It often begins as a case of puppy love.

Or a close encounter of the kittenish kind.Whatever the case, a growing number of singles are finding that dogs and cats are more than mere pets. They have assumed the role of "significant other" - creatures that help fill the void of affection in a world of fleeting relationships.

Singles also tend to spend a small fortune on their animals, said Phil Goar, owner-manager of Phydeaux's pet department in Englewood, Colo.

"These animals are all some people have," he explained, "so they like to spend a lot of money on them. They buy everything from sweaters and boots to fingernail polish for dogs."

While it's not uncommon to find young people embracing pets as a status symbol, equally common are older individuals - often women - who upon separation or divorce find animals the perfect source of companionship.

"Being newly single and having moved from a home to a condominium, I'm basically alone," said Carol Kingsley of Denver. "That's why my children gave me a 5-month-old poodle named Edgar. They wanted me to have a companion."

"He's someone to talk to, to say goodbye to, to say hello to. The best thing about Edgar is the affection he provides and the friendship," Kingsley said.

For restaurant manager Jim Conner, 35, pets trigger a paternal instinct in addition to offering companionship. Conner owns two dogs, Nipples, a 3-year-old dachshund, and Schnikelfritz, a relatively rare Chinese crested hairless.

"Having dogs is just as much responsibility as having a child," said Conner. "I watched her being born and I held her in my hand two minutes later. There's really a special attachment there.

"She's a wonderful companion," he said. "She's loving. She'll put her arms around my neck and give me hugs. She smiles at me. Both of my dogs are great. I get along with them better than with a lot of people."

While there are few nationwide studies detailing pet ownership by singles, John Alberz, executive director of the American Small Animal Association, said focus groups show that singles are playing a pivotal role in Americans' shift from traditional domestic pets.

Cats, for example, may be replacing dogs as man's best friend. A 1987 study by Charles Charles Associates showed that ownership of dogs has declined by nearly 5 percent since 1983, while cat ownership has risen by 3 percent. Part of the reason, Alberz said, is that cats can be left alone for several days without negative effects to themselves or personal property.

But the big boom is toward birds and other caged animals.

"We're definitely seeing an increase in large and small birds as pets," said Alberz, "as well as small mammals like hamsters and even such rodents as rats."