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Rat pups suckling from Ingiequeen.

Rat with teeth, as well as the pile of rat pups, from The Scientific American.

Laughing rat cartoon from MIT.

Tickled rat from ABC News.

Dopamine circuit drawing from ScienceDirect.

Rat larynx from NCBI.

Dog laughing from DailyInfoPics.

Ape laughing from FindingDulcinea.

Phylogenetic tree relating humans to rats from phyloGif.

Phylogenetic tree of laughter from ScienceDirect.

Header image from The Telegraph.