CHICAGO — About once a month, Carl and Ann Christoff visit the cemetery where Mindy and Buttons are buried.

As Carl clips and sweeps grass around their graves, his wife uses vinegar to wash bird droppings off the marble headstones. Before they go, they leave decorations: flowers, an angel statue or a small Christmas tree.

This is no ordinary burial ground. Mindy and Buttons, two Shih Tzus who died in 1990 and 2005, are among more than 15,000 pets — including dogs, cats, deer, lizards, turtles, rodents, monkeys and a 3-foot shark — buried at Hinsdale Animal Cemetery, one of the nation's oldest.

To the Christoffs, of Oak Brook, Ill., these were no ordinary pets. "At one time, every one of the animals meant so much, and brought so much joy into one's life," Ann Christoff said.

Just how much they meant to their owners is evident from the epitaphs. "Our Dear Pet," "Gentle Giant" and "Loyal Friend" are common headstone inscriptions. A mausoleum adorned with a dog sculpture reads: "He gave up his life that a human might live. Greater love hath no man."

"You walk through and read the inscriptions on the headstones and some will make you laugh, some will make you cry and some will make you think," said Bill Remkus, whose family has owned the Hinsdale animal cemetery for four generations. "You can almost understand the story."

Michael Schaffer, author of the book "One Nation Under Dog," said he has noticed the messages on pet epitaphs have evolved over time, reflecting how many people have promoted their pets to "full-fledged members of the family."

"If you visit old pet cemeteries, the oldest headstones might say 'Here lies Fido, a loyal servant,' or 'Here lies Fido, man's best friend,'" said Schaffer. "Nowadays it's 'My little girl,' or 'Mommy and Daddy miss you.' People have developed a conception of their pets as children. That is quite a dramatic development."

Remkus said he did not think the feelings people have for their pets have changed, but instead, modern society has become more accepting of people who love their pets and considered them family.

"Years ago, if you buried your pet in a pet cemetery it would be seen as eccentric," he said. "That's not how it's seen today. Now it is just another way to memorialize."

Hinsdale is not a celebrity pet cemetery, although guide dogs for blind author Bernice Clifton of Oak Park, who died in 1985, are buried here. Rather, the cemetery, established in 1926, is a memorial to many pets who faithfully serve their owners, asking little in return.