Tails Blog

Rat Reproduction: Mating, Gestation, Birthing, and Growth

Steve Coppell - Friday, May 21, 2010




Rats are very easy to breed. In fact, many people end up with unplanned baby rats. Rats can reach maturity at 5 weeks of age, so the sexes should be separated prior to this age. Rats do not recognize incest, so brothers, sisters and even mothers and sons must be separated.

Rats do not have a breeding season, although very hot or cold temperatures will reduce breeding. Females of breeding age come into heat all year round, every 4 to 5 days, unless they are pregnant, and even then they may come into heat once or twice early in pregnancy. Each female usually has a regular schedule that can be marked on the calender, but it can vary. Each heat usually begins in the evening and lasts most of the night.

As a female approaches menopause at about 18 months of age, her cycle will become more irregular until it stops completely, and if she is bred during this time, the size of her litters will decrease as her fertility wanes. It is possible for a female who has stopped cycling to get pregnant, although the preganancy may not develope normally.

It is possible for domestic rats (Rattus norvegicus) to mate with roof rats (Ratus ratuss) although the babies will not survive. The embryos will be reabsorbed, aborted, or born dead.

A responsible decision

Before deciding to breed your rats, you should consider a few things. Do not breed rats who have, or who have had mycoplasma infections.Only breed rats who are free of respiratory symptoms and therefore hopefully resistant to mycoplasma. Second,rats have large litters average is 10-12 so if you do not plan to keep all the babies, you will have to decide how you will find homes for them.
If you have decided to breed your rats you must also consider their age. The best time to breed a female for the first time is at four to five months of age. It can be dangerous to breed a female older than six to eight months of age for the first time since her pelvic canal will be fused in a narrow position. This will put her a risk of being unable to deliver her babies normally. In such a case a cesarean section may be necessary to save her life. If a female of any age has a difficult birth, do not breed her again.

Age is not such an important consideration for males. Males can be fertile into old age. If you plan to breed a female a second time, it is a good idea to wait several weeks after her litter has been weaned to allow her time to recover, both physically and mentally.

The mating process

You can breed rats by simply puting a pair together for ten days, insuring they are together through two heat cycles. But since the female might fight the male, a better way is to put the pair together only when the female is in heat. This workes especially well when you are breeding your rat to one who belongs to someone else because the pair only have to be together for one evening. When a rat is in heat her vagina will gape open; otherwise it's tightly closed. Usually there will be behavioral signs too. Stroking her back will usually cause a rat in heat to perform the mating dance. She may first dart forward or spin around, then she will brace her legs stiffly, lift her head and tail, and vibrate her ears! This display tells the male that she is ready for mating.

Most males will be interested immediately and will sniff and perhaps lick her. When mounting he will grasp her scruff with his teeth. During courtship, mounting will occur numerous times, but most of this is foreplay. Usually the male must mount many times before completing the act, and mating will continue for sometime however, it is possible for a female to get pregnant from a single mounting,so do not let your girls and boys play together if you do not want them to mate. Even if the female is not in heat, a determined and persistant male can sometimes stimulate her into coming into heat, so keep your unneutered males and females seperate! Females in heat will sometimes also escape their cage to visit a male.

Planning for the birth

The gestation peroid is normally 22 days, but can vary from 21 to 23 (and rarely to 26). A post partum pregrancy will last 28 days. Two weeks into the pregnancy the mothers abdomen will usually start expanding, but not always. As the birth approaches, you may be able to see the pups moving inside her, or feel them if you gently feel her abdomen. her mammary glands will also start to enlarge two weeks into the pregnancy. The mothers needs are simple; a nutritious diet, exercise, and extra nesting material a few days before the expected event. If you've been letting the male live with the female you should remove him before the birth. The father would very rarely hurt his babies, but all females come back into heat within 24 hours of the birth. So if you leave them together she will immediately become pregnant again. If the pregnant female has been living with another female or a neutered male, it is alright to leave them together during the birth and the raising of the babies, as long as the cage is large enough to allow the mother privacy. However, it is not a good idea to leave two pregnant females together because although they will not intentionally hurt each others babies, they may steal them from each other. if this turns into a tug of war, the infants tender skin can be severly damaged by the females sharp teeth. Never put a new rat in with a pregnant or nursing female, because she will viciously attack it. The exception to this is babies about the same size as her own. A nursing mother will almost always adopt other babies, even babies of other species, allowing easy fostering.

