Comments & suggestions  should be directed to Visit my website Reproduction Puberty occurs in the rat at about 50 - 60 days of age.  The estrous cycle of the rat is highly light sensitive, and rats housed under conditions of constant light will stop cycling.  Rats are polyestrous and normally cycle every 4 - 5 days.  Estrus lasts for approximately 12 hours and generally occurs during the dark cycle.  Ovulation is spontaneous in the rat  (i.e. it is not induced by mating activity).  The maximum fertility of the rat occurs between 100 - 300 days of age, and breeding activity usually begins at 100 - 120 days of age.  Menopause is reached between 450 - 540 days but breeding efficiency is reduced well before then.  The estrous cycle of female rats may be determined by examining the cellular composition of a vaginal smear.  During proestrus, the cellular population will be composed primarily of non-cornified (nucleated) epithelial cells and lymphocytes with a few cornified (non-nucleated) cells.  During estrus, the vaginal smear consists of primarily cornified epithelial cells.  At late metestrus and during diestrus, the number of cornified epithelial cells is decreased and the number of lymphocytes is increased. Pheromones are hormones produced by one animal that provide a signaling function to another animal, and in rodents may affect reproductive activity.  The Whitten effect is a pheromonal effect that occurs in female rats housed together for extended periods of time and that are subsequently paired with a male.  These females demonstrate a high incidence of mating on the third night after pairing with the male.  In contrast, females that were previously singly housed show an even distribution of mating over 4 nights when paired with a male.  This effect can be taken advantage of to orchestrate the production of timed pregnant animals. Breeding animals may be grouped in pairs (monogamous system) or in harems (polygamous system) containing 1 male and 2 - 6 females.  Pregnant females in harems are moved to separate cages at about day 16 of gestation.  Advantages of separation of the female are increased milk production, higher birth weight, and larger litters.  The presence of a male in a cage with a female and her recently delivered pups may result in cannibalism, litter desertion, or agalactia  (lack of milk production). Gestation length ranges from 21 - 23 days.  Abdominal enlargement is usually detectable by day 13 of pregnancy, and mammary gland enlargement can be seen at day 14.  A clear, mucoid vaginal discharge is often seen 1.5 - 4 hours before the delivery of the first pup, and the female may be seen licking her perineum at this time.  Delivery of the pups normally lasts about 90 minutes, but may range from 55 minutes to almost 4 hours.  A postpartum estrus occurs within 48 hours following parturition, at which time the female may be rebred to maximize production. If a timed pregnancy is required, breeding the female at the postpartum estrus should be avoided because implantation of the embryos is delayed for a variable period of time (3 - 7 days).  Most colony managers do not rebreed at the postpartum estrus because the male may bother the neonates. Mating is detected by the presence of a milky white to yellow, waxy vaginal plug due to secretions from the male's coagulation glands.  This plug normally persists 12 - 24 hours and after discharge may be found in the cage or litter pan.  If a hanging cage system is used for breeding. a piece of dark paper can be placed under the cage to allow easier detection of the copulatory plug.  If timed pregnancies are required by the investigator, the rats should be checked daily for the presence of a vaginal plug.  Mating can also be confirmed by the presence of sperm in the vaginal smear.  Cotton nestlets, tissue paper, shredded paper, wood shavings or another suitable material should be provided so that the female can build an adequate nest.  Avoid the use of materials like cotton or shredded paper in breeding cages because the pups can become entangled in the fibers, and may suffocate or lose appendages.  Pups delivered on corn cob bedding alone may be exposed to the cage floor and may become cooled and the female may neglect the pups.  Cooled pups should be warmed before returning to the female to ensure acceptance. The average litter size is 6 - 12 pups and varies with the strain of rat.  Females should not be disturbed a few days before and after parturition to prevent cannibalization of the pups.  Females should also be insulated from loud noises at this time.  Rat pups are altricial at birth; that is, they are born blind, deaf, and naked.  Newborns are often referred to as “pinkies”.  The “milk spot”, which consists of the stomach filled with milk, can be seen through the pup's thin skin on the left side of the abdomen and can be used to determine whether nursing has occurred.  Fine hair covers the pup by 7 - 10 days of age and their ears open between 2.5 and 3.5 days.  The eyes open between days 7 and 14.  Lactation lasts for approximately 3 weeks.  Pups are generally weaned at 21 days of age, at which time they weigh 40 - 50 grams. Sexing of the pups is done by observation of the anogenital distance, which is 1.5 to 2 times greater in males than in females. Gender determination.  The anogenital distance is 1.5 to 2 times greater in the male (left) than in the female (right).