All living things (organisms) have a life cycle. Different organisms may have very different kinds of life cycles, but they all have these in common: they are born, grow up, reproduce and die. They need to reproduce or they will go extinct. So, what kinds of things have organisms evolved in their life cycles to survive in their habitats?
Mammals are born singly or in a small group. They are small and helpless at birth, but they have a much higher survival rate than other groups of animals. How? Because their parents feed and protect them until they can survive on their own. Some mammals are born into a nest, den or burrow that is prepared and guarded by their parents. Other mammals are born into their mother’s protective pouch (marsupials). Mammals often teach their young how to find food and escape predators before they grow up and go out on their own.
Birds share a similar start as mammals, except that they start as eggs and are kept warm and protected until they hatch. Hatchlings are small and helpless. Their parents will bring them food and keep them warm and safe. Bird parents will stay with their young until they can fly (or run), avoid predators, and find food on their own.
Amphibians and reptiles lay many eggs. Amphibians lay soft eggs in the water, while reptiles lay leathery eggs on land buried in sand, soil or plant debris. Both usually leave their young to fend for themselves when they hatch, though some species, like the American alligator, watch over young for a time. Only a few hatchlings survive.
Insects lay many, many eggs, but most of their young do not survive to be adults. Insects have two very different kinds of life cycles. Some undergo incomplete metamorphosis. This is when young insects hatch from eggs looking like miniature adults, called nymphs. As they grow, they shed their hard outer layer, called an exoskeleton. Each new size is called an instar. They have 5-6 instars before they reach their adult size. Insects that go through incomplete metamorphosis include: grasshoppers, crickets, preying mantises, and cockroaches. Many insects, however, go through a complete metamorphosis. This is when new hatchlings, called larvae, look completely different from adults. They feed and grow until they reach a certain size and then form a protective cocoon or chrysalis. Inside their chrysalis, they go through a process called pupation. During pupation, the body breaks down and changes into the adult form. Insects that go through complete metamorphosis include: moths, butterflies, ants, beetles, and bees
The Next Generation Science Standard for this topic in 3rd Grade is:
LS1.B: Growth and Development of Organisms
• Reproduction is essential to the continued existence of every kind of organism. Plants and animals have unique and diverse life cycles. (3-LS1-1)
Try some of the activities to help students understand these important concepts.
Download the whole bundle from the Exploringnature.org science store for a complete Growth and Development of Organisms UNIT.