Mammals have a number of unique features that have allowed them to adapt successfully to many different environments. They have the most highly developed nervous systems in the animal kingdom, providing them with complex and adaptable behaviors. With the exception of a few species such as the platypus, mammals do not lay eggs like other vertebrates; instead, mammalian embryos develop inside the mother and are not released until nearly or fully developed and equipped for survival. Mammals are also unique in having milk glands that provide nourishment for their infants. In this way, the protection and feeding of their young is built directly into mammalian bodies, dramatically increasing the ability of these animals to raise surviving offspring in diverse environments. Examples of mammals are whales, cows, mice, monkeys, and humans.