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Young adult does come into season every four or five days in summer (less often in colder weather): a doe on heat will leap about with her legs all stiff and her behind in the air, waggle her ears, and usually mount her cage-mates. Does who live together come on heat together.
Up to about 12 or 14 months does are generally much more highly-sexed than bucks: a doe on heat will mate with as many bucks as she can get, as often as they can manage - whereas many bucks have little or no libido. She will often pretend to run away: but if she finds the buck isn't chasing her she goes back and prods him until he does. Most older does aren't really all that interested: but a few retain an active libido into extreme old age.
The best age to mate a doe is from about 5 to 10 months: much younger than that and nursing will be a big strain on her system; much older and she may be sub-fertile. Most does older than about 15 months are sterile, especially if they are overweight and/or no longer appear to be coming into heat. However, some does can and will get pregnant even at 3 years old, though with such geriatric pregnancies the litter is likely to be small and there is a risk therefore that the individual babies will be so big that the doe has problems giving birth, and bleeds excessively and/or has a high proportion of still-born kits.
Many bucks can be mated successfully all their lives, from 7 weeks old until death, but some old bucks withdraw the testicles into the body, leaving nothing but a limp, empty furry pouch: such bucks are both sterile and completely uninterested in sex, and make safe companions for elderly does. If the testicles don't disappear then it's not safe to assume that even the most doddery old buck can't impregnate a doe, unless he's so far gone he literally can't get his leg over: however old he is, if you can still see his testicles he can still use them.
Some rats lose their sex-drive during the winter. Bear this in mind if you want to breed a doe who is nearing the end of her period of peak libido: if you put off breeding her for a few months and that takes you into late autumn, she may lose all interest until the following spring - by which point she may well be infertile anyway.
Despite their terrifying reproductive powers, many fancy rats are unable to breed for one reason or another. You occasionally see (or don't see) undescended testicles and sealed vaginas (called an "imperforate" doe, i.e. one without an opening): though be warned, imperforate does sometimes develop an opening later in life and suddenly turn up pregnant when you'd mistakenly assumed it was safe to leave them with a buck. It's also quite common for bucks to have no libido, and for does to be sterile even if their libido is sky-high.
Some of this is due to pheromones - hormones transmitted through the air from one individual to another. Dominant bucks produce pheromones which suppress the growth of their subordinates, and often suppress sexual desire as well. And the does have built-in birth-control: if a rat-population is too crowded, the build-up of pheromones causes pregnant does to re-absorb embryos rather than carrying them to term.
If you do want to breed a pair of rats you can put the doe in the buck's cage, if he lives alone (if you put her in with a group of bucks she'll probably mate all of them and you won't know who the father was), or put them together in neutral territory such as a holding cage: but if you put the buck into the doe's cage she may thump him. You can if you wish leave them together until just before the babies are due; but bear in mind that if you keep a buck, especially a dominant buck, away from his cage-mates for longer than a few days it may cause fights among the remaining bucks as they reorganize their power-structure, and more fights when he goes back in.
If you have rats of both sexes and you don't want to breed them, don't let them out together if there's the slightest chance of the doe being on heat; don't leave them together unsupervized because some does can over-ride their hormonal cycle and bring themselves into heat by main force of will; and don't keep the doe in a cage which has widely-spaced bars going down to near cage-floor-level, because there have been recorded cases where a does appears to have gotten pregnant through the bars.
Some inexperienced bucks get a bit confused and will e.g. mount the doe's shoulder or flank: the spirit is willing but the aim is weak. Usually, but not always, the doe will eventually manage to get him lined up right. Some does are nearly as bad. The late great Badger-the-Cat-Chaser, whose portrait adorns the top of this page, was never able to be bred from - even though she was very highly-sexed. Not only did she have a thing for cats but, as her intended mate's owner reported, the buck was unable to persuade her that it was her that was supposed to be underneath.
Rats have "induced ovulation", which means that even though the doe may be on heat she won't actually release any eggs until she has mated. A scientific study indicated that in order to be absolutely sure of ovulating does actually need to mate more times and more frequently than an individual buck is up to. This explains why does tend to fight each other in the presence of a buck, whereas bucks usually follow an on-heat doe in a fairly amicable, brotherly group: each doe wants several bucks all to herself, whereas a buck knows that if he waits the doe will work her way round to him eventually.
