Method	Pros	Cons
Ear punching/ notching	Simple, inexpensive, easy to read	Sometimes ambiguous, subject to tearing and healing, limited numbers
Metal ear tags	Unique numbering, relatively inexpensive	Loss of tags, infections, hard to read
Toe clipping	Simple, inexpensive, permanent	Less humane, must be done at an early age
Tattooing	Relatively permanent, easy to read, may be done on newborns	More difficult and time-consuming, may fade with time if done improperly
Microchips	Permanent, virtually unlimited numbers, can provide physiologic data	High cost per mouse, difficult to apply, requires expensive reader

AN EXAMPLE OF LOW-LEVEL MAINTENANCE

Week 1:

Mat. cage 1	Mat. cage 2	Holding cage	Holding cage	Holding cage	Holding cage
7 pups	5 pups	4 males	5 females	3 males	4 males
3 wks old	2 wks old	15 wks old	15 wks old	10 wks old	8 wks old

The progeny from mating cage 1 are ready to be weaned. Three females and four males are found. They are numbered and placed in two cages. The two cages of older progeny are discarded (shaded boxes). Result: no change in cage number.

Week 2:

Mat. cage 1	Mat. cage 2	Holding cage	Holding cage	Holding cage	Holding cage
8 pups	5 pups	3 males	4 males	4 males	3 females
newborn	3 wks old	11 wks old	9 wks old	4 wks old	4 wks old

The progeny from mating cage 2 are ready to be weaned. Two females and three males are found and numbered. The females are added to the cage of 3 females created in week 1, while the males are put into a new cage. The oldest cage of male progeny is discarded (shaded). Result: no change in cage number.

Week 3:

Mat. cage 1	Mat. cage 2	Holding cage	Holding cage	Holding cage	Holding cage
8 pups	No pups	4 males	4 males	5 females	3 males
1 wk old		9 wks old	5 wks old	4&5 wks old	4 wks old

The oldest male progeny can be discarded (shaded). No other action is necessary. Result: cage number reduced to five.

Dos and Don'ts of Mouse Breeding

• Do minimize the amount of noise and traffic in your mouse room. Try to always have the same person(s) take care of your mice. Ultrasonic noise, in particular, can disrupt normal mouse behavior (including mating behavior), and can be produced by something as innocuous as a dripping faucet. Contact the TMF about borrowing an ultrasound detector to test your mouse environment.


• Do get to know your husbandry staff and maintain a good working relationship with them. They will usually be the first people to notice when something is wrong with your mice.


• Do make use of the box at the bottom of the standard cage card to identify the mice in that cage. The information in this box will appear on any Animal Health Reports filed by ULAR (University Lab Animal Resources) staff.


• Don't bang cages around or jostle the mice any more than necessary. Handle them calmly and quietly.


• Do allow male mice a little time (e.g., a day) to get used to a new cage before expecting them to mate successfully. Males like to mark their territory before "getting down to business".


• Don't let mice get too old prior to first mating, if you are planning to breed them. It's best to give them at least some practise within 4 months of birth.


• Don't expect breeding females to be productive after more than 8 months of age, although they can have litters well beyond that. Males are generally productive at least a year, and often well beyond that. Maximize production of pups by replacing your breeders with younger mice on a regular schedule.


• Do keep records of breeding performance so time and resources aren't wasted on unproductive mice. Not all mice will breed successfully, even among standard strains.


• Do cycle females through a male's cage to maximize production from a small number of males. The estrus cycle is 4-5 days long, so females should be left in the cage at least this long, unless you are checking for plugs.


• Don't cycle males through a female's cage - pheromones from new males can disrupt pregnancies initiated by previous males.
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• Don't disturb gestating females any more than necessary, especially a few days before and after birth.
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• Do use "Do Not Change" cards on the cages of gestating females to minimize disturbances a few days before and after birth. If a cage needs changing while a Do Not Change card is on it, you will be responsible for changing the cage.


