Rat Breeding Articles

Rat Breeding Part 1: Biology

© Ann Storey MSc. FIBMS


Many people, who own animals, like the idea of breeding from them. However, when you breed it is your responsibility to to ensure that you only breed from those animals that are likely to produce fit, friendly offspring, that the adults (especially the doe) are fit to breed and that you are prepared to deal with all those extra mouths. Rats can have big litters of 14 or more and two rats can become 100 in under six months without control, and cute babies soon become big rats. You must never, ever release them into the wild. This is just about the worst thing that you can do. Most will end up as food for predators but the remainder will add to what is a serious problem. If it becomes known that these are released pet animals then this is the worst possible publicity for pet rats. No animal should be bred on a whim but after careful thought and research.

This is a general section giving the biology of rat reproduction and detailing what you should look for to help ensure healthy mothers and litters. Breeding for exhibition is dealt with under Part 2.

A rat can reach puberty as early as five weeks although six is more normal. At this age a doe is not physically mature enough to raise a litter so you need to keep her away from any bucks. It is important to remember that keeping them separate means just that. You should not allow them to run around together for even a few minutes as this can be enough for a successful mating to occur if she is on heat. If a very young doe does become pregnant by accident, then unless she is very big for her age it is better to take the litter away from her or at least leave her with only three or four babies. Some people advise spaying of does that become accidentally pregnant but this operation is riskier than allowing her to have the kittens.You would need to discuss it with your vet. Very young does rarely have problems giving birth, they are just too light to feed the babies properly.


Like other mammals, male rats are provided with two testicles which, in the rat, descend into the scrotal sacs from the abdomen at the age of four to five weeks. For another two weeks they are still liable to disappear upwards at times of stress. Testicles have to hang outside the body cavity because spermatozoa, which are produced in the testicles, do not develop as well if they are kept at body temperature. Sometimes one or both testicles remain inside the body cavity. These animals are called CRYPTORCHIDS. Those with both testicles undescended are either infertile or have much reduced fertility. As cryptorchidism is hereditary, these animals should not be bred from. Rat testicles are quite big and are sometimes mistaken for tumours by the uninitiated. The testicles are also a major producer of the hormone, testosterone, which is primarily responsible for the buck looking like a buck. Compared to the doe, bucks have a larger, heavier frame, their heads are broader, muzzles blunter and coats considerably coarser. Adult bucks also have a brown layer under the coat on the surface of the skin. This is caused by oxidation of sebum (buck grease) on the surface and is quite normal. Bucks take a long time to fully mature and can continue growing into their second year. By the time they are seven to eight months old they should weigh about 600g. This should not be fat however, but firm flesh. Fat bucks will probably not live longer than two years and in addition are prone to develop ulcerative pododermatis on the heels. Bucks chosen for stud must be fit and not have suffered respiratory or skin disease, tumours, fits, cataracts, maloccusion etc as these have a hereditary component. In addition they should be good natured without being a total wimp.

Bucks will breed from about six weeks of age although it is unusual for a buck of this age to mount his own mother successfully due to the difference in size. He will certainly mate with his sisters though. It is better not to breed with a buck before four months and even then make sure that the doe is either on heat and good natured as a bad tempered doe may injure or frighten him off. Research has shown that does much prefer to mate with a dominant buck and so young or timid bucks are less likely to sire a litter, especially if the doe herself is quite dominant. If you intend to keep your buck primarily as a pet, it is better not to breed with him more than two or three times and to make sure that there is a big gap inbetween times. This is because animals (not just rats but all male animals) used regularly at stud tend to become more assertive in themselves and more prone to bite. This is because they no longer see you as the boss but possibly as a rival or as one of their harem! All male rats tend to mark their territory with small drops of urine but this becomes more evident in breeding males. This urine contains a sticky molecule called lipocaliene. This molecule is interesting as it contains buck pheromones within it, which it releases slowly over a period of time.

Bucks used for breeding need no special diet. In fact, too much protein or too many calories can predispose to a buck getting kidney disease. Most bucks can handle as many does as you care to give them and regular use is more likely to increase their fertility than decrease it. There is no upper age limit on males. I have had many good posthumous litters from bucks over three years old and one from a buck over four. The only problem with using old bucks is the possibility of genetic mutations. These are more likely (believe it or not) to occur in old bucks than does and happen in the sperm producing cells in the testicles. The mutations occur because by the time the rat is old these cells have divided many times and errors in the DNA eventually creep in. (In does all the eggs have been formed before the doe is born and the cells the eggs originate from have divided far less often. For this reason it is believed to be rare for mutations to arise in does.) The mutation will not appear in the rat itself, nor in the offspring if the mutation is a recessive. However, if the offspring of this male are inbred there is a possibility of it cropping up down the line.


