by Tad Fitzgerald

As herpetoculturists, sometimes we need to breed our own mice. The peace of mind and convenience of always having mice, plus the money you can save are good reasons to take on such a project. Plus having your own mouse colony can be a side business, as you can sell excess mice. There are several ways to successfully house, raise, and breed mice; it is not the intent of this article to mention them all, but to share how I set up, house, and maintain my colony. My goal was to end up with a maintenance system that was not to laborious or time consuming, while producing a continuous supply of healthy mice.

The common house mouse (Mus musculus) is the species predominantly raised by herpetoculturists. Obtain breeding stock from a reputable breeder who is having good results with the colony. If you are unable to locate such a breeder, try pet shops. Another source is biological supply houses that breed mice; contact local university biology departments for addresses. Wherever you get your starter mice, try and obtain subadults, correctly sexed, with no visible health problems.

Purchase subadults (four to six weeks old) because mice have a short life span (under three years), and an even shorter productive breeding period (six months to one year). Younger mice adapt easier to a community-type structure (one male housed with two or more females), while older ones may fight to the death.

Carefully examine mice to ensure they are healthy. Mice should have clear, alert eyes, be very active, and slim. Excess weight indicates the mouse is older and should be avoided. Check for lumps, bald spots,  an arched back, general listlessness, and the anal area for signs of diarrhea. Don't buy the mice if any of these signs are observed. Wait until you find healthy stock.

To determine sex, grasp the mouse by the base of the tail and hold it up so you can see the underside. Note the distance between the anal and genital openings: it is almost twice the distance in males. This works well for mice four weeks and older, but younger mice are more difficult to sex. For mice eight-11 days old you can see the two rows of nipples on females before the belly hair grows. Between two to three weeks they cannot be sexed with certainty. For three- to four week- olds it is more difficult, but can be done by pulling the skin near their genitals forward causing the males testicles to descend into the scrotum.

Many breeders use laboratory-type cages with plastic bottoms and metal lids to house mice. Lids hold both food and water, while keeping it from becoming contaminated. With this type of lid, food and water are easily added without disturbing the mice. Bottoms have rounded edges with smooth sides, which make them easy to clean and do not give mice a chance to chew through the container. Laboratory-type cages may be obtained from a number sources: check the current trade magazines, the internet, and reptile swap meets.

I have 48 cages measuring 19" by 10-1/2" by 6-1/8"; these house one male and three to six females each. Some breeders set up trio's, where one male is kept with two females. I have 10 smaller cages that measure 11-1/2" by 7 1/2" by 5", that house one male and one to to two females each. In addition to the 58 breeding cages, I also keep several holding tanks for excess mice and future breeders  This was the setup and cages I used in the early nineties, I later switched to the setup( utilizing cat litter trays) pictured above, changing again in 2004 to the odor free cages pictured on our facilities page.

If you make your own, use containers with smooth rounded corners and smooth sides, so the mice cannot chew through them. Design an escape-proof lid. Take into account how their food and water will be supplied, and if you will be able to keep it from becoming soiled by the mice. Also, if possible, design cages that are durable and easy to clean and service.

For bedding material, fine pine shavings, cedar shavings, and sawdust all work. There are also several companies who manufacture various pellet-type bedding that works well. You can usually obtain sawdust for free from local lumber yards. I prefer sawdust or one of the pellet-type bedding as it absorbs odors better. I put 1/8" of either pellets or sawdust in each cage and add a few sheets of telephone book paper to give the females material to build a nest with. This may help reduce cannibalism by making the females feel more secure, while satisfying the instinct to build a nest.

Specially formulated lab pellets made specifically for rats and mice can be purchased for feeding the colony. Chunk-style dog food also works (chunk-style pieces large enough so it will not slip through the lids). However dog food is consumed much quicker than lab pellets, and it seems to attract ants. I keep a 30 gallon covered container of food in the room (a plastic rubbermaid-type tub or metal garbage can work well). Note: Use food with a minimum of 15% protein, this may help reduce cannibalism.

