loss. Older mice (older than a year) are susceptible to tumors, especially breast cancer in females, as the mammary tissue is distributed around much of the body. Other common cancers in mice are leukemia and lymphoma. The reason that mice, as well as many other small animals get ill easier is due to their genomes containing more viruses. Persistent problems should be referred to a veterinarian, although finding a veterinarian with experience in treating mice can be difficult, however not impossible. Like people, fancy mice can become obese if they overeat and do not get enough physical activity. This can lead to them developing life-threatening cardiovascular disorders and diabetes as well as arthritis. Activity aids such as tubes and wheels are useful for ensuring mice get enough exercise, as well as mental stimulation. Mice also love to climb, and a wire cage with horizontal bars is perfect for this. A mouse set loose for exercise should be carefully observed, as they tend to scurry into a hiding spot and can be difficult to retrieve. As rodents, mice must gnaw to keep their incisors from growing too long. Overgrown teeth can cause occlusion (blockage) of the mouth, which in extreme cases can lead to starvation. Hard foodstuffs, small pieces of wood or specially prepared blocks can suit this purpose, although some mice can grind their teeth together ("bruxing") to keep them short. In rare cases, a mouse may not be able to gnaw effectively, either from malformed incisors or jaws, and so its teeth must be trimmed by a veterinarian. Mice self-groom and do not need to be bathed, unless they have a skin condition which requires special treatment. Also, like people, mice can get diarrhea. For humans in developed countries with access to clean water, this is usually not a life-threatening condition. For mice though, it can be. Once a mouse gets diarrhea, since it is so small, it can very easily lose too much liquid, causing dehydration, and die. Breeding Mice have a rapid reproduction rate; the gestation period is approximately 19 to 23 days. The average litter size is four to 12 young. In some instances, up to 30 young have been born. However females have 10 nipples so may only feed this many pups at any one time. The pups in larger litters are often weaker, smaller animals that can weaken the doe, increasing recovery time. Litters