It might seem odd to group Cetaceans (whales) with Artiodactyls (hooved animals such as pigs, deer, camels, llamas, and hippos). But that is exactly what the combination of their scientific names into Cetartiodactyla is about. For some time, scientists had proposed that whales descended from land mammals, with the focus on the hooved fossil Mesonychia. In the 1990s, DNA sequences from many different genes revealed a closer relationship between whales and hippos than between hippos and any other hooved mammals. While it is not yet known what common ancestor whales and hippos share, the newly discovered relationship is getting attention. It appears that whales and hippos may have branched off from Artiodactyls as long as 60 million years ago. Scientists have proposed a new group called Whippomorpha (wh[ale] + hippo[potamus]; morphe = form) to include whales and hippopotamus and exclude other hooved animals.

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.