MisterMiceGuy's Guide to the Science Fancy Mouse Breeding


One of my goals as a mouse breeder is to use mouse breeding to help others gain exposure to science and the natural world. I want to encourage and promote realistic animal breeding that’s based on science and I want it to be available to mature and responsible adults interested in these subjects. To accomplish this, I am writing this guide to becoming a fancy mouse breeder.


Mouse pups are cute but there are a lot of things that come with mouse breeding that are not cute. These include witnessing death, cannibalism and euthanasia. It’s not uncommon for a mouse breeder to find a dead mouse who has had its head eaten off by its cute cage mate. It’s also not uncommon to check in on your prized litter only to find that their mother has decided to eat the limbs off all her pups. It’s not for the faint of heart. In addition, there is also the reality of euthanasia. Although some might find it controversial euthanasia is frequently part of a breeding program that is carried out by the breeder themselves. This can be emotionally daunting, but some breeders feel it’s a necessary part of the hobby (Royer 1998). If you’re prepared to accept the realities of mouse breeding, then maybe you’re ready to explore mouse breeding as a hobby.



First thing you will need are appropriate cages for the mice and a place to keep those cages. If you plan on breeding multiple generations of mice you will need multiple cages to support a small colony of mice. Mister Mice recommends a minimum of 4 cages. One to house your breeder male, one to house a female with pups, and two to grow out your pups.

Some mouse breeders make their own cages out of large plastic bins by adding wire mesh to the sides and lid for ventilation.  Suitable plastic or glass cages can also be purchased but hamster-style cages are not recommended (FMBA 2019).  You can find instructions to make these online.  Remember that good cage ventilation is important as this helps reduce the buildup of toxic ammonia fumes (Smith, Stockwell, Schweitzer, Langley, & Smith 2004)

Recommended cage sizes for mice is based on the size of the mouse with larger mice needing larger accommodations. For Mice being housed in groups it is recommended that a 25g mouse have a minimum 15 sq. in. of floor space. The minimum height for a cage is for mice is 5in. The minimum floor space for a mother with pups is 51 sq. in. (National Research Council 2010). To figure out the size of your cage multiply the length (in inches) by the width (in inches) and that will give you total square inches.

You will also need a place to keep these cages. MisterMiceGuy suggests using a wire utility shelf or similar shelving system. The shelves can be adjusted so you have plenty of space to house your mice on top and then you can have space on the bottom to house your mouse supplies such as bedding or food.


Mice prefer beddings that contain large fibrous pieces (Blom, Van Tintelen, Van Vorstenbosch, Baumans & Beynen 1996). However not all large piece beddings are created equal. Untreated softwood beddings have been shown to affect the immune system and Cedar bedding has been associated with tumor development, altered liver enzyme functioning, and cytotoxicity in mice (Vlahakis 1977, National Research Council 2010 and Sabine, Horton, & Wicks 1973). Fancy Mice Breeders generally avoid cedar and other soft wood beddings (FMBA 2019). However, Kiln-dried pine maybe used as the kiln drying process reduces the aromatic hydrocarbons that are found in untreated pine. Special precautions should be taken regarding bedding used for hairless mice as their lack of eye lashes may contribute to conjunctivitis (Research Council 2010).

It’s worth noting that research has compared the ability of different beddings to control ammonia levels. This is relevant because ammonia is associated with disease in mice (Mexas, Brice, Caro, Hillanbrand, & Gaertner 2015). They found that the common paper-based bedding “Care fresh Ultra” performed significantly worse in controlling ammonia levels when compared to pine shavings, hardwood, or corn cob bedding (Smith, Stockwell, Schweitzer, Langley, & Smith 2004). Hardwood beddings, such as aspen shavings, are thought to be the best wood-based bedding for mice and are very good at controlling ammonia levels (FMBA 2019 and Smith, Stockwell, Schweitzer, Langley,& Smith 2004). It is common practice to replace bedding and sanitize cages weekly however some fancy mouse breeders recommend twice weekly cleanings (Mexas et al. 2015, AFRMA 2019)

Food and Water

Water bottles designed for small animals should be used to supply fresh water. Commercial “Lab block” style products designed for mice or rats can be used for food. Some breeders create their own food mixes (FMBA 2019). However, care should be taken as inappropriate macro nutrients, such as too little protein can have adverse effects on mice (Watkins et al. 2008).


