mammifer

例句：

1. 1.Mammifers are the only animals that give birth to live young.
   哺乳动物是唯一能产活仔的动物。
2. 2. The discovery of fossils in the dinosaur era has led scientists to study mammifers.
   恐龙时代的化石发现促使科学家研究哺乳动物。
3. 3. Mammifers play a crucial role in maintaining ecosystem balance.
   哺乳动物在维持生态系统平衡中起着关键作用。