Littermates are animals born in the same litter, which means they came from the same mother at the same time. When a dog has puppies, those puppies are littermates to each other. The same goes for kittens, piglets, lion cubs, or any other animals whose mothers give birth to multiple babies at once.

Littermates grow up together from their very first day, sharing the same den or basket, nursing from the same mother, and tumbling over each other as they learn to walk and play. This early time together is important: young animals learn crucial skills by wrestling, playing, and interacting with their littermates. A puppy learns how hard it can bite during play from the reactions of its littermates. Kittens practice pouncing and stalking by ambushing their brothers and sisters.

Even after they go to different homes, littermates can remember each other. Dog owners sometimes arrange reunions where littermates who haven't seen each other in years recognize their siblings and start playing together.