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Wild Norway rats can make perfectly good pets, at least if you get them young - and the normal way of ending up with a wild Norway rat is by your cat bringing you a baby one. They are shyer than domestic/fancy rats, with a coarser coat and (usually) much smaller eyes and ears: but not so shy as to make them completely unapproachable. They will generally be more excitable than a domestic rat, just as most wolves are more excitable than most dogs; but a hand-reared wild Norway rat is probably no more difficult to manage than a stroppy hamster - though of course their bite is proportionately worse. They are easy to keep clean, as they are more likely than domestic rats to establish a lavatory corner and use only that (I was going to say "and stick to it", but considering how gummy rat-urine is I decided that that was an unfortunate turn of phrase).
If you find yourself e.g. caring for an orphaned wild rat it should have a blood-test for Leptospirosis and be quarantined away from your domestic rats for several weeks, so that any infections and/or parasites it may be carrying can be identified and cured with appropriate medication. Until you know that the newcomer is not carrying Leptospirosis, you should not allow its urine to come into contact with any open cuts or grazes on your skin; and if it tests positive it will need a course of antibiotics.
If it is indeed a small baby which needs to be fostered, or at least kept company and played with, put a suitable nursing doe (plus litter of course) or a couple of playmates in with it and then keep them, as a group, away from your other rats. It's unlikely to have anything life-threatening, but quite likely to be carrying e.g. a skin disease; and this way you don't end up having to dip all your rats for ringworm or whatever.
The National Fancy Rat Society in Britain bans members who take in a wild rat from showing, or from selling kits through the NFRS, for six months - just to make absolutely certain the wild one won't introduce any exotic diseases into the domestic stock.
Extreme care needs to be exercized when introducing wild Norway rats to other rats, and such introductions need to be done slowly and preferably in circumstances which mean the wild rat has space to get away. They are much more sensitive to pheromones (air-born hormones) than domestic rats are, and there are recorded cases where wild Norway does have been introduced to strange bucks under laboratory conditions and the doe has actually died after a few hours; apparently of physiological shock caused by being exposed to the pheromones of a strange male and not being able to escape. In addition to all the usual precautions listed under social behaviour, I would suggest that you begin by exposing the wild rat just to a little of the other rat's soiled bedding and droppings for a few days, rather than to the rat itself, so that they get used to the other animal's smell before they meet it.
Wild Norway rats who live away from man's buildings often make their own: they are great burrowers and construct elaborate tunnel-systems. The Brown Rat by Graham Twigg describes these rat townships in detail, along with some interesting descriptions of wild rat behaviour. I was particularly struck by the fact that the phenomenon of the killer-doe is recorded from wild as well as pet rats. Bucks fight more frequently than does but their fighting is never to the death, at least within their own pack; whereas a small proportion of does will kill other rats quite casually, striking a single lethal blow to the neck, without warning. Twigg calls these killer-does assassins.
There are numerous legends associated with wild rats - although since Norway rats only arrived in Europe around the beginning of the 18thC, many of them probably really relate to the ship rat. Buck ship rats can get so hormone-happy they really will launch unprovoked attacks on males of other species; although if a screaming ship rat leaps straight at your face it is not, as legend has it, because he wants to tear your throat out, but because he wants to pee on your head in order to prove his superiority. Wild Norway rats do not make unprovoked attacks - but may run all over you, and possibly bite as they go, if you suddenly disturb them and you happen to be between them and the exit. If you are working in an area where there may be wild rats and you have to e.g. lift planks or similar which may have rats hiding under them, don't stand between the rats and the door, and turn whatever it is over slowly and gently. That way, the rats should scamper discreetly out by the door - rather than swarming up your leg.
There are a few gruesome tales about wild rats actually killing and eating humans, but it in all or most of the accounts that I've seen the unfortunate victim was handling food at the time, and was probably drenched in it. If there is any truth in these horror-stories they are probably the result of the rat's poor eyesight and reliance on smell: to a rat, a person covered in dough is a loaf until otherwise proven.
