IANAS, but it sure seems like most of these pig fuckers just went fully off-road genetically as soon as their two main external sphincters formed during mitosis, and focused everything else afterward on that core concept: Eat & Shit. (punctuation optional; creative emphasis, arrangement, spelling, etc. promoted)
They are, just like guinea pigs (their smaller relative). It's just that the bauplan is rather close to a pig - stubby legs, roundy body, stubby tail.
I guess that just like non-crustacean peak performance is to become crab-like, mammal peak performance is to become pig-like? Cats*, humans, dogs, we'll eventually get porcinised!
*except my cat. She's a Schweinekatze (pig-cat) so she's already pig-like.