When it comes down to it you have basically two options when it comes to getting animals to start with: pet store or private breeder. It’s common to find mice at pet stores but the quality of these mice can be lacking. It’s likely that these mice will have poor body types and poor temperament. You want to look for mice with broad heads and large ears in the coat color that you want. You will save a lot of time by starting off with a male that has the basic coat type and color that you want or that has exceptional type.

The preferred method is starting off with mice from a private breeder. You can locate private breeders on one of the fancy mouse breed club pages or by doing a google search. Again, its best to start with a male that closely matches the color and type that you are looking for. By purchasing from a breeder not only will you be starting off with higher quality animals but it’s likely that the breeder will know the genetic history of your animals. This can be useful if you have specific breeding goals.


Grasp the mouse by the tail near its base and place the mouse on a surface, such as your palm, that it can grab while maintaining your firm but gentle grip. Young mice should be picked up by scooping your hands around their entire body, or by picking up a group of baby mice along with some nesting material (Connor 2007).


Breeding mice is straight forward. Mice are old enough to breed between 4-7 weeks of age. Pregnancy in mice lasts 19-21 days. Once mice are born, they will wean off their mothers milk in about 21 days. Litter sizes vary from small litters of 4 pups to large litters of 12 pups. A female mouse’s breeding life span is from 6 to 12 months, and total life span can be anywhere from 1 to 3 years (Connor 2007).

Female pups should be separated from their male siblings and father between 3 and 4 weeks of age. If they are allowed to remain longer, they may become pregnant. Female mice that are bred before 6 weeks of age are still immature and may have difficulty giving birth or be more likely to eat her offspring. Additionally, females may start to loose fertility after about 6 months of age (Connor 2007).

When pairing mice you should make note of the pairing by documenting the pair, the date of the pairing, the estimated date of birth (Connor 2007). Once the pups are born you can go back and note the 3 week date and the 6 week date so that you know when to separate the pups and when they can be breed. Mister Mice suggests keeping a small note book for this purpose called a Breeder’s Log.

When caring for a pregnant mouse a higher fat diet may be provided for milk production. Handling of the female should be avoided for the first day or two after birth as this may cause the mother to eat her pups. Actually, any stressful events such as noises, aggressive handling, or overcrowding can lead to decreased fertility and increased chance the mother will eat her pups (Connor 2007).


When selecting mice to breed it’s important to have a goal in mind.  Ask yourself what do I hope to achieve with this pairing?  Generally speaking, there are two basic methods of selecting pairs.  One is that you pair animals with similar trains with a goal on improving that trait.  For example, you pair your largest eared mouse with your largest eared mouse with the goal of producing even larger eared mice.  The other method is pairing animals based on the traits that they don’t have.  For example, you have a large eared mouse with a narrow head, and you breed it to a mouse with a wide head but small ears with the goal of producing a mouse with a wide head and large ears or at least reducing what you feel is a fault in that mouse. 


There is a difference between breeding a pair of mice and breeding a line of mice. In order for your mice to “breed true” you need to develop a line of mice the consistently produces mice with the desired traits. If this is your goal, then you need think about setting up a breeding plan (Greenwood-Dudar 2018).

The first step is to identify the traits that you want to see in the mice you are producing (Greenwood-Dudar 2018). Traits can include any of the observable physical or behavioral characteristics (Barber 2005 and Genetic Learning Center 2016). If you plan on showing mice you might want to familiarize yourself with the varieties of mice and the characteristics associated with them (FMBA 2019, AFRMA 2019, and Greenwood-Dudar 2018). The second step is developing a feel for what those traits should look like in your mice. This includes being able to evaluate a given mouse and determine if it has the traits you are looking for (Greenwood-Dudar 2018).

As a new breeder you can increase your knowledge in these areas by joining a mouse breeders club, contacting local mouse breeders, or reaching out to other breeders on social media. If there are mouse shows in your area attending those might be helpful (Greenwood-Dudar 2018).


Growth: Mice develop rapidly between birth and 4 weeks and you will notice daily changes. On days 3-4 the ears will lift away from the head. On day 6-7 a coat of fine fuzzy fur starts to grow. On 9 day nipples are visible on females only. On days 10-11 their teeth break through their gums. On days 12-14 their eyes will open (Connor 2007).

