FORTUNE — Not everything is about chocolate.
Snickers got the Super Bowl ad with Betty White, but it’s Mars’s 78-year-old pet-care division that is the company’s biggest business, employing nearly half its employees worldwide. In the southern suburbs of Nashville at its headquarters, it’s easy to observe a culture interlaced with the larger Mars ethos but separate and distinct in its “pet-centricity.” That combination — a core corporate identity along with individuality born of decentralization — is as fundamental to Mars as its obsession with privacy.
You want indications of quirky? Meet Tiffany Bierer, Ph.D., the head of health and nutritional sciences. Her job: making sure Rusty and Sheba like the fare. “I’ve tasted everything we make,” she says. The best: doggie biscuits. The worst: moist cat food, although the curiously named “gravies” on some premium dishes aren’t bad. Bierer does make allowances: She digs in with a fork, rather than diving mouth-first into a dish. Yum!
About half the 475 employees in Franklin bring their dogs to work several days a week. “We don’t require people to have a dog, but we do ask them if they have one,” says Ulf Hahnemann, the head of HR for Petcare in the U.S. (apparently without HR irony about what’s okay to ask job applicants). Hahnemann brings in his Newfoundland named Bamse — which means “bear” in Danish. If dogs could talk, 120-pound Bamse would say, “I want Bierer’s job!”
Behold! Dogs, dogs, dogs Yorkies, Labs, and even puppies. Walking around the cubicle-free open offices — with Bamse — one sees and hears dogs everywhere. When one dog barks, others join the canine cacophony, which happens a lot and is why employees try to take advantage of it. Will Turnipseed, a commercial sourcing manager, says it’s a way to break the ice with external vendors: “When you’re on the phone with someone, they’ll ask, ‘Is that a dog barking?’ And that just opens the opportunity to say, ‘Yeah, it is.’ ” Employees love to tell that kind of story, almost as much as they like to show you photos of their dogs on their iPhones.
Except for tiny Mikey, the black-eared Mi-Ki owned by the receptionist Marilyn Gilliland, the dogs are all on leashes. Every desk comes with a nub on which to attach a leash. But that doesn’t prevent the occasional Fido fight. “If there’s a bite involved,” says Hahnemann, “it’s totally back to the ‘responsibility’ principle. My department [HR ] is not involved — people solve it between themselves.”
For nondog folks, there’s a glass-walled “cat room.” It’s not for pets but for four “housecats” — Faith, Brooks, Dunn, and Emmy Lou, named after country singers. The litter box is disguised as a potted plant — though there’s still an air purifier the size of Bamse. The point of the room, Bierer says, is to offer employees “a better understanding of cats.” One way to learn: Watch Brooks, who delights in taunting dogs as they go past.
The employees pride themselves on community involvement. They’re given a few days off a year to volunteer, but many do far more, like at the local animal shelter. And the inhabitants are the ideal kind of customer: They don’t talk back.
This story is from the February 4, 2013 issue of Fortune.