5 reasons why your mice aren't breeding


Have you ever had issues with poor breeding performance from your mouse colonies? Do you ever wish you were Dr. Doolittle so you could ask them why they aren’t mating? Fortunately, our experts are well trained in mouse husbandry and can help you better understand their needs.

1. You’re stressing them out!

Although we have the best intentions, checking on mice every day (or every other day) can cause unnecessary stress, which, ultimately, can affect breeding performance. Between cage changes, daily cage and water level checks, and experimental procedures, mice undergo a lot of disruptions to their everyday lives. Changing cages more frequently than 1-2 times per week can agitate the mice, and if you are also frequently opening the cage to check on the well-being of the mice, they may exhibit reduced breeding performance.

Besides over-handling, removing males from breeding cages can negatively impact breeding performance, too. Separating males from their pregnant mates can stress the female(s) and may induce them to either resorb their litters in utero, or abandon or cannibalize them once born.  In most of our colonies at JAX, males remain with their female counterparts AT ALL TIMES.  If the male must be removed, try to wait at least until the pups are delivered, and do not return him to the cage until the litter is weaned.

Leaving males in the cage also increases a colony’s overall productivity by taking advantage of postpartum estrus – the phenomenon in which females enter estrus and are receptive to mating immediately following a litter’s delivery.  If the male is still present, the female may become pregnant with a second litter while she is nursing her first, and deliver the second 3-4 weeks later, increasing her productivity.  If the postpartum estrus window is missed, the females won’t be receptive to mating until after their litter has weaned.  Be sure to check with your facility managers, however, on how many mice they allow per cage.  If a second litter is born before the first has weaned, you may need to screen and cull unwanted mice from the first litter, or wean the entire first litter early. Housing the mice in larger cages to allow for multiple litters may be an option as well.

Mice are sensitive creatures, so noises and vibrations that are unnoticeable to humans can greatly affect them. Making sure that your touchy breeders are not located near a door or sink where there may be heavy traffic or loud noises, and reminding caretakers to work gently, slowly, and quietly when handling the mice can markedly improve breeding performance. Adding “white noise” to the room or playing music in the background can help to mask the shock of any loud noises that occur unexpectedly or that cannot be reduced or avoided.

Mice also have a highly developed sense of smell, and odors from perfumes or skincare products may cause undue stress in your colonies.  Similarly, odors and pheromones from other cages can be carried on your forceps or gloves and transferred from one cage to another, which could contribute to increased aggressive behavior and less productive matings. For this reason, be sure to disinfect your gloves and forceps between cages.  Disinfecting and changing gloves frequently also helps minimize the spread of pathogens.


2. Give them somewhere to hide (and play)!

Think about it- when mice are out in the wild, they aren’t out in the open for everyone to gawk and point at. They love to nestle and burrow into their nests to hide from frightful human eyes (and brooms!). The same is true of laboratory mice.  In a cage devoid of any enrichment (a fancy term for mouse toys), many strains display unfavorable behavior, including barbering, reduced eating and reduced breeding.  Many enrichment options are available, including Shepard Shacks®, Nestlets, igloos, and biotunnels.  Any of these can greatly enhance not only the mental well-being of your mice, but also their breeding efficiency. In fact, some of our most commonly used strains- C57BL/6J (000664) and NSG (005557), for example – are housed with some form of enrichment to enhance wellness and provide psychological stimulation.


3. Help them set the mood with the right food!

Not all mice are created equal, and the food on which one strain thrives may not be the best for another. For example, C57BL/6J mice are maintained on a 6% fat diet, whereas CAST/EiJ mice (000928) perform better on a diet containing 10% fat while CC059/TauUncJ (025125) perform much better on a 4% fat diet than on a 6% diet. You can also try using so-called “breeder chow” diets, which typically contain 10-12% fat. Just note, however, that some strains (e.g. C57BL/6J) may become obese on higher-fat diets, which, ultimately, may diminish their breeding performance.

A diet supplement that JAX investigators have used to enhance breeding performance in their colonies is so-called “Love Mash.”


42 oz oatmeal

200ml wheat germ

200ml brewer’s yeast

150ml cod liver oil


If you’re not equipped to cook up your own “Love Mash,” one diet manufacturer, BioServ, offers pelleted “Love Mash” containing the same ingredients.


4. Turn the lights down low.

As we all know, mice are nocturnal animals; they are most active when it’s dark. Indeed, female mice typically ovulate and are receptive to mating only in the middle of the dark cycle.  For this reason, it’s important to ensure that they have an adequate dark period for them to mate. At JAX, all of our mouse rooms are maintained on a light/dark cycle of 14 hours on and 10 hours off.  Many institutions use a 12 hour light/12 hour dark cycle, which is perfectly fine too.  Just make sure that the mice have at least 10 hours of uninterrupted darkness.  Once the lights go off, they should stay off!  Make sure, too, that there aren’t other light sources in the room (for example, from equipment) that can disrupt this crucial period.

If you find that you need to enter your vivarium in the early hours before the lights come on for an injection or to collect a measurement, you may need to invest in some night-vision head gear, because you won’t want to disturb the breeding cycles of your mice.


5. Somebody call the doctor; they are sick!

Another factor that can influence mouse breeding success is their health status. If you are using immunodeficient strains, including NOD scid (001303), Nude (007850 and 002019), Balb/c scid (001803 ), and the most immunodeficient mouse strain, NSG (005557), you must be diligent in ensuring that the room where they are maintained is as clean as possible.  For some strains, it may not be as obvious that they are more sensitive to pathogens in the environment, such as B6.129P2-Il10tm1Cgn/J (002251) B6.129P2-Il2tm1Hor/J (002252), and B6.129S7-Rag1tm1Mom/J (002216), but they may not breed as well (and their phenotypes may also be affected) if the health status of the room isn’t sufficiently clean. You can find information regarding the pathogens that we monitor in our mouse rooms on our Animal Health reports, links to which you can find under the “Health & Care” tab of every strain datasheet.  A more comprehensive description of our Animal Health Program is available from our website, too.


Bonus: Know the reproductive performance of your strain!

Despite our best efforts, there are some strains that no matter what we do still breed poorly. Therefore, it is important to know the fecundity of your background strain so you know what to expect. If you’re asking “Where can I find that information?” look no further than our own Mouse Phenome Database! Here you can find superovulation rates, percentage of productive matings, and overall fecundity of some of our most popular strains. And if you still can’t find what you’re looking for, you can always contact your friends in Technical Information for additional help with troubleshooting non-productive mice.