How to Know if Your Mouse Is Obese
To know if your mouse is obese, use your thumb and middle finger to hold it still by its tail. Avoid squeezing or pressing on the tail, which can hurt your mouse. Then, look down at it from above to see if it looks bulky and wide, which means it's carrying extra weight. Alternatively, find your mouse's spine with your fingers. If its spine feels smooth or if you can't feel it at all, your mouse is overweight. Once you determine that your mouse needs to lose weight, ask your vet how to safely reduce its calories while increasing its activity levels.
Position your mouse. Body condition is an assessment of how an animal looks. It ranges from emaciated (extremely underweight) to obese (extremely overweight). To determine your mouse's body condition, you will hold it still and examine a few parts of its body. Hold your mouse still by holding the base of its tail with your thumb and middle finger. Hold your mouse in its cage or on another flat surface, like the floor or a table. The base of the tail is where the tail is attached to the body. Do not squeeze the tail too tightly. This could hurt your mouse and damage its tail. Your mouse may not want to stay still for long. If it starts squirming, let the tail go in the cage or place your mouse back in the cage. Locate your mouse’s spine. How well you can see and feel your mouse's spine is a good indicator of body condition. If your mouse is overweight, the spinal column will feel smooth and continuous, meaning you can't feel individual vertebrae. If the mouse is obese, you may not be able to feel the spine at all. To apply pressure, use one or two fingers from the hand not holding the tail and press down on the spine. Do not press down so hard that your mouse becomes uncomfortable. The vertebrae of a mouse at an ideal weight would be felt with only slight pressure to the spine. For an underweight mouse, you would be able to see the sharp points of the vertebrae without even touching them. Look for your mouse’s hip bones. The ability to see and feel the hip bones is another indicator of body condition in mice. The hip bones are located near the bottom of your mouse's spine. In underweight mice, the hip bones would be very easy to see because they would not be covered with much flesh, causing them to stick out like sharp edges. If your mouse is obese, though, you would not be able to see or feel the hip bones at all because they would be covered with a lot of flesh and fat deposits. For a mouse at its ideal weight, the hip bones would not be visible. However, you would be able to feel them with slight pressure. Look at your mouse’s overall physique. If your mouse is extra squirmy and won't hold still, you could simply at look at your mouse to determine if it's obese. Looking down at your mouse from above, if your mouse looks really bulky and wide, then it is obese. If you look at your mouse from the back, it would essentially look like a large, wide mound if it is obese. You would not be able to make out any bony structures. Consult with your vet. If your mouse is obese, it should lose some weight. Before you start making changes to your mouse's diet and daily routine, talk with your vet first. Your vet can determine your mouse's ideal weight and give you suggestions on how to help your mouse gradually lose weight. In general, you will need to reduce your mouse's caloric intake and increase its activity level. Feed your mouse a small portion of pellets. Overeating is a frequent cause of obesity in pet mice. Give your mouse only 1 tablespoon (14.8 mL) of pellets per day. This may not seem like a lot, but it will be enough for your mouse. Use a measuring spoon to make sure you give your mouse the proper amount of pellets. Add small amounts of fruits and vegetables to your mouse’s diet. Fruits and vegetables will add important nutrients to your mouse's diet. Pet mice don't need a lot of fruits and vegetables each day, though. Think of these foods as treats and feed them in only small amounts. Examples of fresh produce are an apple slice or a few blueberries. Kale, celery, and peas are also good feeding choices for mice. Some fruits and veggies, like grapes, raisins, and rhubarb, are toxic to mice. Do not feed these to your mouse. Wash the produce with water to remove pesticides. Stop feeding your mouse seeds. Mice like eating sunflower seeds. However, because these seeds are high in fat, a pet mouse could become obese by eating too many of them. If you have been feeding sunflower seeds to your mouse, take them out of its diet. Food mixes containing seeds and grains can also make a mouse obese. If your mouse is obese, do not feed it a seed/grain mix. Instead, feed it a completely pelleted diet. Scatter your mouse’s food through its cage. In the wild, mice will forage for their food. To help your mouse become more active, scatter and bury its food in the cage. This will encourage your mouse to move around to find and eat its food. When you clean your mouse's cage, remove bits of uneaten fruits and veggies to keep them from spoiling. Put an exercise wheel in your mouse’s cage. In addition to eating less, your mouse will need to exercise more to lose weight. Exercise wheels are a great way to get your mouse to exercise. If you do not already have an exercise wheel for your mouse, purchase one that has a smooth surface so your mouse's feet don't get stuck between the wires. The wheel should be at least 6 to 8 inches in diameter. If your mouse is obese, it may not spend much time on the exercise wheel at first. However, as it loses weight from eating less, it may use the exercise wheel more and more. If your mouse does not like the exercise wheel, you could instead put it in an exercise ball and allow it to roam around a room. If you try this, make sure to supervise the mouse while it's in the ball. 
Position your mouse. Locate your mouse’s spine. Look for your mouse’s hip bones. Look at your mouse’s overall physique. Consult with your vet. Feed your mouse a small portion of pellets. Add small amounts of fruits and vegetables to your mouse’s diet. Stop feeding your mouse seeds. Scatter your mouse’s food through its cage. Put an exercise wheel in your mouse’s cage. 