Some does, however, can and will become pregnant from a single mating - so don't take chances.
Norway rat semen sets into a solid off-white plug about an eighth of an inch wide in the doe's vagina. This plug is later expelled: some rat-breeders actually search for it on the floor of the cage as proof that mating has taken place. The purpose is presumably to seal the doe up so she can't successfully mate with another buck before she comes off heat; but if so it doesn't work very well, as does who are put in with several bucks quite often end up carrying a mixed litter of kits from two or more fathers.
Pregnancy lasts 21-24 days, usually 22½ days: although in rare cases delayed implantation (or possibly a pause in foetal development) can occur, extending the interval between mating and birth to as much as 28 days. If you decide to breed, take the buck out before the babies are due - some bucks will attack newborns, and in any case the doe comes on heat as soon as her babies are born (called post-partum strus) and can and will get pregnant again immediately.
About a day before the babies are due the doe will usually begin frantic nest-making: make sure she has a deep secure nest-box the babies can't just fall out of, and plenty of soft clean bedding such as soft meadow hay, shredded paper etc.. I generally use a wire-grid floor with cat-grit under it in my rats' nest-boxes, as this soaks up any urine and keeps it away from the bedding and their fur, but if you use this system for a nursing doe the mesh must be no more than ¼"-square - new-born babies can fall through wider mesh and get stuck.
Some does don't begin nesting, or taking much interest in the babies, until they get a hormone surge a few hours after the birth. Less extreme cases build a preliminary nest, and then make a better one once their hormones really kick in. A few are so hormone-happy they get a bit addled and start making nests out of totally inappropriate materials, such as china dishes and chicken bones.
Litter-size ranges from 2-24, normally 8-12. Most rats make good mothers, competent and attentive, but some first-time mothers take a few hours to work out how to feed their babies. Once babies are born, give feed supplements to help her produce milk - Complan, vitamin-drops etc. - and make sure she gets a lot of calcium to prevent eclampsia. If the doe is feeding the babies properly you will usually be able to see a whitish area under the skin on their abdomens: this is milk in the stomach.
If you can see that the babies have no milk in their stomachs even after about 12 hours, then the doe may not be producing enough milk and you may have to foster or hand-feed them (see below). The first time I ever tried to breed a rat I was new to them and imagined, wrongly, that the doe would eat her young if disturbed. So I didn't check: and in fact the pregnancy had gone disastrously wrong. The dam, Fan, had only "fired" one horn of her uterus and still had babies in the other horn: she had no milk, so the babies that were born all died, and she herself developed a uterine infection due to the dead babies inside her, had to have a long and complex hysterectomy, and died of post-operative complications. If I'd known to check that she was in milk I could at least have saved her babies: and I would have realized earlier that something had gone horribly wrong, and Fan herself might have survived if she'd had surgery sooner.
[I should point out that this sort of disaster is very rare. The vast majority of Norway rat births are as trouble-free as shelling peas.]
Babies are born blind and naked and pink, but dark markings (if any) are visible at 3-4 days and fur grows in fully by 10 days. The rate of development varies slightly depending on how big the litter is and how good a milker the dam is: generally speaking the growth of babies from small litters is a few days in advance of that of babies from large litters.
At about 10-14 days their eyes begin to open and by 14-17 days they leave the nest and become very active and playful. They are partially weaned at 4 weeks, fully weaned by about 5 weeks. Their baby-fur is fluffier and duller in colour than their adult coats: they moult into their first adult coat at 6-8 weeks, beginning with the fur along the spine and on the top of the head. The Whispering Wise Rattery in Vancouver has a Watch Me Grow page which charts the developmental stages of a pretty little buck called Reasoning Rio, starting from 7 days old.
The British call baby rats "kits" or "kittens": Americans call them "pups" or "puppies". Which is more appropriate depends on the age of the babies: up to about 3 weeks they are bumbling little things with short noses and short tails, very much like tiny Labrador puppies, and thereafter they get rangier and leggier and start skipping about with their legs all stiff, wrestling and scrobbling each other - exactly like a mob of tiny cat-kittens.
