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.

Mammals are able to make enough internal body heat to keep their bodies at a relatively constant, high temperature. Endothermy (endo= inside; thermic= heat) makes mammals less dependent on outside temperatures, freeing them to be active in a wide range of conditions. Mammals may be active at night (nocturnal) or in the day (diurnal), in climates as extreme as the cold poles or the hot tropics. Adaptations to warm up or cool off, such as shivering and sweating, allow mammals to maintain their preferred body temperatures in the face of these extremes. Mammals also regulate body temperature with behavior: elephants flapping ears to cool off, humans wearing coats to warm up, dogs panting to cool down. The hairs making up mammal fur are raised or lowered to provide more or less insulation as needed.