I store several one gallon containers filled with tap water, and allow to sit at room temperature for 24 hours before use. Empty plastic milk jugs or juice containers are convenient (and inexpensive) water containers. There are also several manufacturers of automatic type systems available for most setups.

Once a week, all cages should be cleaned and disinfected with a 5% household bleach solution (or something equivalent) and rinsed thoroughly. With my colony of 58 cages this takes me three hours.

A more efficient method method is to have more bottoms than tops, so you can have at you side ready-to-go, pre-cleaned and bedded bottoms. This eliminates the need to handle mice twice, plus saves time. The mice are simply transferred from the dirty cage to the clean one, and the same top is used. The dirty cages are dumped out and stacked in a pile to be cleaned all at once, and the cleaning goes much quicker. Water bottles are cleaned with bleach solution every six months, or sooner if needed.

When choosing a location for a room or building to house a colony of mice, keep in mind the importance of good ventilation, temperature control, and odor management.  Because I live in a residential neighborhood with a small lot, I decided to build a separate room within my garage on the south side of my residence; this is not the best location in regards to temperature control, but was the most practical. I could have built a separate building on the north side, but chose not to because of the location of my neighbors house and the risk that there may be a problem with the odor.

The finished room measures 10' by 12' with an 8' ceiling, and a concrete floor. Shelves were designed to hold the maximum number of cages in the minimum amount of space. I used 3/4" particle board 16" wide, with three different lengths cut to match the three wall sizes. To determine the heights of these shelves , I started by establishing the height of the top shelf and worked my way down. The top shelf low enough to easily service the cages, and the bottom shelf high enough off the floor to store water containers (one-gallon plastic milk containers), plus left enough room between all the shelves to easily service all the cages. I ended up with four rows of shelves on each wall. One bank holding 28 cages, one holding 20, and the other 10. Additional shelves were added above the top shelves for storing various equipment. 

In Las Vegas Nevada, the temperature can reach 115-120 F. in the summer and drops below freezing in the winter, so it was a must to be able to control the temperature to get good breeding results. The ideal temperature for breeding mice is 68-72 F, with 50-60% humidity, but I have had good results if temperatures stay between 55-80 F, with 40-65% humidity. In the summer when temperatures reach 115 or higher, the room reaches 85 F. If these temperatures remain for a month or longer I get a 20-30% drop in production.

The room is cooled and ventilated with an evaporative (swamp) cooler, controlled by an inline thermostat set at 72 F. A 1' by 1' louvered vent was installed directly through the wall to vent the air, and was purchased from a store that specializes in evaporative coolers. It automatically opens when the cooler in on and closes when the cooler shuts off. An important hint: locate the went so the exhausted air (odor) is not a problem to your family or neighbors.

In the cooler months, when outside temperatures drop, I drain the water and shut off the water supply to the cooler. I use a 1500 watt radiator heater when the room temperature drops below 65 F. Because of the body heat produced by the mice (300-350 adults) the room will typically reach 65-70 F, even when outside temperatures only reach 45-50 F.

This system works well except when temperatures reach 115 F or above, or when humidity levels outside reach 40% or higher. When humidity levels are this high, an evaporative cooler is basically reduced to a fan. While it ventilates well, the air temperature is not reduced enough to significantly cool the room. Under these conditions the mouse room reaches 90 F. 

An alternative would be to use a small room air conditioner/heater in conjunction with an exhaust fan. I decided to keep this set-up because it is economical, it adds humidity, and it supplies a constant exchange of fresh air. Operating costs for an evaporative cooler are minimal ($20 per month in the summer, when the cooler is running 24 hours a day, and $5-$10 dollars per month the rest of the year). An air conditioner would cost three times as much. I believe the added humidity, plus the constant exchange of air, provides a healthier environment for the mice, as well as for me. An air conditioner does not give the constant exchange of air, plus ammonia produced by the mice can have an adverse effect on an air conditioning system.

