(FORTUNE Magazine) – Pet food is big business; otherwise gorillas such as Ralston Purina, Quaker Oats, Colgate-Palmolive, Nestle (Friskies brand), Grand Metropolitan (Alpo), and Mars (Pedigree) wouldn't be chasing cats and dogs. Last year chow for Bowser and Kitty brought sales of some $6 billion at retail in the U.S. Under normal conditions pet food is more profitable than human food, with pretax margins ranging from 12% to 20%. Except that this business ain't hardly ever normal. Says William Johnson, CEO of Heinz Pet Products Co.: ''There is a pervasive irrationality in the pet food business that is disturbing. People are willing to throw huge amounts of money at a business that is not growing rapidly. It is without a doubt the most difficult consumer products business in the U.S., and I've been in a lot.'' Heinz, with 9 Lives cat food and Skippy dog food, is now No. 3 in the business, having surpassed Quaker. The Friskies PetCare unit of Nestle moved into the No. 2 spot following a strong year. The industry is beset by frequent introductions of unneeded products, fierce price wars, and an abundance of costly promotions that consumers love but managers hate. Companies keep at it because controlling costs is critical to the business, and that means maintaining the tonnage of product running through the factory. During the 1980s the U.S. cat population, now about 58 million, overtook the dog population, now about 51 million. It is not that people took to liking cats more than dogs (how could they?). Rather, the increased numbers of one- person households and two-career families fed the demand for cats because felines are self-sustaining: They eat only when hungry, don't need to be walked, and don't necessarily need you, either. But alas for the industry, cats also eat less, and that hurts total tonnage. Sales of cat food are growing 6% a year, and that has caused some big players to get their claws out. Alpo shook the industry by introducing Alpo cat food, featuring Garfield, the popular cartoon kitty, as salescat. Says Ian Martin, Grand Met food sector CEO: ''The results have been spectacular.'' Sales of the brand, backed by big promotional money, exceeded $200 million within a year, about 10% of market, the most successful new pet food in a decade. Alpo was on the block until the cat food became a hit. Grand Met, like Quaker, figured it had to be a fatter cat to be able to stay in the alley. Says Martin: ''We came to the conclusion that the way the game was being played, it was unlikely that we were going to be able to build up the pet food side, so we decided to put it on the market.'' Nevertheless, Alpo continued with the rollout of the cat food, which ended up turning around the company's estimate of the business. Another industry wild card is the secretive, privately owned Mars, the candymaker that is also the world's largest pet food manufacturer. Although big in Europe, Mars has found it tougher going in the U.S. Its Kal Kan division was losing $50 million annually two years ago, until the company started to throw money around. ''Of all the players, they are the most unpredictable,'' concludes an executive at a competing company. ''They scare everybody.'' OVER THE PAST two years Mars has consolidated its pet food business under three brand names: Pedigree in dog food, and Whiskas and Sheba in cat food, replacing the Kal Kan and Crave labels. The names were imported from Mars's European business with the idea of establishing one global brand name. Such a name change is a dangerous move, but Mars has pulled it off, at least in the dog food business. ''It's gone better than our wildest expectations,'' claimed one Mars staffer at a trade show recently. Pedigree is now neck and neck with Alpo for market share leadership in canned dog food. But Kal Kan's cat business hasn't been able to perform the same trick. The cat business lost market share to Grand Met and Heinz, whose 9 Lives brand has its own famous feline in Morris. Typically pet food manufacturers use their dog food business to offset results from the cat side, and vice versa. That's why the business is subject to alternating market wars. Most executives in the industry believe that of the six major players now in the game, possibly two won't be able to make it long term. Says one: ''Quaker has really been injured in the pet food business. I just don't know what their prospects are.'' But Quaker executives are already on record as being in for the long haul. It promises to be just that.