Shrews typically live on land, feeding mostly on insects, but also other small food such as worms or seeds. Some are specialized for life in trees or underground, but most sniff around on the forest floor. Because shrews have high metabolic rates, even above what you'd expect for such a small animal, they eat nearly their own body weight in food every day. To get more nutrients and calories, they eat their own feces (refection). Their vision is poor, and they use their good senses of smell and hearing to find food. When they find moving prey, some shrews immobilize it with venom dispensed through grooves in their teeth. While a bite from a venomous shrew does not kill a human, it can cause painful inflammation, which explains why people are described as shrews if they are painfully ill-tempered. Shrew venom is potent and has possible medical applications, such as in the treatment of migraines.

All mammals have hair, outgrowths of the skin, during at least some part of their development. While it evolved to insulate animals in cold climates, mammal hair has various functions. Long, stiff hairs (vibrissae, or whiskers) are found in all mammals but humans. Whether on the face, legs, or tail, vibrissae extend an animal's sense of touch. Some mammals, such as porcupines, have enlarged hairs that function as defensive spines. Defense is also achieved by raising individual hairs to make an animal look larger (such as an angry cat). Muscles in the skin make the hair stand up or lie down. Raising and lowering hair changes how much air is trapped under it, which also makes it more or less insulating. Mammals in cold climates tend to have an extra layer of insulating fur, or extra fat under their fur. Mammals in climates that change seasonally shed and replace their hair to create a thicker winter coat and a thinner summer coat.