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.

Thanks to their ability to produce milk, female mammals are able to feed their offspring for weeks, months, or even years after they are born. This sets the stage for the extended childhoods of many mammal offspring. With food and shelter taken care of (by the mother in most species), a remarkable amount of learning occurs for mammal offspring before they become independent. Elephants tend to their young for as long as 10 years, and humans for longer still. Young mammals may learn about food, shelter, migration routes, social structures, or other keys to survival from their parents. While extended childhoods support learning, they are costly, especially for the female. Lactating females must consume enough calories to feed their offspring and maintain themselves. Offspring that stay around after weaning from their mother's milk may still require protection and food. Human offspring are the most extreme case of investment, with parents usually providing intensive support until age 18 and continuing support beyond.