Does can become fertile from as young as five weeks. This is believed to be linked to body weight, the heavier the kitten, the earlier she will become fertile. If you are sharp eyed you will be able to notice that her vagina has opened. However, does are best bred from for the first time at about 16 to 18 weeks. A breeding doe should not weigh less than 300g. They should be in full coat (no moult) and completely fit. They should never have been ill at all. This means no snuffles, diarrhoea, ear disease or anything else. Even a case of spots or scabs is suspect. A fit rat has a glossy coat, looks like she could run a marathon and has a firm, lively feel when you pick her up. She should also be perfectly tame. It is important to realise that a doe at this age is not fully grown as, like the buck, a doe will continue growing until she is about a year old. Also, during lactation, does produce growth hormone from the pituitary gland, meaning that they get bigger with each litter.

Does are less likely to suffer complications if they have their first litter before eight months of age. Young does are more fertile (that is they get pregnant faster), have larger litters, and are less likely to have problems during delivery. Also if a rat proves to be an exceptional producer, then it is better that she has had the litter early as you have more time to breed from her again. Your doe should not be fat as then there is less chance of her getting pregnant, as the eggs sometimes cannot find their way into the oviducts if they are covered in fat. This is another reason for not waiting too long as older rats are more likely to be seriously overweight.Overweight does on second or subsequent litters can suffer from eclampsia and other complications. A good weight for a four month old doe who is ready to breed is about 300-400g. Rats lighter than this may become very underweight while nursing their litters. Rats who have had previous litters will probably be heavier than however due to the fact that as I have already said, does who have had previous litters grow bigger. Some very big does can weigh up to 500g and still not be overweight. Viewed from above, a fat rat looks like a potato, a rat who is the right weight like a cucumber. Try to use long bodied does for breeding as these have more room for the developing embryos and are more typy.

Does have a four to six day oestrus cycle. This means that every 4 to 6 days (av. 5) the doe has a number of mature eggs ready for release in her ovaries. If the doe is mated, these will be released about eight hours later. This is because, like rabbits and cats but unlike humans, the rat is a reflex ovulator. The doe at this time is said to be "on heat" or "in oestrus" and once you know what to look for the signs are unmistakable. While on heat and usually in response to touch or sometimes movement, she will jump about, freeze and vibrate her body - especially the head and neck region. If you examine her vulva you will notice that it is mauvish in colour and the mouth of the vagina will be gaping open. (When not in oestrus the area is pale pink in colour and the mouth is closed) This is when you should mate your doe. During oestrus, homosexual behaviour with other does in the cage is not uncommon and can be a way of spotting the doe on heat. Often if one doe is on heat in a cage then most of the others are too. This is called the "Whitten effect" although this phrase is normally applied to mice. Some researchers think it doesn't happen in rats but most rat breeders could tell them otherwise. Older does that are not been mated may not come in heat regularly. Sometimes housing them with the buck or putting some dirty bedding from his pen into theirs can stimulate normal oestrus, due probably to the presence of pheromones. Taking a doe to a show is another good way to induce oestrus although you should beware of mating does which have just come back from a show in case they have picked up an infection. During the late autumn and early winter many does may fail to go into oestrus at all. This is due to a rise in the levels of the hormone melatonin produced by the pineal gland in the brain. This occurs because of the shortened day length. Raised levels of melatonin can block oestrus by decreasing the production of gonadotrophic hormones by the pituitary gland. However, not all rats are affected, (young does under 6mths normally are not affected in my experience)and research with mice has suggested that it is possible to breed a strain that is not affected in any way. Work with rats has shown that if you use normal lighting turned on for twelve hours a day (you can do this using a simple timer available from a DIY shop) then the effects can be lessened. It is normal in research labs using rodents to use a 12 hour on /12 hour off lighting cycle. It is important to realise that not only are the does less likely to come on heat during the winter but if they do, the litters are normally smaller and the doe more likely to suffer from inertia and other complications. A lot of problems occur because the period between the end of November and middle of January is typically the "off" season for shows. Some breeders then take this opportunity to mate up their ageing winners, occasionally with fatal results. It is just bad luck that the end of October to the middle of January is the 'off' time for rats breeding as well.