Sometimes a pregnant of nursing rat has a change in personnality due to hormone changes. She may become more aggressive, or less interested in playing. In rat society, a mother rat is usually dominant over all other rats, even if she is usually submissive. However, when her job of child rearing is over, the mother will usually regain her former status and personnality. It is also common for a nursing mum to have soft stools.

The birth process

The birth process normally takes about  an hour or two. In general, the mother rat will deliver a new pup every five to ten minutes. In rats, the average litter size is 6 to 13 pups. The first sign is a bloody discharge from the vagina. Next, the contractions will cause her to stretch out while her sides suck in, in a most amazing way. Once the babies start arriving, the mother will sit up and help deliver them with her hands and teeth. Then she will clean off the birth sac and lick the new born. The mother will usually eat the placenta and the umbilical cord. During this process a healthy baby will wiggle and squeak, which inhibits its mother from eating it to. However, if a baby is weak is weak or dead this inhibition will not occur.

Most female rats are wonderful mothers, but rarely there can be problems. If the mother is stressed, either because of pain from a long difficult birth, or from enviromental disturbances such as unusually loud noises, etc, she may kill and eat some healthy babies. A poor diet may contribute to this problem. You can try removing the babies and giving them back to the mum when she calms down.

Difficult births

Birth in rats usually proceeds without need for assistance, but occassionally and especially with first time mums older than eight months, there will be problems. I have seen three rats that have died during birth and have heard of several others. The danger of an obstructive birth is that a mother can go into shock. A cesarean section may be possible if done soon enough.

Once the birth process begins, if no babies are delivered within two hours, there is defenately a problem. The rats uterus is shaped like a Y and a baby can get stuck across the bottom of the Y. Gently massaging the mothers abdomen may help reposition the problem baby. If a bay is stuck in the birth canal, it may be possible to lubricate it with baby oil and pull it out with forceps. Then the rest of the babies can be delivered normally. If the mother survives the birth but has retained one or more unborn fetusus, she may be able to expel or reabsorb them. in this case it is a good idea to treat her with anti biotics to prevent infection.

If the mother dies and leaves surviving babies, or if the mother refuses to nurse them the best chance for the babies is to foster them to another nursing mother.


Growth and weaning


At birth, the pups are born hairless, toothless and have short limbs and tails. They will start to have hair when they are seven days old, and their eyes generally open when they are thirteen to fourteen days old.

Most mum rats know just what to do and take great care of their rats. Occassionally, there will be a tiny runt who cannot compete with his siblings for the nipples especially in a large litter. You will be able to see if each baby has nursed by the white milk in their stomach which is visible through their thin skin. The best solution is to temperally seperate some of the other babies into another container to give the runt a chance at the nipples. Leave about four to five babies with the runt to stimulate the mum to suckle them. If the runt is all by himself, the mum may not pay attention to him. As long as the other babies are kept warm, there is no harm in them being away from the mum for up to four hours. You can put their container on a heating pad on low, or near a light bulb (be careful not to let them get to hot). Rotating the groups of babies with the mum every two to four hours will give the runt the best chance.

Baby rats grow incredibly fast I recommend you hold and look at them everyday to witness this miracle. This handling will also stimulate and socialize the babies. When they are two weeks of age you should play with the  babies as much as possible. The more you handle them, the better socialized they will be. At this age they will also start to eat solid food. They will either walk to the dish, or their mum will carry food to the nest. You do not need to provide special food for them. Babies can be weaned at four weeks but you can leave the girls with their mum for as long as you want. Remove boys before five weeks or they may breed with their mothers or sisters.