Marsupial mammals are best known for the belly pouch of many mother marsupials, such as kangaroos. The pouch is an adaptation to keep marsupial offspring safe, since they are very immature at birth. After no more than a handful of weeks of gestation in the uterus, newborn marsupials' hearts and lungs not fully developed. They may be as small as just a few millimeters (1 inch) long and skinny like a pinky finger. In species with a belly pouch, these puny young have strong front legs for crawling up to and into the pouch. Once there, they do what all mammals do--nurse on the mother's milk. The nourishment supports further development of their bodies, and teeth emerge. Their jaws house more cutting teeth (incisors) on the top than on the bottom, an arrangement that sets apart marsupials from other mammals.

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.