A photographer's assistant squeaks a yellow duck-shaped toy, and Nikki's owner, Cheryl Siewert, coos encouragingly to get the black-and-brown pooch to hold still. He obliges only momentarily, but Siewert is all smiles.

The photo with jolly ol' St. Nick — an animal shelter volunteer in a red suit and white beard — has become an annual tradition for Nikki, whose five photos hang in ornaments from Siewert's tree.

"We take these pictures so we can know just how cute they can be," she said.

Siewert isn't the only one giving her pet the holiday treatment once reserved for just the people in her family. 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 Association. "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 veterinary services, food and supplies, according to the industry group. That's up from $17 billion in 1994, with continued growth during the economic downturn.

The industry doesn't track numbers of holiday-themed products, but Vetere said it has grown substantially in the past few years.

The gift-giving isn't confined to December, 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, 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 buyer for cat toys 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 like family.

Siewert and a string of pet owners who had their dogs' 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 gets extended-family treatment, with relatives in New York shipping gifts.

Even some reluctant pet owners concede they have started treating their animals like family.

"I didn't want a pet, but now that we have one ... I'm growing attached to him," said Norma Shipp, who came from nearby Cave Creek to see Santa Paws with her three children and a fluffy 4-pound Maltese clad in a blue sweater.

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.

And the number of choices keeps growing, said Shawn Underwood, a Petco spokesman. The variety of antlers, Santa hats and sweaters has grown dramatically in recent years, he said.

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

Not all dogs are fond of being so well-dressed, acknowledged Don Cowan, director of Petco corporate communications. "Some of these things we do for ourselves — let's be real," he said. "We humans tend to like our pets to look nice."

But pet supply sellers also offer holiday gifts that dogs and cats might find more appealing, from plastic candy canes full of dog biscuits to animal-shaped catnip toys with Santa hats.

Petco's Underwood said all this holiday gift-giving is affecting people who don't even have pets. His company's stores see 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.