats don't need much more space than one rat.
Rats overheat easily and are prone to chest-problems in stuffy conditions, so tanks are not a good idea unless very large and airy. Wire cages are generally better: they are messier, in that they allow some bedding to be kicked out, but they are healthier, lighter to move, easier to clean, easier to get the rats out of, easier to get food into and much easier to attach perches and other toys to.
Rats need a lot of water to drink, and should never be left in direct sunlight in warm weather. Tanks must not be left in direct sun in any weather at all.
¾" bars of the sort often seen on chinchilla or parrot cages will do to hold an adult rat, but babies can get through these, and need ¾" square mesh or ½" bars: the kind seen on hamster or finch cages.
Rats enjoy things they can sit on top of - the higher-up the better. A series of shelves breaking up a high cage is ideal.
Wire-mesh shelves which the rats have to step on as they get from place to place in the cage are not a good idea and should be replaced or covered with a smooth surface, as they are believed to increase the risk of pododermatitis, a spongy inflammation of the heels, especially in heavy, elderly bucks. However wire-floored shelves which are off the main route and are merely used for sitting on occasionally don't seem to cause problems.
Mesh shelves may also slightly increase the risk of boisterous young rats getting a foot caught in the bars and breaking a leg: this is probably especially true of square mesh, of the type unfortunately used in some of the best cages on the market, as there is less room for the rat to pull its foot free again than there is in the sort of mesh which has long oblong openings. A broken leg isn't the end of the world (see section on common injuries), as the affected animal nearly always makes a full recovery: but it does mean spending weeks being kept quiet and still at the time when they should be most rowdy and active.
Choose a cage with lots of doors giving easy access to every level: or cut new openings and fit doors made of wire mesh. Apart from problems with getting at dishes, cleaning the cage etc., very complex and hard-to-get-at cages make it difficult to handle your rats regularly, and they may become rather wild and wriggly as a result.
Cages should not be placed near curtains, or too close to the wall, as rats will chew both cloth and wallpaper if they can get at them. Also watch out for little hands - and teeth - grabbing your clothes and picking holes in them as you walk past. They will enjoy being given paper, cardboard and cloth to shred - a big piece of rag hung on the outside of the cage, so they have to wrestle with it to pull it through the bars, provides hours of harmless fun.
Rats like toys to play with. As they chew everything in reach, toys should either be very durable - plastic, metal and/or fairly hard wood - or very cheap e.g. cardboard boxes, loo-rolls, twigs. Offcuts of 4"-bore plastic pipe make ideal playthings, and most parrot toys are suitable, if rather expensive - especially the sort of rope and/or wood-block parrot toys which are designed to be slowly shredded. Some very destructive rats will demolish these in a few hours - but generally they last weeks or months. See section on rat merchandise for some suggestions.
They do not generally use wheels - though they may sit next to one and turn it idly with their hands. They enjoy climbing on perches: these should be at least ¾" wide, or they'll fall off.
Variety in the layout of a cage is at least as important as space. Try to provide alternative sleeping and climbing areas (so they can decide to sleep somewhere different tonight), and places where they can lurk if they want to get away from each other. Placing two or three shelves one above the other means they can go and sit on an empty shelf on their own and actually not see their cagemates for a bit.
Sawdust should not be used for floor-litter as it leads to sore eyes and snuffles. Wood-chip, shavings and flakes are OK, but should be as clean, dry and dust-free as you can get: dirty, dusty shavings are not only bad for the rats but can provoke allergies in humans. Also avoid wood-chips which are hard and spiky, as sometimes happens.
Some authorities in America consider that the use of wood-shavings is harmful to rats, since experiments have shown that they can cause identifiable changes in liver-function. However British rat expert Angela Horn, writing in issue #115 (January/February 2000) of Pro-Rat-a, the journal of the British National Fancy Rat Society, had this to say:
"...the shavings provoking worries in the USA were cedar and pine, because essential oils given off by these shavings had been found to cause changes in liver enzyme levels. ...there does not seem to be any hard evidence that this actually causes harm. ...a rat liver expert... explained that rats with raised levels of the enzymes in question would burn off fats more quickly. This meant that they did not put on as much weight as they otherwise would, which on balance means that they were likely to live longer. On the other hand, higher death rates have been recorded amongst baby rats reared on cedar, probably because they were not putting weight on quickly enough. However, all this is slightly academic... in the UK because neither cedar nor pine shavings are widely sold here. Shavings sold for pet litter here are almost exclusively spruce, which in studies has been shown to have little effect on animals compared to pine and cedar. Some rat owners prefer to avoid shavings for cage litter. I use Bio-Catolet recycled paper litter instead, as it is more absorbent."
Most rats like a nest-box but it must be quite big (such as the egg-shaped Rodi Igloo, which will sleep 2 or 3 bucks or 4 or 5 does, depending on how big and fat they are). If no commercial large box is available, use e.g. cardboard box, lunch-box with hole cut in, length of pipe.
Some rats like to have bedding in their nest box - others immediately throw it out, and in any case it's not as important as for, say, hamsters, because except in very cold weather Norway rats tend to flatten their bedding and use it as a matress rather than a blanket. However it's always worth offering bedding until you see how your rats feel about it, especially in cold weather. Does in particular often like to have the bedding put not into the nest-box but loose on the floor of the cage, so they can have the fun of gathering it up and making their own bed.
Soft hay, cloth rags or soft, shredded paper make good nesting material: as with wood-shavings, hay should be of good quality, soft rather than spiky and as dust-free as possible. Never use human-style cotton-wool, and avoid proprietary cotton-wool bedding sold for pets unless it is clearly described as short-fibre and safe, as some of these beddings contain long, indigestible fibres which can lead to cat-type fur-balls if swallowed - a much more dangerous condition for a rat than for a cat, since rats cannot vomit and so cannot sick them up (in the middle of the living-room floor waiting to be trodden in by early-morning bare feet) the way cats do.
Many rats have an unlovely habit of peeing in their own bed. Nest-boxes should include an absorbent layer in the base, otherwise the urine pools, gets on the rats' fur and makes them smell.
Personally I put a grid of ½" mesh in the box, turned down round the edges to make a rim so that the grid stands clear of the floor. Hay or other bedding goes on top of the grid: underneath it goes a layer of cat-litter, to soak up the urine as it runs off the bedding.
Many rats enjoy sleeping in a hammock - a cloth tube suspended from the cage roof by a chain at either end. The ones pet-shops sell for ferrets are great but expensive (especially bearing in mind how fast they get eaten): the sleeve off a discarded item of human clothing, slung up on chains from the local hardware shop, makes a very good substitute (this creative bit of recycling is courtesy of Dr Birgitta Edelman).
Some people build elaborate custom-designed cages out of sideboards, bookcases etc.. Some don't use cages at all, instead letting their rats live "free-range". If you choose to do this then you will have to keep your rats away from cables, cleaning fluids and similar things with which they might injure themselves, and make sure they can't get out of the house. Don't leave windows open unless you are quite sure the rats can't climb up to them and cats can't climb in. And you will have to accept that your home will acquire a slightly nibbled air.