Weaning: At about 3 weeks old mice will look a lot like tiny adults and will be ready to eat solid foods. If pups are separated too early from their mother, they may die so it’s important to know they are eating on their own before separation. One wat to test this to observe the pup’s behavior when you remove the lid to their cage. If the pups lay still, they are too young. However, if they run or jump around the cage then they are old enough to be separated (Connor 2007).


If you are a member of a mouse breeding club then take a peak of the club’s “standards” to see the traits that are desirable in show mice.  These are the traits you want to select for.  It might be helpful to google show mice to get an idea of what show mice look like.

There are two different stages in which mouse selection occurs.  Once when the mice are pups and another time when the mice are adults.  When the mice are adults it should be easier to tell if they have the traits you are looking for.  When you have a new litter it can be hard to tell if the mice will have the traits you are looking for.  Usually when selecting pups out of a litter you want to look at the width of the pups head and overall size of the pup.  Unfortunately, there are some traits (such as angora) you just won’t be able to select for until the mice have matured. 

Sex is also an important trait you want to pay attention to.  Male pups cannot be bred back to their father and they will fight with other males so breeders will often cull all their male pups. In fact, any pup that is small, sickly, or doesn’t have the traits you want needs to be culled from the breeding program.


Culling is simply removing an animal from your breeding population. It is sometimes divided into Hard Culling and Soft Culling. Hard Culling refers to euthanizing ill or unwanted animals. Soft Culling refers to removing of the animals by re-homing or by simply not using them in your breeding program any longer.

It would be nice if all the mice you produce were healthy and you could find homes for them. However, the reality is that mice produce far more offspring that you could possibly rehome. Additionally, in the wild most of these individuals would not survive. This high mortality rate in the wild is why mice produce so many offspring. In the captive breeding environment this makes culling a useful tool for the breeder.

There are two acceptable means of culling are Carbon Dioxide (CO2) for adult mice and Hypothermia for mice under 7 days old. The best way of using CO2 is by means of compressed gas cylinders as this allows you to regulate the flow of gas. Its preferable that mice be euthanized in their home cage but if not, a clean cage should be used (AVMA Guidelines 2013). Some breeders create their own CO2 gas using citric acid and baking soda. A 2-liter soda bottle system designed for aquarium CO2 injection might be used. Additionally, some breeders use CO2 cartridges and valves designed for brewing. (NEED CITATIONS)

Hypothermia is an acceptable means of euthanasia but only for mice under 7 days old (aka pinkies). This is because baby mice do not produce their own body heat. Death by hypothermia can be achieve by gradual cooling in a refrigerator. Mice should not be placed directly on cold or pre-cooled surfaces as this can cause tissue damage and potentially pain (AVMA Guidelines 2013).

Some mouse breeders cull unwanted pinkies in the belief that the pups that remain are healthier (Royer 1998). Research has indicated that pups from unculled litters have a reduced weight of 4% but that this difference disappears by 3 months of age (Paigen, Marion, Stearns, Harper and Svenson 2014). Additionally, some breeders believe that culling the males (which are larger) and leaving behind females will produce larger healthier females (Royer 2014).


There are many factors involved in being an ethical breeder (Royer 2015) Being an ethical breeder involves being concerned with animal welfare. Animal welfare is allowing animals to be healthy, comfortable, safe and free from fear and pain among other things (American Veternary Medical Association 2019). Being an Ethical Mouse Breeder involves breeding animals that are selected for traits that improve the health and appearance of animals and not breeding sickly individuals. It involves humane euthanasia of unwanted animals and using those animals in some resourceful way such as for food or education. Please see my article on Animal Welfare for more information on this topic.


Once you’re setup with your mouse breeding program you might consider joining a Fancy Mouse Breeders Club.  This will allow people to find you as a breeder, puts you in contact with other breeders, and allows you to find and participate in mouse showing competitions.  Two common clubs in the US are the Fancy Mouse Breeders’ Association and the American Fancy Rat & Mouse Association.  When you join one of this clubs you will select your official breeder name.

In addition to Joining a club you will want to setup a simple website, facebook, Instagram, or other social media account for your mice.  It can be simple with your contact information and location.  It should also describe the types of mice you are working on.


