Why is the platypus classified as a mammal even though it lays eggs?

The platypus is classified as a mammal because it belongs to the order Monotremata, a unique group of mammals that diverged from other mammalian lineages approximately 166 million years ago. While the platypus retains primitive, reptilian-like features—such as laying eggs—it possesses key characteristics that define it as a mammal.12

Specifically, the platypus produces milk to nourish its young, a defining trait of all mammals. Although female platypuses lack teats and instead secrete milk through specialized skin patches, this process fulfills the mammalian requirement for lactation. Additionally, the platypus is warm-blooded and maintains a constant body temperature, further distinguishing it from reptiles.21

As a monotreme, the platypus represents an ancient evolutionary branch that provides valuable insights into the early history of mammals. Its classification highlights that the defining features of mammals, such as milk production, evolved early in the lineage, even while other ancestral traits like egg-laying were still present.13