If you disturb a mother fancy rat with young babies she is very unlikely to eat them - but she may have a go at eating you. Some mother-rats are perfectly amiable and will even bring their babies out and show them to their human friends: but some become really savage until the babies are weaned.
Some highly-strung does get fits of moving their babies around from one location to another: but as the babies grow up they start fighting back, pouring out past their mother and back into the previous nest.
It occasionally happens that as the doe climbs out of the nest a baby will be carried out with her, clinging to a nipple. Usually the baby squeaks and the doe realizes what's happened and bundles it back into the nest: but it can happen that a full-fed baby drops off the nipple and off to sleep without any noise, and the doe doesn't notice. Once the kits are fully furred this doesn't matter, but if they are still naked or peach-fuzzed then in such a case there's a risk the baby may die of exposure: so nursing does should be kept in a warm room, and if you find any babies lying in the middle of the floor pick them up and pass them to the doe - who will probably reach out of the nest and grab them from you, glaring at you the while.
Most does like small babies - any and all babies. If you have two or more does with litters in the same cage, or does without babies in the same cage as one with a litter, they are very unlikely to hurt each other's kittens deliberately. However, small kits may be injured accidentally because does like babies so much they steal them from each other and fight over them: so it is safer to separate nursing mothers from other does until the babies are out of the nest and agile enough to take evasive action, at about 2½ weeks.
In large litters it is not uncommon for one or two babies to die due to some congenital defect, and in such a case the doe may eat the bodies to dispose of them. Does won't actually kill sickly babies: but despite their strong maternal feelings, some does can be ruthless about getting rid of them.
I once played host to a nursing doe called Antoinette who had nine apparently perfectly healthy small babies, but from about three weeks old, and for no reason that I could see, she kept carrying three of them away from the nest and dumping them. I of course scooped them up, put them back in the nest and made sure they got fed: but from about eight weeks old it became clear all three had major metabolic problems. One little doe survived but stopped growing, and the other two, a buck and a doe, were unable to digest their food properly and died of malnutrition - despite being put on a special easily-digestible diet, vitamin supplements etc.. It looked as though Antoinette wouldn't actually kill them, and was prepared to take them to the point were they were partially weaned and had some small chance of surviving on their own if they were fit to; but she wasn't prepared to spend any more energy on babies who she could smell were an unsafe investment.
The doe has 12 nipples, although really only 8 breasts. In front of her fore-legs and just behind/between her hind-legs are pairs of breasts, each with a single nipple. Behind her fore-legs and in front of her hind-legs are pairs of larger breasts with two nipples: I don't know if there's a separate mammary gland for each nipple and they are just very close together, or whether these double-nippled breasts only contain one mammary gland. For purposes of "going lumpy" they behave as if they have only one gland, so a doe can develop a maximum of eight mammary tumours.
If there are more than 12 babies then the weaker ones will have to queue up to get their turn at the milk-bar: and even with only 10 or 11 babies it takes a lot of pushing and shoving to find a place. This can present problems if there is a runt in the litter (see below).
If a doe dies, or has no milk or too many babies to cope with, it is perfectly possible to foster babies from one nursing doe to another. Take the new foster-mother out of her cage for a bit, rub the fosterlings in dirty bedding from their new mother's nest, and then leave them in the nest for half an hour before putting the doe back in.
If there is a runt in a fairly large litter then, even if the doe seems to be coping, it may be advisable to take the runt away and foster it into a smaller litter, where it will be able to get at the milk-bar without a struggle - or failing that take it out once or twice a day and give it a hand-feed (see below) to supplement what it's getting from the mother.
If a doe dies or dries and no foster-mother is available, you can attempt to hand-feed. The babies will need to be fed every three or four hours from some sort of dropper. I personally would use a syringe with a wide-bore needle which has had the point cut off and then been filed smooth: in this case it is very important to make sure the milk is flowing freely through the needle/nozzle, as a blockage can cause the needle to be shot off the end of the syringe with some force.