Some breeders opt to separate females from their cagemates when they become pregnant, housing them in another cage to have and raise their young. To cut down on labor and enable females to become pregnant more frequently, I house males and females together. If continually housed with a male, a female is able to become pregnant again a few days after giving birth. If females are separated from the male while raising young, the breeder loses three to four weeks of time when the females could have become impregnated again.

Continually keeping the sexes housed together does not seem to harm them, and in my colony the number of young lost to cannibalism is very small and so this is not enough to justify separating the females. The extra time and labor required to shuffle females back and forth makes it undesirable, as does the extra space and cages needed to maintain such a system.

Start out by obtaining 10 males and 20 females to fill 10 small cages. In a notebook, number the pages 1-10, then number the cages 1-10 (or number the space on the shelf where the cage sits). Place one male and two females in each cage. Record the date the mice were put in the cages on the corresponding pages. When a litter is observed, record the date and number of young. If cannibalism is seen, record the date, and then mark an X next to it. If cannibalism is seen two times in a row or three times total during that trio's breeding career, remove those breeders and start a new trio in that cage. Sometimes females eat their first litter and never do it again. Do not be alarmed, I have had several mice do it once or twice and still go on to produce large healthy litters. Monitor small cages regularly (at least every two to three days) for new litters, weaned mice, and cannibalism.  

Let's assume that the mice are producing and young are ready to be weaned. A mouse is ready to be weaned when it's eyes are wide open and it's eating on it's own (at three to four weeks old). Begin holding back these mice to set up the larger cages. Hold back only from litters of good producers (10-15 young per litter) with no history of cannibalism. With the notebook this should be easy to determine. Utilize mice produced from inferior trio's as feeders.

Avoid breeding related stock. For example take six females from cage #1 and one male from cage #2. You might want to use different-colored males for quick identification. I use white females and colored males. This still gives you different colored young while making monitoring of the males quick and easy.

If a group is not producing well (one litter every six weeks, and 10-15 young per female) remove the old mice and add new ones. This system of record keeping is used only with the small cages of trio's for the purpose of holding back the best possible mice to set up the larger cages. Once that is accomplished, you can monitor all cages with the less labor intensive system described later.

Larger cages are monitored with ordinary clothes pins attached to the cage lids. The clothes pins are color-coded to indicate specific information (blue identifies cages with new breeders that have not produced yet and plain clothes pins with black marks monitor cages that are not producing well). A container filled with clothes pins is kept near the service area, enabling me to quickly and easily attach and remove them. 

Large cages are set up with one male and five females. It is not uncommon for females to fight, and sometimes may fatally wound each other, or a particular female may not be able to become pregnant for various reasons. If you start out with a high number of females, you have a better chance of good production. If after four to six weeks none of the females appear to be pregnant, try a different male. Continue holding back mice from the small cages until all the large cages are full. This may take several months, depending on the number of breeders needed and how well the mice in the small cages are producing. 

When the first large cage is set up, place the cage on the upper shelf all the way to the left and attach a blue clothes pin. As new cages are added, slide the older ones to the right, always placing cages of newly weaned mice in that upper left position on the shelf. When all the cages are filled, you should have the youngest mice starting at the upper left side and the oldest at the bottom shelf all the way to the right.

When a cage of new mice (blue clothes-pin) produces pinkies (newborn mice), remove clothes-pin. This will indicate the cages without cloths-pins are producing. If there are mice in cages to the right that have not produced yet (blue clothes-pins) slide those to the left and move the producing groups to the right. If only a small number of pinkies were produced (less than eight) or if there are signs of cannibalism, attach a cloths-pin with one black mark and remove the pinkies, which are either used now or frozen for future use. 

