Phoenix ? This Christmas season may be going to the dogs — and cats.

Pet supply stores, which have enjoyed strong growth even in the soft U.S. economy, are doing a booming business in holiday and seasonal gifts for pets as owners stock up on sweaters, antler headbands, jingle bell collars and other holiday items.

“This year, it’s just proliferating beyond anything,” said Bob Vetere, chief operating officer of the American Pet Products Manufacturers Assn. “It’s just exponential. It’s beyond anything we’ve ever seen before.”

Americans are expected to spend $31 billion on their pets this year, including vet services, food and supplies, according to APPMA, an industry group. That’s up from $17 billion in 1994, with a market that continued to grow during the economic downturn.

The industry doesn’t track numbers of holiday-themed products, but Vetere said the number and variety had grown substantially during the past few years.

The gift giving isn’t confined to December either, he said. “You can’t believe the stuff they have for Valentine’s Day. It seems like it’s just any excuse for a pet gift.”

When San Diego-based Petco surveyed its customers online, it found more than 85 percent said they would buy their pets a holiday gift. Sixteen percent said they would spend more than $100.

Americans are “adding their pets to their gift lists,” said Alisa Bartmess, the cat-toy buyer for Phoenix-based PetsMart.

Pet supply companies say increasing numbers of pet owners think of themselves as “pet parents,” meaning they treat their dogs and cats more like family than critters.

Bulldogs Sybil, left, and Trudy are next in line to have a photo taken with a person dressed as Santa Paws. They got their picture taken Dec. 7 at a Petsmart in Scottsdale, Ariz. Pet supply companies, which have enjoyed strong growth even in the soft economy, say the demand for holiday-themed products keeps growing.

A string of pet owners having their dogs’ Christmas photos taken at a PetsMart store in north Phoenix agreed.

“He’s an only child,” said Rosemary Connell of her German shepherd mix Bosco. She and her husband, John, brought the puppy in for obedience school and a Santa photo since it’s Bosco’s first Christmas with them.

The Phoenix couple said Bosco received family treatment from extended members of their family, too, with some relatives in New York shipping gifts for Bosco.

Apparel — ranging from red Santa suits to collars with red and green lights — are among the season’s most popular purchases, said Kevin Drouse, a PetsMart store director.

Even upscale retailers have begun offering apparel for the holidays. British clothier Burberry offers $225 lambswool check collar dog coats in its holiday catalogue. Ralph Lauren sells $95 cashmere sweaters in four colors and a toiletry collection that includes dog cologne.

Petco spokesman Shawn Underwood said all the holiday gift-giving even affected nonpet owners. Petco sees an increase in the number of gift cards sold around the holidays, and the company suspects they’re being purchased by family and friends as gifts for other people’s pets.

“What do you buy for the pet who has everything?” he asked.