Usually however, by the end of January rats are breeding again, although an exceptionally dark or cold winter may pospone this.

Just as the shortened daylight hours can affect fertility, so can the longer day length in summer. I have also found that fertility is affected if the summer is very hot.

Does become infertile at about eighteen months old, although a late litter can delay this by a couple of months. Rarely does over two years have been known to produce litters although this is not advisable, as they have been known to suffer strokes and similar problems.


As we have seen, the time to mate a doe is when she is in oestrus. Although some bucks will attempt to mount a doe who is not ready she will soon see him off. During oestrus, nearly all does will mate readily. Some people prefer to put the buck and doe together just while the doe is on heat. This means that both the buck and doe can go back with their cage mates the next day and that the bucks are unlikely to fight as they may do if separated longer. If you are going to do this it is better to put the buck and doe together in a small fresh cage (a show tank is fine) as then they are unlikely to feel territorial about it. Do not put the buck into the doe's cage as she may fight him. Most does come into heat in the mid to late afternoon and remain that way for about twelve hours. During this time the buck and doe will mate many times. It is important to realise that only about one in three mountings is successful and that a doe often needs to be mated several times before ovulation. It is also thought that the number of matings has a bearing on the number of eggs released. (but not one mating per egg!) Although some breeders only put their rats together while they are feeding their rats it is better to put them together all night. However, do not risk even a single mounting with the females you do not wish to breed from, as they of course will be the exceptions!

An alternative method is to put the doe or does you want mated into a cage with the buck and leave them there until they are obviously pregnant. The problem with this is the buck may not go back to living with other bucks without an enormous battle that could leave someone dead or badly injured. However, if you intend to use him frequently or he is by himself anyway this is not a problem. Another problem is that you will not be able to accurately judge when the litter is due. I do think however that people make too much fuss about this, and rushing a doe to the vet just because she is overdue with no other symptoms is pointless. The advantage is that you do not have to examine the does you want mated every night. If you are short of time you may forget to examine them on the very night they come into season!

For about twenty four hours after mating the doe's vagina will swell shut and you may see a white rubbery plug blocking it. This can be quite large and sometimes has bits of sawdust etc. sticking to it! Don't worry about this and it will disappear without any help from you.


If she is pregnant she will probably not come into season again, although this is not always so as I will explain later. Therefore if she does not come into season in the next ten days a litter is probably on its way unless her oestrus cycle has stopped or is irregular. Pseudopregnancies can occur in does mated to infertile bucks. These typically will last 17 days.

During pregnancy the doe should not be overfed, in fact providing she is having a good general diet you don't need to change it at all and her energy requirements are not significantly increased. A good diet is necessary however. Does who are not adequately fed may reabsorb their embryos or suffer problems at kindling or be unable to nurse their kittens adequately. Rats deficient in vitamin E may reabsorb their embryos but deficiency is unlikely as it is added to most ready prepared animal diets.

You should notice a sideways enlarging of the belly in front of the flanks sometime during the second, or occasionally the third week of pregnancy. The uterus of the rat is two pronged or horned, so that she can fit in more babies. The babies lie along the horns of the uterus like beads , one horn on either side of the belly. The size of the belly is not a good indication of the size of the litter as some does with a big belly only have a few kittens and vice versa. Some people like to try to count the embryos by palpating the doe's abdomen. Personally, I can't see much point in this as at worst you might do some damage to the delicate walls of the uterus or the embryos and at best it will make no difference to the outcome. It is unlikely that you will manage to count all the embryos so it will not tell you if any have been left behind. It is best not to let her climb or jump when she is heavily pregnant, so restrict her movements if she comes out. Once you are sure she is pregnant you should cage her by herself although in the winter it may be good idea to let two cage mates stay together. I prefer to put the doe in a smallish cage 45 cm x 60cm floor area, so that any babies getting out of the nest have not got to go on a route march to get back.

The gestation period is normally quoted as being 22 - 23 days. However this may vary from 21 to 28 days or even longer. The reason for normal pregnancies going longer than 23 days is usually due to delayed implantation of the embryos. When the eggs are fertilised, they then develop to a certain point but cannot develop further until they have implanted in the walls of the uterus. This implantation may be delayed for up to 10 days in my experience ( 7 days in the literature). This delay is commoner in older does. If your pregnant doe is overdue but is not showing any signs of being in labour and has no discharge, don't worry about her, she is probably not due yet. (If, during pregnancy your doe begins to bleed from the vagina or has a discharge you should get advice.)

