Here's why:

* Red blood cells (RBCs) are responsible for carrying oxygen throughout the body.

* Mammalian RBCs lose their nuclei during development. This allows them to have more space for hemoglobin, the protein that binds to oxygen.

* Other vertebrates, like birds, reptiles, amphibians, and fish, retain nuclei in their red blood cells.

Let me know if you have any other questions!