Puppy-milk substitute is probably OK. Alternatively, some years ago a vet gave me a recipe for DIY rat-milk. I no longer have a list of the exact proportions, but it was something like a pint of milk, half a pint of single cream, the yolk of an egg, a spoonful (don't know how big, so compromize on a dessert spoon) of extra-virgin olive oil and a few drops of Abidec (vitamin supplement sold for human babies).
Once their eyes open you can offer them some solid food, especially soft food such as human baby-food, and once they are taking it they will need less milk.
Until they are old enough to be running about and grooming themselves, you should massage the babies' stomachs gently with a bit of warm damp cottonwool or similar after every feed, to encourage them to defecate, and then wipe their behinds when they do so.
The babies will need to be kept very warm, at least until they are running about. Being in a warm room isn't enough: they will need to be under an infra-red lamp or on a heat-pad or fresh hot-water bottle. Arrange their box so they have a cooler end - say two-thirds on the heat-pad and a third off it but still close to it - so they have somewhere to go if they find the pad too hot.
If the doe is still alive but just not producing milk, you don't have to worry about making a nest, rubbing the babies' stomachs etc.: you can take them out for their feed and then pass them back to their mum for her to do all the rest. If the doe has some milk, but not enough - e.g. if she's just not a very good milker - or has had 24 babies! - you only need to take the babies out for one or two extra feeds a day, and can let the doe do the rest.
Rat kittens should be handled regularly: say once every day or couple of days from about a week old (if the doe will allow it), increasing to a couple of times a day by the time they are about 4 weeks old, to get them socialized towards humans. Handling should ideally start before their eyes open if the doe will let you: initially it is probably enough just to put your hand in the nest and let them smell you rather than picking them up.
Gorgeous though they are, resist the urge to pick them up every 10 minutes. Like baby cats and dogs, they need some space to themselves. Of all my rats, the baby I handled most - because he had very promising variegation and I kept examining him to see how his markings were developing - was Thorn: who grew up to be the most ferocious rat I've ever met. He once chased my mother onto the kitchen table and climbed up after her.
From about 3 to 10 weeks baby rats are extremely active and should be given lots of space and toys to play with. The clip-together "play-cubes" sold for hamsters are particularly appreciated, especially if several are slotted together to make an elaborate rat-castle.
Baby rats shouldn't be sent to a pet-shop before 6 weeks. Ideally they should be left with their mother until 5 or 6 weeks, but can be removed at 4 weeks if e.g. the mother is ill. By the time they are 4 weeks old the doe will often appreciate being let out for a run every night, to give her a break from being bounced on. In the wild, after all, she could go several fields away if they started to get on her nerves.
Buck-kits must be removed from their mother by 7 weeks, as they are then fertile and will get their mother and sisters pregnant. Doe-kits are fertile at 5 weeks and must then be kept away from all males aged 7 weeks or over.
Doe-kits whom you are going to keep can of course be left with their mother all their lives. Buck-kits can be put in with their father or other adult buck as soon as they leave their mother, provided the cage bars are close enough to hold them: see under Social Behaviour for advice on how to introduce babies to adults.
It is easiest to sex babies when their fur is just growing in - does have nipple spots, males do not. Once they are fully furred it is hard to see the nipple-spots, and females have a projection called the genital papilla which looks superficially like a penis: but by the time you need to separate them the bucks' testicles will have dropped and it will be easy to see which are the males. If there's one thing you can say for sure about rats, it's that they've got plenty of balls...
Most female mammals do not ovulate - and therefore cannot conceive - while they are producing milk (this applies to humans as well provided they breast-feed the baby every few hours, including halfway through the night). Rats get round this by coming on heat - called post partum strus - as soon as their babies are born and before coming into milk. This way, the doe can be pregnant with a second litter while nursing the first one, and produce the second lot just as the first lot are weaning. This means that in theory, given enough food and space, a Norway doe can have a litter of - on average - 10 or 12 kits every 3½ weeks from 8 weeks old until death. However, conveyor-belt reproduction is a big strain on most does' systems, so it is advisable to rest the doe for a month or two between weaning a litter and mating again.
My vet tells me that several years ago he successfully put a rat doe on the pill. He can no longer absolutely swear to the dosage but he believes it was 2ml of Delvosterone, administered by injection every eight weeks. Unfortunately this not only stopped the rat getting pregnant but prevented her from coming on heat at all, so that she was not able to revel in safe sex - but it did enable her to continue to share a cage with her brother without creating a population bomb.