When the cage is cleaned the following week, the clothes-pin alerts me that the mice in that particular cage are not producing well. If it still has not produced it receives a clothes-pin with two black stripes. This is continued for another week, when I attach a clothes-pin with three black stripes if that cage is still not producing. The next week mice are removed if they haven't produced, as four weeks is ample time for mice to produce. The exception being if there are obvious pregnant females ready to give birth. The cage is then set up with new mice and moved to the upper left position with a blue clothes-pin attached. Most of the time the above scenario doesn't get that far, as young mice usually produce within that time period. To keep it simple, the important factor is to first establish that the cage is producing, then simply remove the blue clothes-pin. Until that point that particular cage is on "trial". Most cages that start out producing well do so for several months. Once you have established a cage is producing it can be monitored easier by purposely leaving a few young in the cage. This tells you from week to week that the cage is still producing. It will not tell you exactly how many, or if every female is producing, but it does tell you your getting somewhere. 

When you removed the first blue clothes-pin and did not attach one with a black mark, you determined in was a good producing cage. Now, for the remainder of that cages time in service, you simply monitor it weekly to confirm it is still producing. If at any time there are no young in the cage, it get's a clothes-pin with one black mark, and so on as described earlier until, if necessary, the mice are retired and new ones are added. Even if all the young were removed from a cage the day before you serviced it, therefore receiving a clothes-pin (one black mark) the next day, the system still allows ample time ( three weeks) for additional litters, or to detect obviously pregnant females.

Although this system does not keep a record of the exact age of the mice, it does give you an estimate by cage location on the rack. This would enable you to retire a set number of breeders, say every month, which would ensure you always will have young, healthy breeders. Remove bottom cages, slide other cages over, and set up the top shelf with new breeders. If this was done with four cages per month and a 24-cage system, you would never be breeding mice over seven months of age.

I do not retire breeders this regularly, because I am only interested in whether mice are producing or not, so age alone does not determine if I keep a cage in service, but I do pay closer attention to the production of the older breeders. Also, if adult mice are needed as feeders, I take the older ones that are not obviously pregnant from a cage that is not producing well (clothes-pin with two or three black marks) before randomly removing just any adult.  

You may choose to continue using the time-consuming monitoring system used for the smaller cages indefinitely. This would enable you to continue holding back potential breeders only from the best producing females. I do not do this because it is important to me to reduce the time and labor required to monitor the cages, and I want to free up a number of small cages so when the number of mice in a large cage drops to three (one male and two females) I have a cage to house them. It is not productive to tie up a large cage with only two females, and on the other hand if those two females are still producing well or pregnant, I do not want to use them as feeders at that point. This set-up is efficient when used this way. I keep a minimum of five small cages available for that purpose and use the remaining five for normal use. This system, including feeding and watering, takes me approximately one hour per week, the the three-hour weekly cleaning for a total of four hours a week to maintain the colony.

With appropriate housing, nutrition, and care, mice rarely become ill. If you encounter a small number of mice with health problems, it would be more practical to cull them from the colony than to treat them. The cost of veterinary care, along with the time required to diagnose problems, makes it impractical. Some of the more common health problems encountered are mange, tumors, infectious conjunctivitis, and respiratory problems.

Mange (loss of hair) can be caused by mites. Over-grooming or aggression by cagemates can be mistaken for mite problems. If it is determined that mites are the problem, the colony can be treated by adding either Ivermectin to the drinking water, or a pyrthrine-based flea powder to the bedding. You can obtain these from a vet, along with the correct dosages. If the flea powder is used, do not feed the treated mice to your reptiles for at least two weeks.

Tumors are fairly common, especially in older mice. During the cleaning period I remove mice with obvious tumors and euthanize them. Tumors are not contagious and do not seem to harm the reptiles they are fed to. If juvenile mice are seen with tumors, euthanize them and do not hold back any of their young as breeders, as this could be as inherited trait. A low-fat, high protein diet may help prevent or reduce their frequency. 