• Do leave a female with the same male continuously to take advantage of postpartum estrus. The female can get pregnant within 24 hours of giving birth and normal males will not harm their pups. If you have plenty of males, 1:1 matings are recommended to maximize production while minimizing labor.


• Do use dirty bedding from a male's cage to stimulate estrus in females. Females group-housed for long periods can go into anestrus, i.e., they will stop cycling normally. This is reversed by exposure to male pheromones present in the urine.


• Do check for vaginal plugs if you want to know exactly when a female has mated successfully. This is best done in the morning because the plugs will start to fall out after a few hours. The presence of a plug is not absolute proof of pregnancy, but it is a very reliable indicator. On the other hand, the absence of a plug is not a guarantee that the female isn't pregnant.


• Do communicate with the vivarium manager and husbandry staff about any special precautions for your mice, e.g., autoclaved water, cages, and bedding, and irradiated (or autoclaved) feed for immunocompromised mice. Special diets are available and those that aren't can be custom made by the manufacturer.


• Do give your mice some environmental enrichment. Nestlets are an inexpensive choice - they are compressed cotton squares that mice love to tear apart and fluff up into a warm nest. Other options include plastic or cardboard tubes, or "shelters". Nesting material of some sort should be given to all breeding females.


• Do keep your mice on a regular light/dark cycle with at least 12 hours of light. The TMF uses a 14-hour light cycle. Periodically check that the lights actually come on and go off as scheduled. Avoid entering mouse rooms during dark periods. A red light can be used to minimize impact on the mice if you have to work in the room during dark hours.


• Don't transfer mice from one room to another without permission from a UCI veterinarian. It is ULAR policy to have all animal transfers performed by vivarium staff.

Cross-fostering of Mouse Pups

Many factors affect the survival of newborn mice, including the mother’s nurturing instincts, the number of pups in the litter (too many and too few are bad), amount of milk production, tendency for cannibalism, etc. Some problems can be overcome by moving the pups to the cage of another lactating female that will take better care of them. This is known as cross-fostering. Our protocol for cross-fostering is as follows:

1. Donor and recipient litters should be within 2 days of age, and neither litter should be more than 4 days of age, for best results.
2. If possible, use a recipient strain with a different coat color from the donor strain, for easier identification of the cross-fostered pups. The recipient strain is normally an outbred strain such as CD-1, ICR, or Swiss, because these tend to make the best moms.
3. Remove the recipient mom from her cage. When you pick her up, try to get her to urinate on your gloves, and wipe the urine on the pups you are transferring. (Most mice will urinate if turned upside down.)
4. Remove some (or most) of the recipient’s pups if she has a large litter. After cross-fostering, the total number of pups in her cage should be 5-10.
5. Mingle the transferred pups with the recipient’s pups and rub them with dirty bedding to give them the same scent as the recipient’s pups. Add nesting material if necessary.
6. Return the recipient mom to her cage and place it back on the rack.
7. Observe the recipient mom’s behavior after a few minutes. If she settles down to nurse the pups, or is grooming them, she will probably accept the transferred pups. If she scatters them around the cage, it is unlikely she will take care of them.

Pheromone Effects

Mouse reproductive behavior is governed to a large extent by pheromones. Three important effects of pheromones are described here.

Lee-Boot Effect
Housing female mice in groups will result in synchronization of their estrus cycles. Prolonged absence of male pheromones results in a state of anestrus (lack of a normal estrus cycle).

Whitten Effect
Estrus can be induced in most group-housed females by adding male mouse urine (or dirty bedding from a male’s cage) to their cage.

Bruce Effect
Pheromones from a strange male can prevent the implantation of embryos into the uterine walls of a recently bred female. This is why one should not move a female from one male’s cage to another.

The Lee-Boot and Whitten Effects can be utilized to produce closely synchronized pregnancies. After group-housing females separately from males for some time, put them in a male’s cage. Generally at least 75% of the females will become pregnant within 3 days.

Males prefer to have their territory scent-marked before they will breed efficiently. Therefore, it is better to leave males in their “home” cage and shuffle females in and out to maximize the production from a small number of males.