The main problem of small mammals is their high surface to weight ratio: they have a big surface area for such a small mass. Since heat is radiated from the surface, they have adaptations to help them keep their temperature up to normal. They need to eat frequently, and live in areas where they are protected from the coldest weather. Easy access to high-quality vegetable food (such as grains) and a protected habitat helps them survive and multiply. That is why they are found in numbers living in human settlements.