Updated: Dec 26, 2022
First and foremost, if you don't already have mice, get some and get used to caring for them before you go into breeding. It might turn out that you aren’t all that thrilled about the smell, or you are allergic to most beddings. Even if everything goes well, you want to have a good understanding of how to house and care for mice before you jump into the added information about breeding! Breeding seems really simple, and on some level it is. Put a boy and a girl together and babies will happen naturally. That is true! Most of the time. But there is a lot more that goes into it than that, and many factors that make breeding not right for everyone! It is okay to love mice and realize that breeding them is just not your idea of fun. You can always go back to having them as pets if you want! If you decide you want to breed, it’s helpful to know what you are breeding for. Do you want to produce amazing show mice that other show breeders will want to buy from you, or just for you to show? Do you want to breed pet mice to sell to others? Do you want only one litter to give yourself more pet mice? Do you want to breed feeder mice for yourself or others? Each one of these is completely valid, but they don’t all have the same methods and needs from you. There is also no reason you have to choose just one of these options. For instance, I breed for both pet and show purposes.
THINGS TO KNOW FOR ALL BREEDING TYPES
FACTOIDS Does have a 5 day heat cycle, and are sexually receptive for less than a day of that cycle. This is called estrus. Mice have a 19-21 day gestation period. The natural nursing period is about 21 days, though many will nurse for a little longer than that if the mom allows it. Does have 10 nipples (bucks don’t have any!)
Mice can have litters of 20 or more babies each, though the average is closer to 12 and is different from line to line. Culling means to remove from your breeding program. This can be from euthanasia or from making that mouse pet only. Some form of culling is required to have a healthy breeding program no matter what your breeding purpose is.
PICKING WHICH MICE TO BREED
Get the healthiest and friendliest mice you can find to start your breeding program. Even for the feeders, it's no fun to deal with super flighty or aggressive mice. It's not worth the effort, stress, and pain. Most of mouse temperament is genetic, so if you don’t start with friendly mice you will have to work VERY hard to get them, and it will take several generations of breeding. Whatever level of friendliness your breeders are, always pick the friendliest of the offspring to keep as breeders and you will get friendlier mice going forward. You shouldn't have to work on taming them for them to be handleable. If there are certain colors or coat types you want to work with, look into what genetics you will need in your breeding program to get them. Some things are easy to find or just need one gene to get. Others are hard to find, or are a combination of genes. In those cases, you might not be able to start right in with your ideal mice, but you might be able to get there eventually if you start with all of the parts. I will get more into genetics in other blog posts, but it is a thing to keep in mind.
Mice breed better in smaller spaces than they do larger. Get used to looking for info from breeders rather than pet owners, because there is a very different mindset for a reason. Mice can be really picky and not breed for you if they are stressed, so breeders work to limit stressors (unless you don't want a group to breed, in which case they will breed no matter what is happening, seriously, one accidental 30 second exposure with a pair you don’t want to breed will 100% result in pregnancy.) I am not saying go super tiny, just not huge. For a group of 3 or more, 50 to 75 square inches per mouse is plenty of space.
Mice will need a hide and bedding like usual, but once the does are about ready to give birth, it is good to remove things like wheels and most of the distractions. Moms have been known to put babies on wheels and then fling them off when they start running. It is hilarious to think about, but not so great to have happen. Some does will also get so distracted with the other stuff that they will neglect their babies. In their place, provide lots of different types of options for your mice to make nests out of. This can be hay, paper towels, different types of bedding, or even pieces of cardboard. These will provide enrichment in the form of tearing things up and moving them around, without distracting your does from their litter. Since you will not have a lot of clutter to take up extra space, it is good to provide a fairly small nursery habitat. For a single doe, you don’t need anything bigger than a shoe box until the babies are getting close to opening their eyes. They will feel a lot more secure this way. Once babies are up and moving around, you can move them into a larger space and add all the other enrichment back in with them!