Some American authorities recommend routine spaying and castration of rats that aren't going to be bred from, claiming that this reduces the incidence of tumours, especially mammary tumours. It seems to me that what this means is that the rat is being subjected to an extremely invasive, expensive and potentially dangerous operation in order to reduce the risk that it will some day require a minimally invasive, cheap and safe one (though admittedly some does go for the whole set - and several lumpectomies can seriously damage your financial health).
Some people also believe castration reduces the incidence of kidney-failure in bucks, and thus significantly increases their average lifespan: but personally I am very dubious about this. I know someone who has had a large number of bucks castrated for aggression (because she has kept many hundreds of rats, rather than because an abnormally high proportion of her bucks are fierce!), and she has not observed any significant increase in lifespan among her castrated males: indeed, two of them died unusually young. I myself have only once had a buck castrated (because he was excessively over-sexed and kept mounting his brothers and getting beaten up for it), and he not only became very depressed but died young - of kidney-failure.
Personally, I would strongly recommend against castration, which is a major operation in the rat because the testes extend right into the body-cavity. It carries a small but not negligible risk of death, and is unlikely to have any significant health benefits (except possibly in strains with an excessively high incidence of severe kidney-failure), since bucks rarely develop tumours anyway and have almost no mammary tissue. It is likely to turn the rat into a bland slob, and in some cases it can also lead to chronic depression. Almost the only time it may be useful is in the case of very aggressive bucks: personally I wouldn't do it even then, but if you can't cope with a biter, or have small children in the house, castrating such a buck is certainly better than having him put down.
I wouldn't recommend routine spaying either, since it is a far worse operation than the simple removal of the tumours it is intended to prevent. But it may be worth doing in some American strains which have an excessively high incidence of aggressive mammary tumours arising in endocrine tissue (whereas most British rat mammary tumours arise in fatty tissue, and are hardly more difficult to remove than an abscess).
Neutering does of course mean that the animal can live with members of the opposite sex, and for some very fierce bucks who won't live with other males it may be a useful option. But you always have the alternative of putting an entire buck in with an un-spayed doe who is on the pill.
There is, however, one situation in which castration is strongly advisable: indeed, pretty-much mandatory. Bucks who are cryptorchid (having both testicles retained inside the body-cavity instead of descending into the scrotum) or monorchid (having only one testicle descended into the scrotum) need to have the undescended testicle(s) surgically removed, as if left they are likely to lead to intestinal complications which can prove fatal.
N.B. New varieties of fancy rat are being developed all the time. Because new varieties almost inevitably start off from a very small number of animals, for the first few years such varieties may have genetic problems due to inbreeding. It is also possible for a new variety to have a problem which cannot be bred out because it is a consequence of the same gene which makes the variety what it is.
One very pretty new variety which has become popular since the early 1990s is the coat-colour blue. British blue rats come in two shades: a dark slate-blue and a pale, definite blue-grey similar to the grey of an African Grey parrot: neither should be confused with the long-established brownish-grey colour mink. Unfortunately blue does from some of the paler lines have a greatly increased risk of haemorrhaging severely while giving birth - sometimes fatally.
Until such time as this problem is bred out (if it ever can be) it is advisable not to mate light-blue British does, unless you are an experienced breeder and very sure of your stock. It is still possible to produce this attractive colour safely, however, by mating a doe carrying blue - that is, a doe one of whose parents was blue, but who is not blue herself - to a blue buck or a blue-carrying buck. Any slight increase in haemorrhaging from such a blue-carrying doe not likely to be dangerous. On average half the kittens from a blue buck to blue-carrying doe mating will be blue, and a from a blue-carrier to blue-carrier mating a quarter will be blue (unless both parents are carrying albino genes, in which case the colour of some of the blue kittens may be masked: and bear in mind that an albino doe both of whose parents were blue or carrying blue may herself also be genetically blue and at risk of haemorrhaging).
In America there are also two shades of blue, a dark and a light, but these shades are due to different genes and it is not clear which if either matches the British blues. I have not heard of any haemorrhaging occurring in American blues.