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Nearly all rats like the company of other rats, and should be kept in same-sex groups of two or more wherever possible. They spend much of their time grooming each other, and sleep piled up together in a convivial heap. A solitary rat will be very attached to you because it has no-one else, but if you are going to keep just one rat then you must be prepared to give it the same sort of intense attention you would give to a dog: and even so there are things such as social grooming which only a rat-friend can really provide.
Extremely large groups of rats can lead to social friction, whilst pairs occasionally fail to hit it off: the optimum number in a group is probably between three and eight individuals, either of the same sex or infertile (naturally or artificially: see section on reproduction). For anyone who wishes to keep just a small group of rats, I would recommend beginning with three or four rats of the same sex. When you get down to only two left, add two babies of the same sex. That way, you never end up with more than four, or with a rat left on its own, or putting just one baby in with no-one its own age to play with.
Rats frequently form strong friendships with particular packmates, and can become depressed if separated from them. Wherever possible, avoid splitting up rats who are getting on well.
By the same token, they sometimes develop strong animosities, especially between bucks, and it may become necessary to separate cagemates who really seem to loathe each other.
Bucks and does can nearly always be introduced together (if not then the aggressor is usually the doe), but will of course have kits unless they are sterile due to age or other cause (see reproduction). You can also introduce does of all ages together. It is rarely possible to introduce strange adult (i.e. much over 3 months) bucks together unless they are very old, but kitten bucks can be put with each other or, usually, with adult bucks. At a rough estimate I would say introducing young adult bucks to young adult bucks is only successful about 20% of the time, whereas the success-rate of other combinations is about 90%.
When adding new animals to a group, introduce them on neutral territory, e.g. in a carrying-box, and leave them together for some hours to get acquainted. Meanwhile, clean their cage thoroughly before they go back in.
When introducing new rats together, it can be helpful - especially with bucks - to dab the animals with something strong-smelling such as Vic or lavender water. This distracts them from the smell of the strange rat, and by the time the lavender smell has worn off they already half know the new animal and are less likely to attack it. In extreme cases, with very stroppy bucks, slipping them a mild sedative beforehand can also facilitate introductions. Your vet should be able to supply the sort of tranquilizer which is used to calm nervous cats and dogs before long journeys.
Rubbing a little of the existing group's dirty bedding over the new animal's fur may also facilitate the introduction-process by making the new animal smell vaguely familiar.
Does are usually very highly sexed and want to have several bucks each, so they fight over sex. Bucks don't need to fight over sex because they know the doe will work her way round to them: but they do fight over territory.
The younger a doe is, the more highly sexed and fertile she is. Therefore, established adult does will generally be more aggressive and resentful towards a strange young doe than towards a new adult, though they will accept her eventually. But see section on aggression for a word of warning about the occasional savage doe.
The bigger a buck is the more of a territorial threat he is - so bucks will usually accept a new male kitten fairly happily, but will rarely accept a strange adult buck unless they are all so old they've stopped caring. Bucks who are so old their testicles have retracted into the body can usually be introduced together without difficulty.
When you first introduce rats on neutral territory they are usually perfectly OK together, and may even ignore each other. Often it is only after a few hours that they begin to sniff and possibly fight each other. It seems they are prepared to ignore strangers who they think are "just passing through", but once they realize they are sticking around then the stranger looks like a challenger/invader and they have to sort out issues of territory and dominance. Very, very rarely a tiny minority of does can become really dangerous at this stage: see section on aggression.
It is usually better to put two kittens in with adults, rather than one - two kits play together, but one on its own will try to play with the adults, and annoy them.
New additions will be bullied and chased a bit at first, but this should only go on for a couple of days. If a resident animal seriously attacks a new one you may have to split them up.
Norway rats are very clean animals, and much of their social life is spent washing themselves or their pack-mates. The commonest way of washing someone else is to scrub their shoulders and the back of their neck - areas which are difficult to reach for oneself. Many rats solicit such grooming - slinking up to their companions in an ingratiating way and thrusting their head at them - and most go into a trance while being groomed, and will lie there with their eyes shut and a dozy expression on their face for quite a long time after the grooming-session has finished.
The self scrub-up routine is the same as for mice - washing the face with the hands; pulling bits of belly into range; waving one leg in the air - with the exception that mice wash their tails assiduously: whereas Norway rats like to let theirs get good and smelly and then waft them at each other.
The more dominant animals commonly groom their subordinates. In force-grooming, a subordinate animal is pinned down and scrubbed rather roughly; often being turned over onto their back first. Some rats resent this: some, especially socially-adept babies, flip over on their backs and do a full submission routine - and then flip upright again and go on with whatever they were doing as if nothing had happened.
Norway rats have scent-glands along their flanks. When fighting, bucks often rub their sides against each other to transfer their scent: they will also rub scent all over their cages and other objects, leaving a pale brown greasy smear. Some very possessive bucks also urinate on items they wish to own - including you.
Most Norway rats are "thigmotaxic", which means they like to be touching or close to a solid object such as a wall most of the time, and therefore tend to slide around the edges of the room and under the furniture rather than swaggering straight across the middle of the floor (as ship rats do). They combine this tendency to stick close to solid objects with their scent-marking behaviour, rubbing the scent-glands in their flanks against whatever they happen to be sticking close to. Then everything else sticks to it, too.
When Norway rats fight they blow up their fur, hump their backs and prance sideways at each other, making themselves look as big as possible. They then scratch at each other with a back foot and and try to rub the scent-glands on their sides against each other. They also often get into a clinch where they stand up on their hind-legs and waltz nose to nose, holding each other off with their hands.
Eventually somebody's nerve breaks - and then they get chased and lightly beaten-up.
A whistling squeal, like a thin high scream, is generally a sign of aggression rather than nervousness. It's equivalent to tomcats singing at each other.