COMBINING MICE When you are combining a buck with does, you can put the does in with the buck, or put them all into a neutral territory. If you put the buck in with does in their space, they might become territorial and attack the buck. This is more likely to happen if they are not sexually receptive at the time. I leave a buck with does until I see they are visibly pregnant, but that is a personal choice. I like knowing for sure that a pregnancy has happened, because even if I see a mating take place, that doesn’t mean pregnancy will happen from that mating. I also like giving my bucks company for as long as possible, since they *cannot be housed with other bucks. It is important to separate the buck from the does before the does give birth or you will have back to back litters (possible exception being feeder breeding, will go into that later), because does go into estrus and will mate the day they give birth.
Some people choose to keep does together to raise their litters, others separate them. There are pros and cons each way, so this is very much a personal preference situation. If does are kept together, they will share feedings and other caretaking responsibilities. They also keep each other company and often end up with babies that are more socialized. On the other hand, some will fight over where to put the babies and this can result in injury as they pull newborns in different directions. If one of the moms turns out to be a baby muncher (yes, mice are sometimes cannibalistic) you will lose the whole group. You also usually will not be able to tell which mom gave birth to which baby, and that makes tracking down any health or genetic issues much more difficult. If you have the moms raise babies individually, you will be able to keep more of the babies than if you combined the litters without risking the health of mom and babies (more about this later on.) There are fewer things that can go wrong and it’s easier to find the source if they do. It will take up more space, though, and the moms will be lonely between when they are removed and when the litter is born.
ONCE YOU HAVE BABIES
You can and should check babies soon after they are born, even within 24 hours. Remove obviously unhealthy babies and do a head count. You can also do sex checks and start reducing the litter size (I will go into this more when I talk about differences in breeding goals). Reduce to the amount you want to keep on the doe by day 5 at the latest.
Learn what failure to thrive (FTT) looks like. Even if you do everything right, some babies just don't make it. FTT happens because something is not right in their bodies. Either they can’t process food the way they should, they have severe anemia, or they have some other internal problem, these babies will never be healthy and usually die a slow and painful death. If they are FTT, it's better for them to be humanely euthanized than to suffer.
If the moms are aggressive with you during this process, do not breed those moms again, and those babies should not be kept. Some mice are super sweet until they give birth, and then hormonal/maternal aggression kicks in. This is still a form of aggression that can be passed onto the offspring, and you do not want aggressive mice in your lines!
Prepare for HOPPERS!!!! They will go from tiny pink aliens to adorable silky puppy things to nightmares. One day they will be sweet little wiggly things about to open their eyes, and then next day eyes are open and the whole world is new and terrifying. They will fling themselves, they will suicide jump, they might try to nip. You can choose what you will tolerate in hoppers, but generally, they grow out of that in a week or two and start transitioning into their adult behavior. Occasionally you get lucky (or work hard with breeding) and get a super chill litter of hoppers. It is wonderful!! It isn't likely going to happen right out of the gate, though. Don’t worry, they will get better and they aren’t like this because you did something wrong. The stage of development is called hopper for a reason.
DIFFERENCES IN BREEDING PURPOSES
The size of your breeding groups will be different depending on what you are breeding for. With pet and show breeding, you do not want to end up overrun with mice. Having one buck and 1-3 does per breeding group is common. You don’t need to breed them all at the same time, and you might have multiple groups if you are working on multiple things, but you want controlled numbers. Many people have a spare buck just in case something happens to the main buck in a group. This is advisable, but you also need to consider how much space you want to devote to each group.
Feeder breeders often want large numbers, especially if the goal is to sell them. When large quantities of mice is the goal, you want much larger groups! This is where many groups also come in handy, even if they are all being bred for the same goal. One buck and up to 10 does or more per group, and as many groups as needed to make up numbers the goal numbers will be used. If you are just starting out, though, it’s good to start small and grow, so you can build your breeding facility as you build your customer base.
I mentioned before that you don’t want back to back litters. In general, it wears out the does and the second round of babies do not get enough nutrients in the milk. Feeder breeders, however, will often colony breed. The means they leave the buck in with a larger group of does and will often have back to back breeding. There is debate on how ethical this is, but there are better and worse ways of doing it. By reducing the litter size as I suggest, the does won’t get worn out as quickly as if they are left all in together. It is also possible to pull breeder does out to be used as feeders before back to back litters wear them down. If the youngsters are going to be used as feeders anyway, the shortened lifespan of not getting the best milk is not a factor. Just make sure the holdbacks to be bred are the ones in better shape.
