Why do mature red blood cells in mammals lack nuclei?

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polar bear and cubs — mammal Mother polar bear nursing her cubs (*Ursus maritimus*).

Mature red blood cells in mammals lack a nucleus as an evolutionary adaptation to maximize their efficiency in oxygen transport.1

This structural change provides several critical advantages:

Increased Capacity: The absence of a nucleus and other organelles, such as mitochondria, creates more internal space to pack hemoglobin, the protein responsible for binding and transporting oxygen.1
Metabolic Efficiency: By lacking mitochondria, these cells do not consume the oxygen they are transporting for their own metabolism. Instead, they generate ATP through glycolysis, ensuring that nearly all carried oxygen is available for delivery to body tissues.1
Flexibility: The lack of a rigid nucleus, combined with the cell's biconcave shape, allows the red blood cell to remain highly flexible. This enables it to deform and squeeze through capillaries that are often narrower than the cell itself.1

During development in the bone marrow, the nucleus and other organelles are gradually lost as the cell matures from an erythroblast into a functional red blood cell.1