Breeding Considerations

Sexually mature mice and rats must be properly paired in order to enjoy success in breeding them. 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 house together for breeding purposes within the same enclosure with aggressive displays.

 

Sexually mature male mice and rats usually exhibit a prominent scrotum. Sexually mature female mice and rats will 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.

Breeding-Rearing Cycle

Mice

Female mice must not be bred before 50 days of age. They are continuously "polyestrus," which means that they come into heat at fairly regular intervals (every four to five days) throughout the entire year unless they are bred. The actual period during which the female is receptive to the male and will allow breeding is approximately 12 hours and usually occurs at night. Female mice are capable of coming back into heat within 14 to 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 three weeks but can be extended as much as 10 days longer if the pregnant female is suckling a previous litter. Litter sizes average 10 to 12 pups, although it is not unusual for a female's very first litter to be smaller in number. Litter sizes will also be decreased as breeding females age. Although mutilation and cannibalism of the young are rare occurrences, it is wise not to disturb mice for the first two to three days after giving birth. Pups are usually weaned when they are approximately three weeks old. The female will resume her breeding cycle between two and five days after her pups have been weaned (unless she was bred during her postpartum estrus.)

Rats

Female rats must not be bred before 65 days of age. They are continuously "polyestrus" which means that they come into heat at fairly regular intervals (every four to five days) throughout the entire year unless they are bred. The actual period during which the female is receptive to the male and will allow breeding is approximately 12 hours and usually occurs at night. Female rats are capable of coming 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 strong probability of injury to the new pups by the male.

 

Pregnancy lasts an average of three weeks. Litter sizes average six to 12 pups, although it is not unusual for a female's very first litter to be smaller in number. Litter sizes will also be decreased as breeding females age.

 

Female rates must not be disturbed for the first few says 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 when they are approximately three weeks old. The female will resume her breeding cycle between two and five days after her pups have been weaned.