Animals must consume food to obtain the energy and nutrients necessary for growth, maintenance, and reproduction. Energy provides fuel for movement, digestion, and other bodily processes. Nutrients are used to build and repair body structures and to produce enzymes and other essential molecules.
Nutritional requirements vary among animal species. Herbivores consume only plant material, while carnivores eat only other animals. Some animals are omnivores, consuming both plants and animals.
Animals have evolved various adaptations to obtain food efficiently. Some have sharp teeth and claws for capturing prey, while others have long necks and tongues for reaching foliage. Some animals are able to store food in their bodies, allowing them to survive long periods without eating.
Animal Function #2: Reproduction
Animals reproduce to ensure the continuation of their species. Reproduction involves the production and fertilization of gametes, or sex cells. In many animals, the sexes are separate, meaning that there are males and females. In some animals, the sexes are combined, meaning that each individual is both male and female.
Fertilization can occur internally or externally. In internal fertilization, the male gamete (sperm) fertilizes the female gamete (egg) inside the female body. In external fertilization, the sperm and eggs are released into the environment, where the sperm fertilize the eggs outside of the female body.
Animal Function #3: Growth and Development
Animals grow and develop throughout their life cycles. Growth is the increase in size, while development is the increase in complexity. Development involves the formation of new organs, tissues, and cells. Growth and development are regulated by a variety of hormones and other signaling molecules.
The growth and development patterns of animals vary greatly among species. Some animals grow quickly and reach sexual maturity early in life, while others grow slowly and take longer to mature. Some animals undergo a dramatic metamorphosis from a juvenile stage to an adult stage.
Animal Function #4: Movement
Animals move to obtain food, find shelter, avoid predators, and find mates. Movement requires energy and is facilitated by a variety of adaptations, including muscles, tendons, joints, and skeletons.
Animals move in a variety of ways, including walking, running, flying, swimming, crawling, and slithering. Some animals are capable of great speed, while others move very slowly.
Animal Function #5: Response to the Environment
Animals respond to the environment in a variety of ways, including behavioral, physiological, and evolutionary responses. Behavioral responses are changes in behavior, such as moving away from a threat or seeking out food. Physiological responses are changes in the body's internal environment, such as an increase in heart rate or blood pressure. Evolutionary responses are changes in the genetic make-up of a population over time, such as the development of resistance to a pesticide or the ability to use a new food source.
Animals respond to the environment to maintain homeostasis, or a stable internal environment. Homeostasis is necessary for survival, and animals have evolved a variety of adaptations to help them maintain it.