Litter Size can vary from as little as 1 ritten up to as man as 24, however, 10 is the average size litter.
Birth Weight – 6 Grams
Weaning generally occurs between the 4th and 5th week.
Puberty is normally reach at around 8-10 weeks, however, it has been reported that puberty can occur as early as 6 weeks. Oestrus Cycle normally occurs every 4-5 days. Oestrus duration generally lasts for between 12 and 24 hours. Postpartum Oestrus – can be fertile 24 hours after parturition (giving birth). Gestation is generally between 21 and 23 days.
Sexing
It is possible to determine the sex of young Rats. In males the anogenital distance is twice as long as that of a female. In Males the testicles descent early, and if a male is held with his head up the testes are visible in the scrotal sac. Most males can draw there testicles up for a short period of time, making it difficult to an untrained eye to determine the sex of that individual.
Females may start to show nipples at around 10-15 days old. Males and Females will need to be separated between the ages of 4-5.5 weeks to prevent unwanted pregnancies. Females can become pregnant at around 5-6 weeks of age.
Housing Option
Rats can be housed together in breeding pairs, or one male can be kept with a group of between 2 & 6 females in a polygamous system. Because of the social structure of Ratties, it would be unkind to keep a Rat on its own, weather that be male or female. For non-breeding purposes we recommend that you keep at least 2 males or 2 females in a cage so they have the companionship of other Ratties while you are away. If you have decided to keep your Ratties in a group then you must be willing to allow each Rattie enough space to avoid overcrowding and possible stress, aggression and cannibalism of litters (if kept in mixed groups).
Puberty & Breeding
Young Rats average growth is around 5-6 grams per day. Rats reach puberty at between approximately 2 and 3 months of age, when their body weight is between 250-300 grams. They are best mated after the age of around 4.5 and 6 months. Female Ratties have oestrus cycles all year around, each cycle lasting around 4-5 days, during which time they are receptive to the male for approx 14 hours. If several females are housed together it is not uncommon for them to exhibit oestrus cycles simultaneously. Signs of oestrus can be a willingness to be mounted by the male (some females exhibit rapid ear movement when touched near their lower back), the vulva will be open and can be a purple colour. After mating a white waxy plug (the copulation plug) fills the vagina and come sometimes be visible. This plug, which contains secretions from the male accessory sex glands, prevents leakage of seamen from the vagina. The plug usually drops out within 24 hours and if found is a sign that mating has taken place. There are no other early clear signs of pregnancy. Mating between a male and female can take as little as a few seconds, if this has occurred then you will be best advised to prepare for a litter. Females tend not to show pregnancy signs until late in their pregnancies, sometimes only giving you a few days notice before the delivery of her young, her girth will become noticeably large and she will start to look very uncomfortable, her nipples will also start to become more exposed from under her fur in preparation for feeding her young. Females should not be bred from more than 3-4 times in their lifetime, and not past the age of 1 year. Males tend to be able to breed for their entire lifespan, however, some males do become lazy and uninterested in their later years.
Gestation
Gestation can last anywhere from around 21 to 24 days. Pregnancy may be determined, by a professional, by gentle abdominal palpation from around day 15 or the females weight can be recorded daily to keep track of her weight gain. Mammary development becomes more noticeable in the last week of gestation. By the 16th day of gestation it is recommended to remove the male from the females cage as he may cannibalise the young or impregnate the female shortly after the delivery as females have been known to become pregnant as early as 24 hours after delivery.
The female will start nest building 3-4 days prior to delivery day. If the pregnancy is the result of a post-partum mating, and the pregnant female is lactating, the gestation period may be extended by a couple of days. Delayed implantation can occur for up to 7-10 days, giving an apparent gestation period of 28-30 days. As long as the female is well and there is no abdominal vaginal discharge, a longer gestation can be considered to be the result of delayed implantation, rather than an overdue pregnancy.
Complications During Pregnancy
This may occur if the female is kept in poor environmental conditions, is poorly nourished, or has an underlying disease. Abortion and miscarriage may go unnoticed as the female will eat any dead fetuses or fetal matter. The female may develop a vaginal discharge, which if unpleasant is a symptom of a uterine infection which will require treatment by a professional animal Doctor (Vet).
Vaginal Bleeding – Female rats do not menstruate. If there is vaginal bleeding, take her to the vet to establish the cause of the bleeding before commencing any kind of treatment. Vaginal bleeding can be a symptom of uterine tumor, genital myco, a urinary tract infection, a miscarriage or the beginning of labor.