Before giving birth most does start to build nests. Give them lots of bedding so that they can decide for themselves how big a nest they want. The size varies with the air temperature. The best bedding is hay, shredded paper or straw. Do not use cotton waste sometimes sold for hamsters as this will wrap itself around the babies and amputate toes etc! It is a good idea to clean your doe out two or three days before you think she is due as it is not a good idea to totally clean the cage out for ten to fourteen days after the birth. Do not use shavings or sawdust made from red cedar as this has been linked with high kitten mortality. There is no good evidence however that the normal white sawdust sold for pet bedding in the UK is harmful.

The next thing you may notice is a change in the doe's shape. The bulge, which up to now has been carried to the sides, suddenly drops down so that her sides are thinner but she will have distinct bulge underneath. This normally happens within 1 or two days of the birth. The next sign you are likely to notice is a staring coat and occasional twinges of the abdomen. This indicates that the doe is in labour.

The uterus of the rat does not have strong muscles like those of humans and she mostly relies on her abdominal muscles to expel the babies. Birth is usually preceeded by 1. the doe stretching, 2. violent contractions of the abdominal muscles, 3. doe sitting back on her heels and licking her genitals and maybe delivering a baby. Rats are born head or tail first, both are normal. The amniotic sac that each is born in is often ruptured during delivery and the doe will lick the rest of it off. At delivery the doe will eat the placenta of each baby in turn and at the same time the unbilical cord will be severed. Dead babies are either ignored or eaten. You can watch the doe giving birth but do not poke her about unless there is a problem. Afterwards make sure she has food and water but otherwise leave her to get on with it. A few hours later you should examine the litter to make sure the doe is feeding them. A healthy kitten will be lively, a good bright red colour and will have a large pale patch showing through the skin half way down the abdomen on the left. This indicates that the kitten has recently fed, the pale patch is the stomach. This is sometimes called the 'milk band'. Don't disturb the litter more than is necessary, but you should remove any dead or sick kittens. Sometimes does accidently bite kittens during delivery or when they are cleaning them, these bites usually heal without incident and it is not necessary to treat them. (Kittens are quite tough. They can survive severe chilling for several hours and can go without food for 24-48 hrs).

You should also check the doe's vulva. Some discharge is normal but heavy bleeding may indicate a problem that requires veterinary attention. Does sometimes bite their perineum during delivery, possibly in panic. These injuries often become infected and should be cleaned and treated with antiseptic or antibiotics if necessary.

The doe will often eat only sparingly on the first day, so make sure you give her something light but nourishing such as vitalin or porridge. She needs plenty of water as suckling her litter will make her thirsty.

Don't leave the buck with the doe as she will come on heat immediately after she has had her litter(post partum oestrus). It is not a good idea to mate her up now as it puts too much strain on the doe for her to be carrying one litter while feeding another. If a post partum mating occurs and is successful, the new litter is often delayed (probably due to late implantation), by a week or two.

Very rarely a doe who has had a litter will have another two to three weeks later even when the buck has been removed before the birth of the litter. This is due to SUPERFOETATION. This condition is common in some mammals (hares) but very rare in most. However it can happen in many species including humans. Basically, the hormonal system that is meant to suppress oestrus during pregnancy has not worked and has allowed the doe to come into season again. She has mated and the eggs have been fertilised. I have not been able to find out if, in the rat, the eggs have implanted but their development has been delayed or if they simply fail to implant until the first litter is born. Both systems operate in different species. Alternatively I have heard that in rats one litter is held in one horn of the uterus and the other in the second horn. I must repeat however, that I have known only a handful of cases where this has happened. It can be an explanation for the so called pregnancies that happen from rats "mating through the bars of the cage" something that I would have to see to believe!

A doe will have an average litter size of eight to twelve in a first litter and more in subsequent litters, until the age of fifteen months when the numbers will drop. Maximum litter size are in the mid twenties but twelve to fourteen is more normal. A doe has only twelve nipples so it is sensible not to let her rear more than twelve whatever your viewpoint on culling, I know that people will tell you how the doe will split the litter but in my opinion it is not good for her. Any small, sick, deformed or runty kittens should be culled. Breeding is not easy and not for the faint hearted.

