In the classic Star Trek episode “The Trouble With Tribbles,” the titular balls of fluff wreak havoc on Space Station K-7 with their large appetites and reproductive rate that makes rabbits look like eunuchs. They end up helping foil a Klingon plot, but still, they’re like ambulatory hamster bodies.
So what makes a tribble tick? Boy, are you in luck. Because anything Star Trek-related exists somewhere, this diagram from the Star Fleet Medical Reference Manual shows you in great detail.
Note that the uterus takes up a great portion of the tribble’s body; it would have to to crank out almost as many babies as the Duggars have.