mono
Explanation
This is a single-panel comic showing two echidnas (spiny, hedgehog-like mammals) in bed together. One says to the other: "We're monotremes, Tracy. We already do it in the butt."
The joke is a biology pun. Monotremes (echidnas and platypuses) are a group of egg-laying mammals named for having a single opening -- a cloaca -- that serves for both excretion and reproduction. The word "monotreme" literally derives from Greek meaning "single hole." So when one echidna suggests an adventurous sexual act, the other points out that for monotremes, all reproductive activity already involves the same rear opening by default.
The humor comes from applying human bedroom-conversation dynamics to animals whose biology makes the proposed "adventurous" act simply their standard anatomy. It is a compact, well-executed zoology joke that rewards anyone who remembers what makes monotremes distinctive.