Jon Preston was enjoying a visit to the National Zoo with his family this winter when he came upon a surprising, unofficial exhibit: a spotted gecko belly-up on the sidewalk.

Puzzled to see a tropical species on a cold day in February, Preston picked up the small lizard and held it in his hands.

"As soon as it warmed up, it became quite active -- much to the delight of my 4-year-old daughter," said Preston, a National Park Service ranger at Olympic National Park in Washington.

But how, Preston wanted to know, did a gecko that thrives in warm weather end up coping with winter in the nation's capital?

Bill Xanten, the zoo's general curator for animal programs, said the gecko is part of a small, self-sustaining population of lizards, the offspring of geckos that escaped from an exhibit in the early 1980s.

The lizards were Mediterranean geckos that zoo researchers brought from Florida. During renovations to the Reptile House, the geckos and other reptiles and amphibians were moved to a building along Rock Creek at the lower end of the animal park, where the zoo's genetics lab is now located.

"A few of them got loose," Xanten said. "I remember they migrated up to the second floor, and you would see them once in a while on the wall."

Normally, tropical geckos wouldn't have survived their first winter outside. But these lizards -- the zoo isn't sure how many -- discovered the zoo's steam tunnels, where it is warm and dark.

"They started hanging out down there and produced more geckos," Xanten said. "They're seen occasionally. But they're pretty small, only about three to four inches in length, and nocturnal, so you don't see them that often."

The wild lizards don't pose any problems. If anything, Xanten said, "they're kind of good for the zoo in a way because they eat bugs."

Geckos live three to four years, so none of the original group of escapees is alive today.

"They are somewhat territorial, so they space themselves out," Xanten said. "We're not talking large numbers. They are susceptible to the cold, so they are not going to go outside."

The zoo used to have other Mediterranean geckos on exhibit, but no longer. The current Reptile House collection features some gecko species, including the leopard gecko, the Tokay gecko and the Madagascar giant day gecko.

Preston said that his February visit to the zoo was his first. He happened upon the gecko while chasing his 2-year-old son, Archer, who had wandered down a stairway near the lion and tiger exhibit.

"I found it at the bottom of the stairwell, near a steam vent," Preston said.

He said he thinks the gecko he saw had been chased out of the vent by another gecko. There were still little patches of snow from a previous snowfall, and the lizard probably got cold very quickly.

It would have died if he hadn't picked it up and warmed it in his hands, Preston said.

After the gecko revived, Preston's daughter, Erin, began playing with it. She held it in her hands and let it crawl on her arm. Preston's wife, Debbi Ross-Preston, a photojournalist, snapped several pictures.

"If the steam pipes don't leak too much and the steam flowing through the valves isn't making too much noise, I'd be willing to bet those male geckos make an interesting racket defending their territories and looking for girlfriends," Preston said.

"I grew up in Hawaii and listened to the geckos in my house my whole childhood."

That day at the zoo, Preston said, he wanted to find out more about the geckos roaming the grounds. The Reptile House seemed like the logical place to start.

"My daughter, who was quite taken with her living possession, took the gecko out to show someone," he said.

But small lizards, when grabbed, have the ability to lose part of their tail and flee. The tail continues to wiggle after the lizard has made its getaway. It's a defense mechanism to divert predators -- and one this gecko handily employed.

"It made its escape, appropriately, in the Reptile House, complete with the wiggling tail left behind," Preston said.

During a visit to the zoo this winter, Erin Preston's dad found a gecko, which eventually made Erin's acquaintance.