On a hot day the week before school started, a polar bear swam nonchalantly on his back in the pool and children with a gray-haired lady stopped to laugh and guess at Bryan McKenna's questions about the secrets of the Buffalo Zoo.

Who's got the longest tail? What would a tiger look like if you shaved its fur? Who's the boss of the hyenas? Answers: The giraffe. Tiger skin has stripes, too. Female hyenas won't let the males sit on the big heated rock in winter. The bear paused in the water and rested his head on the ledge, as if listening, too. McKenna, a docent who has taken every training class, went on.

"He could smell a hot dog cooking from Niagara Falls," he said of the bear's power to detect scents for miles in the Arctic. McKenna has a habit of sharing what he knows with zoo visitors he meets even when he's not scheduled to stand at a popular spot and talk. On this day, he had come to give a tour to a young family friend.

Breanna Gerner, 12, knew his routine. As McKenna went on -- Do you think bears are lonely by themselves? They're happier that way. They don't like sharing food. -- she began to peer into his pockets. She wanted him to show the bear claws.

In the four years since he retired from a 30-year career driving and unloading trucks for as many as 60 hours a week, McKenna went from being a quiet guy to surprising his wife and friends by talking all the time about the creatures he'd been wanting to know more about since he was a boy living nearby and walking to see Samson, the zoo's old gorilla.

First laid out as a deer sanctuary in 1875 by park designer Frederick Law Olmsted, the zoo is now surrounded by neighborhood houses and streets, where a person can hear a lion's roar from bed and think life here is so unusual.

Giraffes lope just beyond the fence along Parkside Avenue. Peacocks wander into yards. When neighbors call, unfazed zoo officials tell them not to worry. The birds, McKenna said, know how to get home.

Stories of the 1,200 animals living behind glass and fences in the pools and rocky habitats on the zoo's 23 acres do get out through the people -- 69 full-time staff, 120 docent volunteers, 460,000 visitors last year -- who take the time to get to know the lesser-known quirks.

Hum and curious sea lions swim to the window. Sometimes the other snow leopard cub hides in the rocks. McKenna listens for the deep howls of the "loudest land-mammal" -- howler monkeys -- and reveals that vampire bats in the rain forest exhibit's cave drink blood supplied by a farmer's cow.

Kids who come back year after year have learned his facts, so he keeps collecting, studying online, watching Animal Planet on TV and talking to zookeepers and docents.

"I know some really cool stuff about reindeers that you probably don't know," he told Breanna as they walked. Females are the ones that keep their antlers in winter. "All Santa's reindeer are girls," he said. "Or, Santa Claus is magic."

Since McKenna, now 52, first called to offer to clean up after the elephants and wash windows, the zoo has gradually changed his life. Cleaning was union work, he learned. To be a volunteer, he would have to take classes.

After 13 weeks of six-hour lessons on Saturdays, he was fascinated to see the zoo's private places: the TV that plays Sesame Street for the gorillas, the vet clinic by the giraffe house, a close behind-the-exhibit look at how big the tigers really are. He soon took animal handling and other courses so he could take pythons, turtles and blue-tongued skink lizards to nursing homes, schools and children's hospital wards. Even though ferrets leave behind a stinky, musty smell on the hands, Dakota and Cupid delight people so easily with their willingness to be held that he now considers them some of the most important animals.

Last winter when zoo docent assignments dropped along with visitors, McKenna also got a new appreciation for amphibians. Restless without a full schedule, he considered finding a regular job until he was offered a spot in a special training class. To learn how to collect frogs and find if they had a similar kind of amphibian chytrid fungus that was killing the golden frogs in Panama, he learned to distinguish local frog calls by listening to recordings he kept in his laptop.

A visiting scientist marveled that McKenna could hear the scarce "chorus" frogs call -- like a finger running across the top of a comb -- near his North Tonawanda backyard along the Erie Canal. Then when he and a team of two students went to a swamp thick with poison ivy and hatching mosquitoes, they found a toad with the fungus.

While it's too soon to tell how dangerous it is locally, McKenna worries about disappearing habitats and disease that lead to extinction. He wishes he could do more. "I know there's nothing I can do to save the planet," he said, "but the zoo can." Over by the bear pit, McKenna reached into his pocket.

He always brings his "zoo things." He thinks they help get people curious, and public interest should help keep the animals safe. When he talks, he may pull out a lipstick case with the rattlesnake rattle. Or, the 18-inch string as long as the tongue the giraffe uses to clean its ears. The family this day examined the sharp plastic claw that shows how a bear climbs ice. The family thanked him and left. "We have learned so much," they said.

McKenna was ready to go, too. He knew Breanna would love seeing the Australian parrots' bristled tongues that look like paintbrushes. Just before they hurried to the other side of the zoo, the polar bear stuck his nose up in the air and slipped back into the water.