Fur starts to grow from birth, which is why dark coloured rats have a greyish colour from the age of about two days. The coat is not really noticeable until the rat is about a week old. At ten days the kitten will be completely covered in a plush fur coat and resemble a young puppy ( the laboratory name is pups.) At fourteen days their eyes will be open and they will start to crawl around the cage. Encourage them to begin to eat by putting soft food such as mashed potato, soaked dog meal and bread and milk into the cage. Mother will also put food into the nest to encourage them. Rats transport their babies by carrying them by the scruff of the neck in their mouths. After about three weeks, however they get too heavy and boisterous to be carried around. Some mothers are quite poor at this but the babies have a good sense of smell and get back to the nest alright by themselves.

Rarely a doe will eat her litter, or parts of it. This may be because she has been disturbed, or because she has not got enough food, bedding or water. Normally this is done by first time does during the first few hours after birth. Occasionally a doe will turn on her litter when they are much older, three to four weeks old, and kill the lot. I've had this happen twice and can only assume it is hormonal, a type of post natal illness. In fact it is possible that most savaging and litter abandonment is due to hormone imbalance. If a doe does abandon her litter or appears badly frightened by the babies, you can sometimes encourage her to take the babies by putting them in her bed. If the babies manage to suckle, the doe sometimes changes her mind. This can take a day or two however. If she will not take them you can try handrearing them (using Lactol or better Esbilac from your vets) or foster them to another doe who has a litter of the same age. Handrearing is not often successful if the babies have had no milk at all from the doe. This is because for the first two or three days she produces a type of milk called colostrum which is rich in antibodies. Without these the kittens usually sicken and die. Fostering is easy providing you have another nursing doe. Take her out of the cage and pick up her babies. Mix the new babies in with them and put them back in the nest. Put her back and feed her. Most does take the newcomers without fuss. It is not necessary to rub them in the dirty end of the pen. Rats have a sense of smell orders of magnitude better than ours and are not fooled by this trick. The reason why they take strange babies is just because their maternal instinct is very strong at this stage. I normally just drop the babies into the nest and find that they are accepted without fuss.

If a doe fails to rear a litter the first time I would try once more. If however, she failed on the second attempt I would not try again. It is possible that there is a genetic reason for her lack of success and, if she did succeed finally, she may pass on her poor mothering.

With practice it is possible to sex a litter from birth. The way to tell bucks from does is that does have nipples and bucks do not and the distance between the anus and the top of the urethra is longer in bucks than in does.

Without going into a moral argument here if you are going to cull the litter it should not be done before four days of age. At this time her milk will have "come in" properly. If you cull earlier, she will produce less milk. It is quite true that mammals produce milk geared to demand. This is often used as a reason for not culling by some people. However, a large litter can take more calories from the doe than she can eat, leading to a thin doe and quite frequently weedy kittens. Also if you cull at four days, there will be an excess of milk for a short time. Kittens will usually respond to the excess by drinking more, thus keeping up milk production. Kittens from culled litters are bigger than those from unculled and there is less strain placed on the doe. If you are opposed to the culling of litters then you need to be extra careful in the way you raise the babies. During lactation make sure that the doe is always well fed and has plenty of fluids.

If you have not culled the litter then you need to feed a higher nutrient diet to the kittens than if you have culled them. Kittens from culled litters should not be fed in the same way as kittens from unculled litters or they will make fat adults and stand a higher chance of infertility.

My personal preference for feeding litters is 50% Alpha Herbal or other good quality rabbit mix and a 50% Vitalin mixed with cow's milk, table scraps and about 10% EMP.

You should separate the mother and litter at five weeks. If you leave them together any longer the mother will eat the rich food meant for the babies and get fat. Separate bucks from does at five weeks as you will have lots of unplanned pregnancies if you do not! Before weaning you should get the babies used to being handled. If you leave it later it will be harder. Even show rats need to be tractable. Tame rats grow faster than nervous ones. I would advise that all babies should be handled in the nest, after the age of four or five days.

Kittens from more than one litter, sexed of course, can be run on together when they have been split up, providing that they are not overcrowded. Watch for any signs of illness as this can be a problem in groups of rats.

The doe can be mated again when she has returned to full fitness, usually about a month later.

The general timetable of the litter (based on Donaldson) is as follows:

Gestation 22 days(normally)
Opening of ears 2-4 days
Incisors through 8-10 days
Genitals covered in fur 16 days
First molars through 19 days
Second molars through 21 days
Third molars through 35 days
Migration of testicles 21-41 days
Opening of vagina 42-72 days
Average lifespan 1000 days