American Fancy Rat and Mouse Association (2019) Retrieved from: AFRMA.org

American Veterinarian Medical Association (2019). What is Animal Welfare? Retrieved from: https://www.avma.org/KB/Resources/Reference/AnimalWelfare/Pages/what-is-animal-welfare.aspx

AVMA Guidelines for the Euthanasia of Animals: 2013 Edition.  AVMA Guidelines. Retrieved from https://www.avma.org/KB/Policies/Documents/euthanasia.pdf

Blom, H.J.M., Van Tintelen, G., Van Vorstenbosch, C.J.A.H.V., Baumans, V. and Beynen, A.C., 1996. Preferences of mice and rats for types of bedding material. Laboratory animals30(3), pp.234-244.

Connor, A.B. (2007) Aurora’s Guide to Mouse Colony Management at MIT. Retrieved from https://ki.mit.edu/files/ki/cfile/sbc/escell/mouseManagement.pdf

Hermenegildo, C., Marcaida, G., Montoliu, C., Grisolía, S., Miñana, M.D. and Felipo, V., 1996. NMDA receptor antagonists prevent acute ammonia toxicity in mice. Neurochemical research21(10), pp.1237-1244.

Mexas, A.M., Brice, A.K., Caro, A.C., Hillanbrand, T.S. and Gaertner, D.J. (2015) Nasal histopathology and intracage ammonia levels in female groups and breeding mice housed in static isolation cages. Journal of the American Association for Laboratory Animal Science54(5), pp.478-486.

National Research Council (2010) Guide for the care and use of laboratory animals. National Academies Press.

Paigen, B., Marion, M.A., Stearns, T.M., Harper, J.M. and Svenson, K.L., 2014. The effect of culling on health and physiology of mouse litters. Laboratory animals48(3), pp.207-215.

Royer, N. 1998. Culling: The question that has plagued the fancy for many years. American Fancy Rat and Mouse Association. Retrieved from: http://www.afrma.org/culling.htm

Royer, N. 2015. A Responsible Breeder’s Code of Ethics. American Fancy Rat and Mouse Association. Retrieved from: http://www.afrma.org/breedethics.htm

Royer, N (2014) Culling: The question that has plagued the fancy for many years. American Fancy Rat and Mouse Association. Retrieved from http://www.afrma.org/culling.htm

Sabine, J.R., Horton, B.J. and Wicks, M.B., 1973. Spontaneous tumors in C3H-A vy and C3H-A vy fB mice: high incidence in the United States and low incidence in Australia. Journal of the National Cancer Institute50(5), pp.1237-1242.

Smith, E., Stockwell, J.D., Schweitzer, I., Langley, S.H. and Smith, A.L., 2004. Evaluation of cage micro-environment of mice housed on various types of bedding materials. Journal of the American Association for Laboratory Animal Science43(4), pp.12-17.

Vlahakis, G., 1977. Brief Communication: Possible Carcinogenic Effects of Cedar Shavings in Bedding of C3H-Avy fB Mice. Journal of the National Cancer Institute58(1), pp.149-150.

Watkins, A., Wilkins, A., Cunningham, C., Perry, V., Seet, M., Osmond, C., Eckert, J., Torrens, C., Cagampang, F., Cleal, J. and Gray, W. 2008. Low protein diet fed exclusively during mouse oocyte maturation leads to behavioural and cardiovascular abnormalities in offspring. The Journal of physiology586(8), pp.2231-2244.

Genetic Learning Center (2016). Inherited Human Traits: A Quick Reference. University of Utah. Retrieved from: https://health.utah.gov/genomics/familyhistory/documents/Family%20Reunion/reference%20guide.pdf

Barber, N. (2015) What Behaviors Do We Inherit via Genes? Psychology Today. Retrieved from: https://www.psychologytoday.com/us/blog/the-human-beast/201509/what-behaviors-do-we-inherit-genes

Greenwood-Dudar, A. (2018) Establishing A Bloodline: Selection Of Brood Stock And Breeding Styles. American Dog Breeders Association. Retrieved from: https://adbadog.com/establishing-bloodline-selection-brood-stock-breeding-styles/


Published by Ryan David Tuttle

PhD Graduate student studying Behavioral Neuroscience, Addiction, Stress, Behavioral Economics, and Individual Differences. Former member Ministerial Servant and Pioneer in a Spanish speaking congregation of Jehovah's Witnesses.

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