In general, mammals share several key characteristics. They are endothermic vertebrates, meaning they can regulate their internal body temperature higher than their surroundings. Mammals possess functional mammary glands in females to produce milk for their offspring. Additionally, they have a neocortex in the brain and three middle earbones.

Now, take a look at this picture:

A whale nursing her young.

Observing this, it becomes evident that whales also exhibit these distinctive mammalian features. They possess a neocortex, three middle earbones, and their fetuses develop hair during gestation, which is later shed.

Considering these characteristics, it becomes evident that whales belong to the diverse group of mammals. It is worth noting that the presence of middle earbones has proven instrumental in identifying and classifying many extinct mammalian species since mammary glands, being soft tissues, do not fossilize easily.

This fact is truly mindblowing, as it highlights the importance of examining the fossilized middle earbones to understand and recognize ancient mammalian species