Depending on your breeding goals, different litter sizes are needed. Show breeders tend to keep only 4-5 babies per litter on each doe. The reason for this is so each baby is able to get as much milk as they want, with no fighting over nipples. This allows for them to grow as big as possible and be as healthy as possible.
Pet breeders don’t necessarily need the biggest mice, but health is still very important! For feeder breeding, health might not be as important, but you also don’t want to have runty and sickly mice. The health of the mom is also something to consider, because nursing a litter takes a lot out of them, and they can’t provide the needed care if their own resources are depleted. In these cases, 8 babies is what I suggest as the maximum for a single doe, but 6 is what I would recommend for pets.
These numbers change if you keep your does together to give birth and raise the litter! You can’t just multiply the number of moms times the number of babies per mom. Mice do not take turns well, and not all moms buddy up well. Often, one mom will do most of the feeding, and even if it is split, many babies will try to nurse each time either is there. Does will not consistently feed at the same time, so smaller and weaker babies will get kicked off the teat, which will lead to an increase in size and strength differences and can cause FTT from inadequate nutrition. For 2 does together, 10 is the maximum I would recommend for pets, and 6 is the maximum I recommend for show.
Okay, so now that we know what litter sizes should be, the big question is how to make that happen. This is the worst aspect of breeding by far. It’s up to you as the breeder to reduce the litter size. That means humane euthenasia. It is horrible to think about, and it’s worse to actually do, but there are so many reasons why it is needed. I will expand on the need to cull litters down in another blog post. The most common method of culling pinkies in the United States (where I am located) is freezing. This can only be done with pinkies, not older mice. Pinkies have not developed their pain receptors or thermoregulation yet, so it is fast and does not hurt them. There are other methods that you can look into as well if this does not work for you.
To determine which babies to euthanize, we go back to breeding purposes. In general, cull bucks first. *They need to be housed alone once they reach sexual maturity, plus they are milk hogs as babies. They are usually the bigger ones that kick the smaller ones off the teat. All of the mice you keep will be healthier if the bucks go first, so only keep the ones you actually need. For pet and show breeding and working with marked varieties, do your final selection on days 3 to 5, when you can see their colors. This will allow you to keep the best looking of the bunch. If you are hoping to get specific colors, you can start selecting based on eye color from day one, since that can be seen through the skin despite eyes being closed. For show, you want to take into consideration the comparative size of the skull, shape of the nose, and length and width of the tail. The best ones should be held back. Feeder breeders don’t need to look at any of this, so other than getting rid of those milk hog boys first, it doesn’t really matter!
ONCE BABIES GET BIG
As I mentioned in factoids, mice will nurse until they are at least 3 weeks old. They should absolutely be kept with their moms until that point, but I like to give extra time for social development. I move any bucklings away from the group at 4 weeks, because that is the time where they can start to become sexually mature. Some take longer, and if you know your line, you can extend that time to what works for you. While doelings take longer to be sexually mature, bucks can and will impregnate their mothers as soon as they are ready. Doelings, on the other hand, can stay until there is some reason to move them. If you are going to sell your mice as pets, show mice, or breeders, it is good to wait until they are at minimum 6 weeks old. By then, they have been on solid food long enough that you will know they have successfully made the switch, and they should be out of their hopper stage. Any earlier than that, you might miss an unhealthy mouse, or you might send someone with animals that have not shown their adult temperament yet. People tend to be unhappy when they get mice they can’t handle because they still occasionally fling themselves into space! This also gives you as a breeder time to assess which you will hold back for your breeding program, which might not be up to your standards for your goals, and what personality matches might be good for the needs of your buyers. For the ones you are adding to your breeding program, wait until they are 8 to 10 weeks (for pet) or 12 weeks (for show) before breeding. This way your mice have time to grow and mature, and you can continue to assess them for how well they match your needs and goals.
HAPPY BREEDING EVERYONE!!!!
*Going back to my comments about bucks needing their own spaces, this is very important to consider. Bucks are territorial of other bucks by nature. When they are freshly weaned, they can often stay together for a short period of time, but will almost always become aggressive and can easily kill each other if left together. Unless you are experienced with breeding and are working specifically toward a breeding line where bucks can cohabitate, please do not ever put adult bucks together.