From whales swimming in the ocean to bats flying through the night sky, mammals are one of the most diverse and adaptable groups of vertebrates on Earth. Like all vertebrates, mammals have a backbone that supports their bodies. However, mammals are set apart by several advanced traits. They are endothermic, meaning they regulate their own body temperature, allowing them to survive in a wide range of environments. Mammals also have lungs, a four-chambered heart, and a two-loop circulatory system, which efficiently delivers oxygen throughout the body. Their large brains and complex nervous systems support learning, memory, and behavior. All mammals reproduce through internal fertilization.

Another key feature of mammals is how they care for their young. Most mammals give birth to live young and nourish them with milk produced by mammary glands. This milk provides essential nutrients needed for growth and development. Mammal offspring are born with the instinct to suckle, allowing them to feed from their mother shortly after birth. Compared to other vertebrates, mammals typically spend a longer time caring for their young, increasing their chances of survival.

One defining characteristic of mammals is the presence of hair, which all mammals have at some point during their development. Hair is made of keratin and serves several important functions. First, it provides insulation by slowing heat exchange with the environment. Second, specialized hairs, such as whiskers (vibrissae), act as sensory receptors that help animals detect their surroundings. Third, hair can offer protection, either by shielding skin from the sun or by forming sharp structures that deter predators. Finally, hair color and patterns can serve as camouflage or communicate, such as attracting mates or warning other animals.

Mammals also have specialized teeth adapted to different diets. The shape and size of teeth can reveal what a mammal eats. Herbivores, like deer and horses, have incisors for cutting plants and molars for grinding them. Carnivores, such as lions, have sharp canines for tearing meat. Omnivores, like chimpanzees and humans, have a combination of teeth that allow them to eat both plants and animals.

Mammals can be divided into three main groups based on how their young develop. Marsupials, such as kangaroos and koalas, give birth to underdeveloped young that continue to grow in a pouch. Monotremes, including the platypus and echidna, are unique in that they lay eggs rather than giving birth to live young. They do not have nipples but release milk through pores in their skin. The largest group, placental mammals, includes animals like rabbits, lions, and humans. In these mammals, the developing embryo is nourished inside the mother’s body by a placenta, which is connected to the embryo by an umbilical cord.

Review:

1. Compare herbivores to carnivores and omnivores.
2. Identify two characteristics of marsupials.
3. What is the function of the placenta?