If you can eliminate miscarriage or labor as the reasons for the bleeding, and antibiotics have been unsuccessful at treating the bleeding, then the only likely treatment left is to spay your female.
Parturition
Parturition generally occurs at night. The first stage of labor begins with the loss of the clear vaginal discharge which the female will lick. The second stage, the delivery of the young, generally takes 1-2 hours. The young are born naked, with their eyes and ears closed. The female should not be disturbed for the first 2-3 days after delivery as she may abandon or cannibalise her young if she feels that her young or herself are not safe. Opinions differ at this point.
Problems After or During Parturition
Perineal Tears – A slight vaginal discharge after delivery is normal, but if there is heavy bleeding, or if the doe repeatedly licks her vulva , she should be carefully examined. Does may aquire perineal tears during birthing which may require treatment with antibiotics either orally or topically.
Mastitis – This is not common and must be differentiated from other mammary issues. The affected gland will be swollen, warm and may ulcerate. Treatment that may be required to ease the swelling and warmth is bathing with warm water. Antibiotics will possibly also be required to help reduce any infections that may be present. If there are areas of ulceration, they will need to be kept clean and a topical preparation should be applied.
Maternal Behaviour
Maternal behaviours generally begin immediately after parturition (Delivery). The female rat nurses her pups by crouching over them and making her nipples available, you will also observe her licking her pups to encourage and stimulate the pups to go to the toilet. This is an important part of her maternal behaviour because without it the pups will not and cannot yet toilet themselves without her assistance. Straying pups are retrieved and placed back in the mothers nest, when she is presented with new or additional nesting materials, the female will respond by building a nest or adding to her already made nest.
Oxytocin is the hormone responsible for “switching” on the material behaviours in the female mother. Oxytocin is very concentrated in the milk of nursing rats and helps to promote attachment to the nipple and may also contribute to the bonding between mother and her infants.
Weaning
Rat young develop rapidly, and by ten days of age have their eyes and ears open and a covering of hair. Weaning can take place anywhere between the 3rd and 5th week. The young will be taught how to eat by their mother, a higher protein diet should have been offered to the mother during and after her pregnancy to help her cope with feeding her young. Once the pups are starting to try solids these same high proteins should be made available to them also. You can offer young rats human baby formula or puppy replacement milk for the first 2-3 weeks after weaning to help promote health and growth in the young. Cows milk should be avoided for the pregnant or nursing rat and her offspring, as it does not contain the essential nutrients that rats require and can cause an intolerance to develop. The female rats returns to oestrus cycling 2-4 days after weaning.
Male Reproductive Tract
The testes perform two major functions that are largely complimentary. The production of sperm and the secretion of steroid hormones. The steroid hormones influence the physiological state of accessory ducts and glands and condition the appearance of the secondary sex characteristics. In most mammals, the testes normally descend into the scrotum shortly before or after birth and remain permanently in that position. The testes are generally 2-4 degrees lower in temperature than that of the abdomen. To produce healthy sperm the testes need to have this lower temperature otherwise the necessary transformation can not be made meaning possible infertility.
Testicular Degeneration
Testicular degeneration is a very important cause of infertility in male animals of all species. It tends to progressively develop with age. It is difficult to find the exact cause in the majority of cases. Specific causes for testicular degeneration include elevated temperatures, frostbite, systemic infections, nutritional factors, hormonal deficiencies and excesses, vascular occlusions, obstruction of the flow of sperm, autoimmunity and age.
Fostering
Orphans can be fostered successfully onto a mother with a litter of about the same age. The orphan should be rubbed in the bedding of the foster family so that they all smell the same and are less likely to be rejected. Hand rearing is also possible, but difficult; Aspiration Pneumonia or chilling are very commonly caused by hand rearing efforts.
Infertility
Rats are generally most fertile between the age of 3 and 10 months of age, after which time they may take longer to conceive, and have smaller litters. Female rats may stop having oestrus cycles at around 15 months of age, however, rats older than this have been documented as still having their regular cycle.
Male rats are capable of reproducing for their entire life, unless they are suffering from an ailment that stops the production of sperm such as testicular degeneration. Occasionally, a male may have one or both testicles which remain in the abdomen undescended, these males may have reduced fertility or be infertile. This condition is thought to be hereditary and they should not be used to breed.
Overcrowding or poor nutrition may also contribute to low births or the lack of any pups. Generally once conditions are improved the chances of breeding will improve.
