The Rat Fan Club

Rat Reproduction

by Debbie “The Rat Lady” Ducommun

updated 12/27/13

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

Rats don’t 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 or nursing. Each female usually has a regular schedule that can be marked on the calendar, 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 pregnancy may not develop normally.  You can’t count on a female being completely infertile until she is 2 years old.

A Responsible Decision

Before deciding to breed your rats, you should consider a few things. Do not breed rats who have or have had active mycoplasma infections. You should only breed rats who are free of respiratory symptoms and therefore hopefully resistant to mycoplasma. Second, rats have large litters(average is 10 to 12) so if you don’t plan to keep all the babies, you’ll have to decide how you will find homes for them. Keep in mind that most pet shops sell 50-90% of their rats for reptile food. In my opinion, the only reason to breed rats is to produce healthy, well socialized pets with great temperaments.

Many responsible rat breeders don’t breed any female rat until she is at least a year old, and males until they are at least 1 ½ to 2 years old to make sure they are healthy and that the males have no tendency for aggression.  If you plan to breed a female a second time, it’s a good idea to wait several weeks after her litter has been weaned to allow her to recover, both physically and mentally!

The Mating Process

You can breed rats by simply putting a pair together for 10 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 works especially well if you're breeding your rat to one who belongs to someone else because the pair only has to be together for one evening.

When a rat is in heat her vagina will gape open; usually it is tightly closed. Usually there will be behavioral signs too. Stroking her back will usually cause a rat in heat to perform the mating “dance” which is quite interesting. 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 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 the 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 some time. However, it is possible for a female to get pregnant from a single mounting, so don’t let your girls and boys play together if you don't 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 separate! Females in heat will sometimes also escape their cage to visit a male.

Planning for the Birth

The gestation period is normally 22 days, but can vary from 21 to 23 (and maybe even to 25). Two weeks into the pregnancy the mother's 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. The mother's 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 never hurt his babies, but all females come back into heat within 24 hours of the birth (called the post partum estrus) so if you leave them together she would immediately become pregnant again.

If the pregnant female has been living with another female, or a neutered male, it’s all right 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's not a good idea to leave two pregnant females together because although they won't intentionally hurt each other’s babies, they may steal them from each other. If this turns into a tug-of-war, the infant’s tender skin can be severely damaged by the females’ sharp teeth. Never put a new rat in with a pregnant or nursing female, because she will viciously attack them.

Sometimes a pregnant or nursing rat has a change in personality from the 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 personality. It is also common for a nursing mom to have soft stools.

The Birth Process

The birth process normally takes about an hour or two. The first sign is a bloody discharge from the vagina. Next, the contractions will cause her to stretch out while her sides suck 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 newborn. Each baby is delivered attached to its own placenta. The mother will usually eat each placenta and the umbilical cord. During this process a healthy baby will wigggle and squeak, which inhibits its mother from eating it too. However, if a baby is weak or dead, this inhibition may 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 environmental disturbances such as unusual loud noises, etc. she may kill and partially eat some healthy babies. A poor diet may contribute to this problem. You can try removing the babies and giving them back to the mom once she settles down.

After the birth is completed and the mother settles down to nurse her litter, there is much less danger of her eating them. If you want to look at the babies, wait until she’s off her nest and first remove her from the cage. Some mothers will rush to defend their babies if they squeak when handled. Don’t worry about putting your scent on the babies as this won’t cause the mother to reject them. However, if the mother seems very nervous, then you should look only and wait a day or two before handling the babies. It’s a good idea to examine the babies every day to identify any problems or remove any that have died. Rarely, on the first day the umbilical cords will get entangled and you'll have to separate them.

Difficult Births

Birth in rats usually proceeds without need for assistance, but occasionally, and especially in first-time moms older than 6-8 months, there will be problems. I've seen 3 rats who died during birth. The danger of an obstructed birth is that the mother can go into shock. A Caesarian section may be possible if done soon enough.

Once the birth process begins, if no babies are delivered within 2 hours, there is definitely a problem. The rat's uterus is shaped like a Y and sometimes a baby can get stuck across the bottom of the Y. Gently massaging the mother's abdomen may help expel the problem baby. If a baby 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 usually be delivered normally or with the aid of oxytocin. If the mother survives the birth with unborn fetuses, she may be able to expell or reabsorb them. In this case it is a good idea to treat her with antibiotics to prevent infection.

If the mother rat does not care for her new babies, it’s possible that her hormone levels are not where they should be. The best thing to do is confine her with the babies in a very small cage so she will be exposed them as much as possible, which will hopefully cause her hormones to kick in. In the meantime, the cage should be on a heating pad to keep the babies warm. If the mom doesn’t start caring for the babies within 12 hours, they will need to be fostered on another mother or hand raised. (See the article about Raising Orphans.)

Umbilical Abscesses and Hernias

Occasionally, a baby rat will have an umbilical hernia, or will develop an abscess right over the belly button. An umbilical hernia will appear as a lump at the belly button. It is caused when a hole in the abdominal muscle allows part of an intestine to protrude under the skin. What marks it as a hernia is that gentle pressure will cause the lump to recede, as the protruding intestine is pushed back into the abdomen. An umbilical abscess will appear as a white “pimple” at the belly button.  These are not serious problems. In most cases, an umbilical hernia will eventually close up on its own. An umbilical abscess can be lanced with a sharp needle and the pus squeezed out. The mother rat will then lick it and keep it clean and it should heal right up.

An umbilical abscess. (Photo by Megan Casey.)

Growth and Weaning

Most mom rats know just what to do and take great care of their rats. Occasionally, there will be a tiny runt who can't compete with his siblings for the nipples, especially in a large litter. The best solution is to temporarily separate some of the other babies into another container to give the runt a chance at the nipples. Leave about 4-5 babies with the runt to stimulate the mom to suckle them. If the runt is all by himself, the mom 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 mom for up to 4 hours. You can put their container on a heating pad on low or near a light bulb. Keep a thermometer next to the babies so you know exactly what temperature they are experiencing. Use a small weather thermometer, not a medical thermometer, as the latter can't record drops in temperature. Keep the temperature around 100-102 degrees F. Rotating the groups of babies with the mom every 2-4 hours will give the runt the best chance.

Baby rats grow incredibly fast. I recommend you hold and look at them every day to witness this miracle. This handling will also help stimulate and socialize the babies. Once their eyes open at 2 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'll also start to eat solid food. They'll either walk to the dish, or their mom will carry food to the nest. You don't need to provide any special food for them.

Babies can be weaned at 4-5 weeks, but you can leave the girls with their mom as long as you want. Remove boys before 5 weeks or they may breed with their mother or sisters.

Warning: If your mother has a wheel in her cage, be sure the clearance between the wheel and the bottom of the cage is at least 1 1/2". If it is less then this, you need to remove the wheel while the babies are 2-3 weeks old. Otherwise, a baby can be trapped under the wheel and suffocate.

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