Why do dog and cat farts smell so bad?

A carnivore’s protein-rich diet produces relatively small amounts of intensely stinky gas because proteins contain lots of sulfur. A dog’s or cat’s farts are rarely audible, but the odor is overwhelming. I have asked biologists why dogs and cats generally fart silently, and their theories include: (1) the amount of gas produced is small, but potent, (2) the horizontal orientation of their gastrointestinal system puts less pressure on the anal opening, so the gas is expelled more slowly, (3) their anal sphincters don’t close as tightly as humans’ because it takes less force to hold in the contents of the colon — again because of the horizontal orientation of the gastrointestinal system — and a loose anus makes less sound, and, my favorite (4) dogs and cats don’t feel embarrassed about farting, so their sphincters are more relaxed, leading to less noisy flatulence. My vet tells me that if a cat farts audibly, it could be a sign of pathology.
Mike F. points out that many dog foods are soy-based, so on top of all the above factors, add beans and stand back!
Large herbivorous animals such as cows, horses and elephants, on the other hand, produce vast quantities of relatively non-stinky fart gas. The farts of these animals are noisy and can go on for astoundingly long periods of time. Cows in particular are productive, in part because they swallow huge amounts of air. They need oxygen in their guts for the various protozoa employed there as digestive aids.