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Mice & Rat Reproduction

Sexing: Sexually mature mice and rats must be properly paired to breed successfully. A single male mouse may be included in an enclosure with one or more female mice without difficulty. Including more than one male mouse in this situation invites fighting between them. By contrast, more than one male and female rat may be housed together for breeding purposes, within the same enclosure without aggressive plays. Sexually mature male mice and rats usually exhibit prominent scrotum. Sexually mature female mice and rats usually exhibit a prominent double row of nipples. Furthermore, the distance between the rectal opening and the penis of the male is greater than the distance between the rectal opening and the urinary opening of the female in both mice and rats.

Mice: Female mice should not be bred before 50 days of age. They are continuously "polyestrous," which means that they come into heat at fairly regular intervals (every 4-5 days) throughout the entire year until they are bred. The period during which the female is receptive to the male and allows breeding is about 12 hours and usually occurs at night. Female mice can come back into heat within l4-28 hours after giving birth to a litter. This is called a "postpartum estrus," which means that they can be nursing a litter and pregnant at the same time!. Pregnancy lasts an average of 3 weeks but can be extended as much as 10 days longer if the pregnant female is suckling a previous litter. Litter sizes average 10-12 pups, though it is not unusual for a female's first litter to be smaller in number. Litter sizes decrease as breeding females age. Though mutilation and cannibalism of the young are rare, it is wise not to disturb mice for the first 2-3 days after giving birth. Pups are usually weaned at about 3 weeks of age. The female resumes her breeding cycle 2-5 days after her pups have been weaned (unless she was bred during her postpartum estrus).

Rats: Females rats should not be bred before 65 days of age. They are continuously "polyestrous," which means that they come into heat at fairly regular intervals (every 4-5 days) throughout the entire year unless they are bred. The period during which the female is receptive to the male and allows breeding is out 12 hours and usually occurs at night. Female rats can come back into heat 48 hours after giving birth to a litter. This is called a "postpartum estrus." This period of receptivity is not used when breeding rats because the breeding male is removed from the enclosure just before the female delivers her litter because of the high probability of injury to the new pups by the male. After mating, a white, waxy substance, called a coupulatory plug, is visible within the female's vulva for 12-24 hours. It is not uncommon to find these plugs within the enclosure after they have been discharged. Pregnancy lasts an average of 3 weeks. Litter sizes average 6-12 pups, though it is not unusual for a female's first litter to be smaller in number. Litter sizes decrease as breeding females age. Female rats should not be disturbed for the first few days after delivery because stressed females may destroy their pups. Excessive handling, loud noises, and even insufficient nesting material have all been implicated with this destructive behavior. Pups are usually weaned at about 3 weeks of age. The female resumes her breeding cycle 2-5 days after her pups have been weaned.

Cannibalism: Female rats (mice much less often) disturbed shortly after giving birth to a litter may destroy the pups and eat them. Male rats also engage the same behavior. For these reasons, it is important not to disturb female rodents for 2-3 days after they have given birth. Male rats must be removed enclosures just before females deliver their litters.