関連ワード:  why is platypus a mammal    why is platypus a mammal if it lays eggs    why is platypus considered a mammal    why is a platypus a mammal when it lays eggs    why is platypus not a mammal    why is platypus called a mammal    why is duck billed platypus a mammal    is a duck billed platypus a mammal    why is a platypus a unique mammal    why is a platypus an unusual mammal