Mating

     Mating, or copulation, is usually nocturnal. The male will mount the female from the rear. Copulation lasts for several seconds. After mating, a white, waxy copulation plug remains in the vagina for up to 12 hours, after which time it will fall out. The copulation, or vaginal, plug is formed mainly from serections of the male accessory sex glands, that is, the seminal vesicles, prostate gland. If you have wire mesh bottom cages, the plug can be often found in the pan below the cage.

Pregnancy

     Pregnancy, or gestation, lasts for 21-23 days. If the rat was mated at the heat after whelping, simultaneous lactation may delay implantation 3 to 7 days and therefore lengthen gestation to 30 or more days. Abdominal enlargement becomes evident at about two weeks. Mammary development is evident at 14 days. False pregnancy, or pseudo pregnancy, is rare in rats.

     When using a harem system of mating, pregnant females are often removed to a separate cage prior to whelping. They may not tolerate the other adult females in the cage while nursing. THe adult male may be kept with the adult female in the cage, but it is safer for the litter if the male is removed shortly before parturition and during the lactation period.

     Prior parturition, females show nest building activity, especially in cooler environments. Expectant mothers pile bedding material to one side of the cage and will shred paper to construct a nest.

Whelping

     Newborn rats, or pups, are delivered one by one at intervals of five to ten minutes, usually in the early morning. Difficult whelping, or dystocia, is uncommon (and less than 1%). The pups may be born head first or in a breech position. The mother, or dam, cleans the pups and eats the placenta. Stillborn pups and weak pups may also be eaten or they may be left scattered in the bedding. The dam will place the strong, pink colored pups in the nest area.

Pups

     At birth the young are hairless, blind, toothless, with closed ears and eyelids, undeveloped limbs, and short tail, and weigh five to six grams. Without fur, the pups rely on the dam and adequate bedding for thermoregulation. By five days, they have doubled in body weight and have opened ears. A thin, silky hair is present after seven days. As the hair coat grows, the young become less dependent on maternal body heat. By the tenth day, the pups are covered with fur and may wander around the cage. On days 11-12, their eyes are open, and they have teeth and can eat solid food consumption, while still suckling, until they finally become independent of the dam's milk by 21 days of age, when they can be weaned.

     Most dams will allow you to handle the pups from the first day onward, but it is best to minimize disturbances such as handling, changing bedding, and loud noises. Use latex, rubber, or vinyl gloves or paper towel when handling young. This minimizes foreign odors on the young. Replace any handled young within the nest. 
 

-A hooded rat nursing her litter. During their first few weeks of life, the young receive all of the nourishment that they need from their mother.

-These pups are fully haired, but their eyes have not yet opened.

Lactation

     The young begin to suckle within the first three hours. They are guided to the teats by smell. The stomach of a nursing pup can be seen through the thin abdominal wall as a beige colored structure.

If you do not see milk in the stomachs and the pup appears to be gray at day two, you have problems. You may need to foster the pup. Fostering pups to another dam is relatively easy and described below. Hand feeding of rats is difficult but sometimes done in research colonies, utilizing milk collected from lactating rats.

     Providing the pups with some semi soft feed or mash reduces their dependence on their mother's milk and increases their growth rates. Milk can be mixed with ground diet in a jar lid placed on the floor of the cage.