Other stories about wild rats usually concern their capacity for intelligent cooperation and planning - such as the oft'-repeated claim that wild rats will team up to carry eggs. The idea that whole packs of rats sometimes desert one area and move to another en masse is so established that there's a special name for it in Scots - a "rattan flitting".
The usually accepted wisdom nowadays is to say that such stories are mere fantasy, because domestic, laboratory rats do not behave in this way. However, this presupposes that these behaviours, if they existed, would be instinctive; and that lab. rats haven't been domesticated for long enough to have lost such instincts. I know someone who successfully fostered an orphaned litter of wood mice onto a pet mouse doe, and the babies all grew up behaving just like their foster mother - not just in being tame but in having lost the agility and skittishness of their blood family. This suggests that quite major aspects of rodent behaviour are learned, not instinctive.
We already know that many animals - wolves, lions, baboons, macaques - have local "cultures": learned behaviours which are specific to a particular pack, and have to be actively taught by older members to younger ones. So just because a given group of domestic rats, separated from their mother at five weeks and never given the opportunity to learn from older pack-members, don't know how to team up to carry an egg, that doesn't prove that no rats anywhere know how to do it.
Some of the legendary rat behaviours might also exist but be misunderstood. The "rattan flitting", for example, might be not so much a sudden mass migration as an exodus of surplus youngsters hoping to find territory elsewhere.
I am told by someone who has kept several wild rats that they are more socially responsible than the domestic strain, such that healthy wild Norway rats will bring food to sick members of their pack - rather than swiping food from them, as pet rats tend to do.
The reason wild rats can be a health-risk to humans is not because they are "dirty and diseased", but because they are very like humans in their metabolism and diet, and so get a lot of the same diseases. A feral cat may have fifteen infections of which you might be able to catch one if you are very unlucky: a wild rat may have three infections and you can catch two of them.
Many people call rats "dirty" because they see them rummaging around in rubbish, but it's humans who create the rubbish. The rats are just converting man's litter into tidy, bio-degradable rat-droppings: if they didn't it would lie around growing maggots, which turn into flies which are more likely to spread disease than rats are. And where wild rats do carry diseases communicable to humans, it must often be because they caught them from infected human waste in the first place. The main cause of disease is human overcrowding and poor hygeine, and the animal most likely to give nasty diseases to a human is another human.
Make sure wild rats or mice cannot get at the stored grain you use for your domestic rats - not only because they may spread infections, but because they may eat rat-poison and then pass it on in their urine.
If you have free-range wild rats in your house or garden and don't want to have, the first thing to do is to discourage them passively by cutting off their food-supply. Make sure all your food - and rubbish - and soap! - is securely covered at night. E.g. if you have a swing-top rubbish-bin put a heavy weight on it at night so it can't be swung, or tape it up. Close all food-cupboard-doors securely and make sure no little entrance-holes have been gnawed in the corners. One useful method is to keep food as far as possible on shelves and in cupboards which are quite high up the wall, and not leave anything lying around which could be used to climb up to them. Unlike ship rats, Norway rats can't run up vertical surfaces - though they can jump fairly high.
Secondly, discourage them actively by lacing the areas they frequent with things they don't like the smell of. Possible suggestions would include a thick layer of pepper, or eucalyptus oil.
You could also try claiming the territory from them by putting down the urine of a male human or an unneutered male cat or dog around the edges of the area you want to keep rat-free. In Britain garden-centres sell a product called Renardine (I think it's fox-pee - or smells like it) which is used to discourage animals (usually cats) from entering a garden without hurting them, so if you don't fancy personally urinating all round the garden you could try using that.
I've never actually tried doing this, but scent-marking in this way does discourage other scent and territory-conscious animals such as badgers and wolves, so it seems highly likely it would work with rats too. But be warned that if you put Renardine directly onto grass it will turn it yellow - and that's probably true of urine as well.
