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 named for their special milk-producing structures called mammary glands. A female mammal may have two (like humans) or as many as 18 mammary glands. Each gland is full of small spaces lined with milk-secreting cells that all drain into a nipple. Newborn mammals have an innate sucking reflex that stimulates the mother's mammary glands to release milk. Milk is a superfood for young mammals, containing a nourishing mix of fats, proteins, vitamins, minerals, and even immune-system chemicals. The first milk (colostrum) contains antibodies from the mother to protect against disease. As the offspring grows, milk changes to maintain the right balance of nutrients. Production of milk (lactation) makes most female mammals temporarily unable to get pregnant, resulting in natural gaps between offspring. The spacing of births benefits both the offspring and the mother in terms of survival. Mother mammals stop nursing (wean) their babies after several days or years, depending on the species and environmental conditions.