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Mouse Breeding Information

This page contains a few hints to help you with your breeding.  It is based on observations made by myself and others with whom I have worked.  Much of the information comes from experience and is not based on any one text.  If you desire more information, please refer to Manipulating the Mouse Embryo-  it contains valuable information about mice, particularly in the field of transgenic research.

Gender Determination:

To identify the gender of a mouse, the anogenital distance (space between the anus and the external genital area) is used.  Males have a larger space between the two structures than females. 

If the mouse is very young and you have difficulty determining the difference, the presence of nipples indicates a female.

Male                        Female

Breeding Schemes:

Harem:                One male paired with two or more females.

Monogamous:        One male paired with one female.

Estrus:

A female mouse is only receptive to mating when she is in estrus.  This stage of her reproductive cycle occurs every 4-6 days.  When choosing a female in estrus, there are three important signs to look for in the vaginal area.  These are:

• open
• red (not bright pink)
• swollen

If all three signs are evident, the female is in estrus.  Having only one or two of these signs signify a different stage of the cycle and will not produce a plug or pregnancy.

The CD-1's albino coloring makes it relatively easy for the practiced eye to identify the signs of estrus.   Look for signs of estrus in dim, rather than bright, light - it will be easier to see the coloring.  

 

The  female on the left is  in estrus:  note the swelling around the top of the vagina and the presence of a small opening as well as the bright red color.  



The female on the right is not in estrus:  note the elongated shape of the vagina, the absence of an opening and the lack of swelling around the top of the vagina.  

 

Plug:

If checking females for timed pregnancy, the presence of a plug in the vaginal opening indicates that the female has mated and may be pregnant.

The plug is usually waxy white to yellow in color.  It may also be clear (if it is new) or difficult to see. 

 

 Insert a blunt pair of forceps into the vaginal opening if no plug is visible.  A plug which is deep within the vagina will impede the forceps from entering fully.

 

Pregnancy:

The gestation time for a mouse is 18-21 days.  A pregnant female has characteristically bulgy sides.  



 

Pups:

If you are not checking your mice on a daily basis, it may be difficult to determine the age of pups.  A few observable traits may help you make a relatively accurate guess as to the birth date.  

Newborn pups are bright red in color.  

At day 1, the pups are a lighter pink and will show milk-spots on their abdomens.



At 4 days of age, the pups' ears will be at a 90 degree angle from the head rather than flat.

At 1 week, a fine coat of hair will be visible on the pups.



At 10 days, the lower incisors will be visible.  

At 11 days, the upper incisors will join them. 

At 2 weeks, the eyes will be open and will appear slit-like.

At 3 weeks, the eyes will appear oval in shape.

.

 

Breeding Hints:

Never put more than one male per cage when introducing females (or after retiring breeders).  They will fight and possibly kill each other.

In general, breeding males may remain in the cage after females litter.  The female will undergo estrus immediately after giving birth (allowing for another pregnancy) and males usually help raise the pups.  Certain stocks or strains of mice are more aggressive than others, so watch for signs of abuse and, if necessary, separate the male (leaving the female and litter undisturbed) into a new cage.

Fellow females housed in a cage with a mom and her litter may begin lactating as well and will supplement the feeding of the pups.

Wean pups between three weeks and a month after their birth.  This is especially important if the female appears to be pregnant again or has birthed another litter - it will reduce stress on the mother and protect the new pups from being trampled.

 

All pictures have been taken by Dr. Kevin Barton.