。
Bats are actually mammals, just like us.
Literal
Bats [topic-って] us [same-as-と同じ] mammals [explanatory-なのです].
って is the casual quotative/topic marker — here functioning like は to introduce コウモリ as the topic. コウモリ (bat) is written in katakana as is standard for animal names in Japanese. 私たちと同じ ('the same as us') draws the comparison. なのです is the explanatory copula for nouns/な-adjectives, adding a 'you see / actually' nuance — the speaker is sharing a surprising or little-known fact.