Placental mammals are a group of mammals that give birth to live young, which develop inside the mother's womb and are nourished through a specialized organ called the placenta. This group includes a wide variety of species, such as humans, dogs, and whales, showcasing the diversity of life forms that share this reproductive strategy. Unlike marsupials, which give birth to underdeveloped young that continue to grow in a pouch, placental mammals have a longer gestation period. This allows their offspring to be more developed at birth, increasing their chances of survival in the wild. The placenta plays a crucial role in this process, providing essential nutrients and oxygen to the developing fetus.