How to say "mammal" in Japanese

• 哺乳類ほにゅうるいCommon
  nounThe word for a mammal as an animal class: creatures that nurse their young. The common term and the default choice, whether you are talking about whales, dogs or humans; it also covers 'mammalian'.
  イルカは哺乳類ほにゅうるいの一種いっしゅです。↗

  A dolphin is a kind of mammal.
• 哺乳動物ほにゅうどうぶつ
  nounAlso a word for 'mammal', built on 動物 'animal' rather than the class word 類. It is a true synonym of 哺乳類 but less common, so 哺乳類 is normally the word you want.
  コウモリって私わたしたちと同おなじ哺乳ほにゅう動物どうぶつなのです。↗

  Bats are mammals, just like us.

There is really one sense here: 哺乳類 and 哺乳動物 both mean 'mammal', so take 哺乳類, the common word, as your default and treat 哺乳動物 as a less common synonym.

Usage comparisons are AI-assisted and cross-checked by multiple models; for study reference only

• 哺乳類ほにゅうるいmammal
• 哺乳動物ほにゅうどうぶつmammal