Once you are reasonably sure the rats have vacated your premises, crawl around your house on hands and knees checking along the skirting-boards and around all pipes looking for gaps, and block them. There are often quite large gaps where pipes enter the floor: these can be closed by making a little collar of hard wood or metal and screwing it down firmly around the pipe. Be on the lookout for gaps higher up as well - such as chunks of plaster fallen away behind skirting boards, leaving a gap that would allow something to climb up the back of the board and out over the top, and high-level gaps connecting water-pipes to cavities inside the walls anywhere from floor to ceiling. But don't seal the gaps until you are fairly sure the rats have left, or you could end up locking them into the house rather than out of it.
If you find you still have a few stragglers you can then live-trap them and release them outside: if you don't want them in your garden either then it's best to take them at least half a mile away, otherwise they'll probably make it home before you do. But watch out for nursing does (no testicles, and obvious, prominent pink nipples): if you catch a nursing mother you will have to let her go and put up with her, otherwise her babies will starve to death. Leave it for four weeks and then catch her again: her babies should be weaned by then, and you should be able to catch the babies as well before they have babies.
If you are releasing live-trapped rats into the wild, try to avoid doing so in an area you know already has a lot of rats, as they may be attacked and even killed by the resident pack. Better to release them in an area with few or no rats, where they have a better chance of carving out their own territory.
Suitable live-traps can be obtained from, for example, Havahart. Remember that if you are going to be able to check the trap less often than every twelve hours or so, you should attach a water-bottle to it: or if that isn't possible then along with the bait (peanut butter, a little meat or whatever) you should include some very moist food, such as grapes or cucumber or tomato.
If you have just the one wild rat in the garden, it's best to put up with it. Your one solitary rat will hold the territory and keep all others away: get rid of him or her and you may find twenty more have moved in. And a wild rat in the garden is just as interesting and pretty to watch as a squirrel.
They do of course have a distressing tendency to eat wild birds' eggs and even baby birds: but so does the red squirrel which everyone thinks is absolutely wonderful.
One method of control which has been tried successfully with mice, and would probably work with rats provided you can afford to keep it up, is actually to feed them - at a location convenient to you. If you put out food every night at the far end of the garden, then provided it's a fairly long garden they're unlikely to bother coming up to the house.
Because of their awe-inspiring reproductive capacity, wild rats do have to be controlled. In most cases this is best done by tidying up human rubbish and storing food in secure containers: but since they also take food from the fields it is desirable to have some direct way of limiting their numbers, especially in poorer countries.
Ideally this could and should be done by non-lethal means: for example, the rat-pheromones which naturally cause does to re-absorb their litters when a colony becomes overcrowded could probably be synthesized and used to reduce the breeding-rate of wild populations. Even if no non-lethal method is available and it becomes genuinely neccessary to kill wild rats, it should be done as humanely as possible. Unfortunately because wild rats have a bad press most people still think it is OK to slaughter them, and to do so without regard to their suffering; when they probably wouldn't dream of using inhumane methods to control wild dogs even if it did become neccessary to kill them.
Adlai Stevenson defined a free society as one where it is safe to be unpopular. Until humanity accepts that just because an animal is inconvenient and doesn't have a fluffy tail that doesn't make it evil or deserving of ill-treatment, we will always be some sort of tyranny.
Killing wild rats is ineffective, anyway - a waste of time and energy. Their breeding strategy is to produce enormous numbers of young, on the understanding that most will die; so that if there is a sudden increase in food supply, or a sudden epidemic, there are always spare babies ready to move in. As fast as you kill rats, the surplus young from three fields over move into the vacated space. Limiting their population by saturating the area with rat-pheromones to make them think the territory was taken, or dosing the adults with contraceptives and then leaving them to hold their territory against outsiders until they die of old age or predation, would be more effective - as well as more humane.