* Give birth to live young: Blue whales do not lay eggs. They give birth to live calves, which are nourished by their mother's milk.
* Have mammary glands: Female blue whales produce milk to feed their calves.
* Breathe air with lungs: While they live in the ocean, blue whales cannot breathe underwater. They must surface regularly to take in air with their lungs.
* Warm-blooded: Blue whales maintain a constant body temperature, even in cold water.
* Have hair: While they may not be as furry as land mammals, blue whales are born with a few hairs on their upper lip.
* Possess a four-chambered heart: This is a key feature that distinguishes mammals from other animal classes.
Although they live in the ocean and share some characteristics with fish, blue whales are undeniably mammals due to these defining traits.