The male rules of physical engagement have changed.

Did prehistoric hunters perform an ancient version of the high five upon making a good kill? Did Roman gladiators do some variation of the chest bump to celebrate victory?

We may never know, but today, male greetings, displays of affection, affirmation or just plain jubilation run the gamut and have evolved -- or devolved, depending on one's perspective -- beyond the basic handshake.

Two weeks ago, President Bush bumped chests with U.S. Air Force Academy graduate Theodore Shiveley at graduation ceremonies in Colorado Springs, Colo.

"How do you say, 'Hey, Mr. President, let's chest bump'?" said Mark T. Morman, a Baylor University associate professor of communication studies who has done research on male-to-male displays of affection. "How does that happen?"

Last week, presumptive Democratic presidential nominee Barack Obama exchanged what now has become a much-discussed dap -- horizontal fist bump -- with his wife moments before claiming the nomination in front of an 18,000-plus throng at the Xcel Energy Center sports arena in St. Paul, Minn.

Even "Doogie Howser, M.D." got into the act. Last week, actor Neil Patrick Harris, also of "How I Met Your Mother" and "Rent" fame, shared a dap and later a man hug with a "Million Dollar Password" contestant during the game show's bonus round.

But the Obamas' public display of the dap to the masses may have ruined its cool quotient. Yesterday, Sen. Dianne Feinstein, D-Calif., and Sen. Kay Bailey Hutchison, R-Texas, did a sideways dap on CNN's "Late Edition with Wolf Blitzer." The dap is surely dead now. It's so over.

History of the handshake is imprecise, but it's believed to have evolved from the medieval custom of extending one's hand to another to demonstrate one was unarmed, Dr. Morman said.

The dap went mainstream in the 1960s after African-American soldiers in Vietnam popularized the greeting, which can include a whole series of hand motions besides just the fist bump.

Men's physical demonstrability scale used to run from formal handshakes to hugs, which were to be dispensed only on special occasions -- births, deaths, weddings, graduations and hellos or good-byes after extended separations.

Today, the antiseptic handshake, the pound (vertical fist tap atop another vertical fist), the dap, the sportsmanlike slap on the backside, the chest bump, the high five, the handshake with the follow-up forearm chest bump, the arm-wrestling grip handshake with half hug and the man hug all peacefully coexist in the male affection arsenal.

What now is acceptable for men ranges somewhere between the stoic thank-you-for-the-loan handshake and the rhythmic Miss Mary Mack hand-clapping games.

"It is symptomatic of the breakdown of the formality of our culture -- everything from casual dress at work and sort of the whole deconstruction of every structure that we have, including the presidency," Dr. Morman said.

"Just like with everything else, this sort of deconstruction of all things formal has extended even to the way we greet each other, particularly amongst men."

Women rarely go through chest thumpings, high fives and other greeting rituals, he said, except in the case of female athletes who he has seen trade pounds whenever they greet each other, not just out on the field or court.

"Women emulate what men do," said Linda Lindsey, a sociology professor at Maryville University of St. Louis who has studied gender roles and nonverbal and verbal communication. So, it's not surprising that women athletes would mimic male athletes' displays of affection and celebration.

Still, this ritualistic type of physical interaction is more prevalent with men because women always have been more comfortable with physical affection, touch, expression of emotion and intimacy.

"All of these elaborate handshakes and chest bumping and butt slapping, this is covert affection," Dr. Morman said. "It doesn't look that way, but men do it to express affection and do it that way because it doesn't look overtly feminine."

Men tend to be three times more homophobic than women, he says. That's one reason it had been the norm for men to greet each other quite formally -- to mitigate any suspicion or misunderstanding about their sexuality.

"Now that we're more open, accepting, comfortable and sensitized to male-male displays of affection, we're not quite as worried about this as we once were," he said.

These days, men are more likely to have shared hugs with other males -- friends and family -- than in the past.

"You can see it, if you watch 'The Tonight Show' with Leno even compared to Johnny Carson," Dr. Lindsey said. "Men would always shake hands, but now they hug. They don't do the real long hug, but that's very, very different than it was a decade ago."

Men tend to be more affectionate with each other in emotionally charged public contexts than in private contexts.

"The public context mitigates any sort of homophobic or negative interpretations," Dr. Morman said.

That's why men hugging, high five-ing and piling on top of each other in jubilation at home plate in front of a stadium crowd of thousands is all right.

"Whereas if you're hugging and kissing and butt slapping and rolling around on the floor with your best buddy at your house, it's 'Wait a minute!' " he said.

Generally speaking, younger men are more physically demonstrative than older men.

There also are some general cultural, ethnic and racial differences in how men physically interact. For example, there tends to be more openness about hugging and touching within the African-American community. And Middle Eastern men tend to be much more comfortable with platonic same-sex interaction, in terms of touching and greeting.

"Like President Bush, when he goes to Saudi Arabia, they kiss on both cheeks and hold hands," Dr. Morman said.

While the standard handshake is still de rigueur in business circles, the more familiar arm-wrestling grip handshake -- sometimes with the bonus half hug -- shows up at Wimbledon's Centre Court as well as on basketball courts, before and after games.

The Cleveland Cavaliers are notorious for their intricate pre-game introduction handshake rituals -- a custom they protect.

"No handshake questions," Cleveland guard Damon Jones told a Washington Post reporter. "I can't tell you anything about the handshakes, I'd have to kill you. ... I do a handshake with each individual guy, and that's sacred."

Fortunately, the man hug isn't classified information. Online, eHow.com offers step-by-step man-to-man hug instructions at www.ehow.com/how_2049802_give-great-mantoman-hug.html.

And the key, it seems, is to keep it testosterone-laden.

"Pat the back. This is the man-to-man way of saying the hug is macho and not about making out," according to Step Five of eHow's "How to Give a Great Man-to-Man Hug."

"The more pats the better. This will keep the hug masculine the entire way through."