Infectious conjunctivitis (pink or crusty eye) can be caused by viral or bacterial pathogens. This can usually be avoided with proper ventilation and regular cleaning of the cages. Bacterial conjunctivitis can be treated with antibiotics added to the drinking water; viral are more difficult to treat. As previously mentioned, obtain proper medications and dosages from a veterinarian. If treated mice do not respond, them them (along with cagemates) and disinfect the cage and watering equipment with a 5% bleach solution; non-bleach disinfectants are also on the market and the hobbyist may try these as well.

Respiratory infections can occur when mice are exposed to wet, damp or inclement conditions, or other mice that are contagious. The symptoms are sneezing, followed by anorexia and death. If a large percentage of your colony is infected, euthanize the entire colony and start over. Try and determine if it was caused by environmental conditions or they they were somehow exposed to the infection from new mice added to your colony. Improve these conditions or avoid that particular supplier in the future.

This brings us to the question of adding new bloodlines to your colony to reduce possible problems associated with inbreeding. In an established colony that is doing well, I do not think it would improve the colony, and it may very well hurt it by introducing negative traits and / or infectious diseases or parasites. My colony has improve production under the same conditions  without adding "new blood". I have also not had a problem with mites or infectious diseases.

During the weekly cleaning process young are removed and / or left behind. This is determined by several factors, such as what specific amounts and sizes are needed now and in the near future. For example, if snakes have just come out of hibernation, more adult and weaned mice will be needed, and few newborns. In summer when there are hatchlings to feed, a large supply of newborns plus adult and weaned mice are needed. These factors change according to the season and your specific needs.  The size mice colony described in this article supported a colubrid snake collection of 100 adults, plus 200-300 hatchlings per year, additionally it made a small profit by selling excess mice to local pet stores.

Remove feeder mice in a manner that helps reduce overcrowding and to separate different sized young. Whenever there are large numbers of young living together at the same time, the weaker ones suffer because of the competition for food. The cages become fouled quickly when overcrowded. Thin out cages, leaving a reduced number (20 or less) behind.

I use red clothes-pins attached to the cages that have excess pinkies (one-three day old mice). This comes in handy when feeding a large number of hatchlings. Plain cloths-pins are used to mark cages that have excess fuzzies (one-two week old young) to those almost weaned (three-four weeks of age). This save me from having to look through several cages to get those particular sizes. Also cages with plain clothes-pins can quickly be spot checked during the week to remove weaned mice.

Adult feeder mice are pre-killed to ensure they do not harm the snakes. I use a homemade gas chamber (cost is $100-$150) to euthanize the mice, as I believe this is the most efficient and humane way. Purchase a small CO2 gas cylinder with a regulator and a 1/4"-1/2" (6-12mm) rubber hose. Once the regulator and hose are connected, place the hose in the bottom of a one gallon bucket, place the mice in the bucket, open the regulator all the way, and turn on the cylinder for five seconds. This will painlessly and quickly kill the mice. I connected the hose to a typical ice tea container via a quick connect coupling. I removed the spout and, with a combination of washers and silicone, attached the female end of the quick connect assembly. The male end fit into the end of the hose and I secured it with a hose clamp. I can separate the container from the gas cylinder for easier cleaning. Once mice are euthanized, if they are not immediately used they are dried with a small fan, then put into dated freezer bags and frozen for future use.

This article was written in 1995, so is a bit dated. I decided to include it because there is still a lot of valuable information in it. Since that time there have been many caging systems designed that are readily available to the casual hobbyist or the professional breeder. Most are stackable, come with automatic watering capability, chew proof caging materials, and large food holding bins. Lab caging as discussed in the article is still a viable option, and used lab caging is often available at a very reasonable price. Also, it's been my experience that using a high quality lab chow with a minimum of 15% protein will give optimum production plus decrease cannibalism.