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.

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.