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Fav十万个为什么
为什么鸭嘴兽是哺乳动物却下蛋？
简单的摘要：鸭嘴兽是哺乳动物却下蛋，是因为它属于单孔目动物、保留着卵生特征，并通过乳腺分泌乳汁哺育幼崽。
1、鸭嘴兽属于单孔目动物。单孔目是哺乳动物中最原始的类群，具有卵生特征，而鸭嘴兽正是这一类群中的典型代表。
2、保留着卵生特征。鸭嘴兽保留了卵生这一古老的繁殖方式，其幼崽需要通过孵化从蛋中破壳而出，这是它与其他哺乳动物显著不同的地方。
3、通过乳腺分泌乳汁哺育幼崽。尽管鸭嘴兽下蛋，但它仍然具有哺乳动物的特性，通过乳腺分泌乳汁哺育幼崽，不过没有乳头，乳汁通过皮肤渗出供幼崽舔食。
总结：鸭嘴兽是哺乳动物却下蛋，是因为它属于单孔目动物、保留着卵生特征，并通过乳腺分泌乳汁哺育幼崽。
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