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Is this a runt, or is this a failure to thrive??

One of the questions that comes up regularly with new breeders is how to tell if a baby mouse is a failure to thrive (FTT) or a runt. The short and simple answer is that while runts are smaller than others, failure to thrive mice are unable to live healthy lives because of some sort of major problem. 


Most of the time, runts can grow up to adulthood and be fairly healthy. I never recommend breeding runts because they don’t have the strongest genetics, but they usually make perfectly fine pets. Runts will meet every milestone at the same time as the rest of their litter, they just don’t grow as quickly. Sometimes they catch up in size later in life, and sometimes they don’t. 

Runts can be less thrifty than other mice. They might get sick more easily, or be the first ones to show symptoms of dehydration if the water bottle is stuck. In general, runts tend to be weaker than their full sized siblings. This sometimes leads to a slightly shorter lifespan than others in their lines and litters, but not always. There are plenty of times where runts are just as healthy as others even if they don’t catch up in size.

For breeding programs, many breeders choose not to keep runts if they show signs soon enough. Often the goal is to only keep the biggest, strongest, and healthiest mice in a litter. If you have more babies on a doe than she can comfortably take care of, it makes sense for runts to be among the ones reduced from a litter. This is a completely ethical practice.


Failure to thrive mice, on the other hand, do not meet milestones.  Sometimes they show health problems as soon as they are born, with visible anemia or other abnormalities. Other times the problems don’t become obvious until the mice are being weaned off of milk and have trouble digesting solid foods. Either way, these are mice with health issues.

In pinkies, FTT can look very pale or malformed. These are easy to spot and are often cleaned up by the mothers if they are left in the nest. I do not recommend leaving them for the doe to eat, because once she starts cleaning up after the ones that won’t survive it can lead to over grooming or reducing the litter beyond just the unviable pinkies.

In older babies, FTT often presents with shows of pain, dehydration, or malnutrition. Malnutrition in young mice can be seen as being significantly skinnier than the others in the litter, or sometimes with a distended or bloated stomach that is out of proportion with the rest of the body. Dehydration presents as visible tail bones. If you can see the bumps of the vertebrae in the tail, there is something wrong. With both malnutrition and dehydration, you can and should check to see that appropriate food and water are still accessible to them. Check water bottle nozzles, see if they are out of food, see if the doe is showing signs of illness that might be reducing the quality of her milk. Failure to thrive is not the only cause of these and sometimes you can make necessary adjustments and save the mice.

Pain in mice can show in a lot of different ways. With failure to thrive, it’s common to see a bobble-head type appearance. The body is a bit sunken, the fur on the head is a bit puffed out, and the muscles supporting the head and neck are weak so they get a bit wobbly. It can also look like squinting eyes, pinched face, and ears held mostly pinned back. Signs in the body include sunken in sides, heavy breathing, puffed out hair, or a hunched up back. The hunched back generally comes with stiff joints in the legs as well, so they walk with a side to side sway.

If there isn’t an obvious and fixable reason for your youngsters to be in pain or have dehydration or malnourishment issues, chances are good that you have a failure to thrive. To be cautious, you can try to make adjustments or monitor them for a few days to see if their condition improves. If it doesn’t improve or gets worse, you have a more solid answer.

With failure to thrive mice, the kindest thing you can do for them is humane euthanasia. These are mice that will not get better. They have something fundamentally wrong that is causing them to not mature the way they should and will almost always die a slow and painful death. There has been one case I have seen where the mouse survived, but was unhealthy, malnourished, and showing signs of pain in every picture I saw. Please do not cause your mice to live their lives in pain. Do what is right for them, even though it is hard for you.

Good luck with all of your litters!!

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