SAO SEBASTIAO, Brazil - The sun sparkles on the Atlantic, waves lick the beach, children frolic on the sand. Suddenly, a dorsal fin slices through the ocean's surface.
Men trip over each other as they stampede into waist-high water with hands outstretched. Mothers clutching their infants plunge in, too.
The lovelorn dolphin is making his daily visit.
A woman with red shorts gets there first. The 7-foot-long dolphin rubs against her knees. She pats his belly and flippers. Anyone watching would swear that he flashes a mirthful grin.
"You are so beautiful, Tiao," the woman coos.
Biologists around the world are debating whether people should be allowed to swim with dolphins, but this is a new wrinkle: Should a dolphin be allowed to swim with people?
What is happening in Sao Sebastiao, South America's largest oil terminal, has stumped the experts. Tiao, a young, apparently healthy bottlenose dolphin, refuses to venture to deep water. Instead, he makes the rounds of polluted inlets, nuzzling against bathers.
The dolphin is "sex-starved" and looking for any kind of affection, said Andre Rossi, a marine biologist studying the dolphin.
"Tiao is a very confused teenager," said Paolo Cirilo, a former dolphin trainer who, with Rossi, works for the nonprofit environmental group Fundamar.
The dolphin clearly wants affection, not vittles, because he spurns fish when they are offered.
This could all be entertaining, and in fact it is quite a sight when the dolphin cuddles up to people's legs and backs, even the bottom of jet skis. But Tiao is not trained - and neither are the folks who come to see him. The result can be a dangerous mix.
Last year, several bathers tried to stuff popsicle sticks down Tiao's air hole as he lolled near the shore. Other tried to clamber aboard his back for a ride, as if he were a pony. Tiao's reaction - a mighty tail-smashing - left 29 people injured.
Then, just days before Christmas, as a group of drunken bathers prodded and taunted him, Tiao lashed out. One man stumbled to the beach, complaining of pains. Friends took him to the hospital, but he was dismissed because doctors thought he was suffering from a hangover. Hours later, the man died of internal hemorrhaging.
Still, every time the dolphin cruises a beach, crowds close in. They cluster around him, pointing video cameras. Children who can barely swim trail after their parents.
"It's an accident waiting to happen," one man said as he watched 30 people join the woman in red shorts in shallow water.
After a while, the dolphin disappeared. An elderly man, oblivious to the crowd, swam on his back 30 yards from shore. The dorsal fin surfaced and the man waved his arms and let out a blood-boiling scream. Tiao, apparently, thought he had discovered a new game. He slapped his tail on the water and bumped the swimmer.
The man clenched his fists and started to pummel Tiao, yelling "Tiburon!" - Portuguese for shark.
Cirilo, the former dolphin trainer, swam out to calm the man and distract Tiao. Back on shore, the man trembled and his family worried he would have a heart attack.
Such incidents have prompted the Brazilian wildlife agency, IBAMA, to order humans to stay clear of Tiao. Others in the federal government have proposed capturing Tiao and taking him to the open Atlantic, or at least near an unpopulated island.
Researchers are trying to find out why Tiao avoids his own species. He might have been attacked by an aggressive male, or spurned by a mate, Rossi said.
Rossi has heard from oceanographers and biologists as far away as New Zealand who have told of more than 40 incidents of "lonely dolphins" approaching people. But almost all the animals were older males who apparently had been ousted from their groups by young contenders. Biologists estimate that Tiao is 10 years old, the bottlenose equivalent of an adolescent.
A lonely, but potentially dangerous, adolescent.
"In many ways, the worst thing to ever happen to dolphins was that TV show `Flipper' " Rossi said. "Because of that, people think that every dolphin is trained like a household pet."