How Do Animals Make More Animals?

-Karthik Gurumurthy

Animals are born, they grow up, and eventually they die. So the most crucial thing they need to do is reproduce—create new life to carry on after they’re gone. Without reproduction, animal life would simply end.

There are tons of different ways that animal species create new life, but they all share one fundamental rule: each individual can only produce babies of its own species. A sponge can’t suddenly pop out a worm. A fish can’t give birth to a parrot. That’s just not how it works.

Some animals reproduce by literally splitting themselves apart. Sea anemones stretch their bodies lengthwise and then pinch off in the middle, creating two brand-new animals. When flatworms reproduce, they break themselves into multiple pieces, and each piece can grow into a complete new worm. When animals divide like this, they’re basically creating exact copies of themselves.

Many animals, including us humans, reproduce through fusion—the joining of two cells from two different individuals. In these species, there are two sexes: males and females. A male sperm cell meets up with a female egg cell (called an ovum) to create the very first cell of a new baby animal. That cell splits in two, then those two split into four, and the cells keep dividing and specializing, eventually forming different tissues and organs. The new animal ends up being a unique combination of both parents.

For many animals that reproduce through fusion, babies develop inside a protective casing called an egg. Water-dwelling animals like fish and amphibians have thin, soft eggs. Land animals like birds and most reptiles have eggs with hard, rigid shells. Most mammals take a different approach—their babies develop inside the mother’s body until they’re ready to be born.