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Explanation
This comic is set entirely in darkness -- the panel is completely black. One speech bubble screams "AAAAA! AAAAA! AAAAA!" while another responds: "I know what you're doing Hank. CUT IT OUT!" The caption below reads: "The worst thing about being a bat is that if you want to sneak a peek of boobs you have to echolocate."
The joke is a straightforward biology pun. Bats navigate and perceive their environment through echolocation -- they emit high-pitched sounds and listen for the echoes to build a picture of their surroundings. The comic imagines bats as having human-like desires (wanting to look at attractive things) but being constrained by their biology. If a bat wanted to surreptitiously "look" at something, it would have to scream loudly, since that is how echolocation works. There is no way to echolocate quietly or discreetly -- you have to blast sound waves and wait for them to bounce back. So the act of "peeking" is inherently loud and obvious, immediately giving away what you are doing.
The entirely black panel is a clever visual choice, reinforcing that bats live in darkness and cannot see. The humor is in the contrast between the desired stealth of "sneaking a peek" and the unavoidable loudness of the only sensory method available. The name "Hank" gives the bat a mundanely human identity, which adds to the comedy of the situation.