We’re Nude York, Nude York!

After the big January blizzard, many buckets of purgatorial rain, a chilly, leafless spring-summer, suddenly. Greenhouse gas has cooked Manhattan into a tropical isle; all the hot, half-dressed girls have returned like robins. It’s getting so there’s no place you can rest your eyes without being assaulted by a salvo of flesh. The subway poles are like strippers’ poles, encircled with the most marvelous and terrifying variety of breasts; but don’t look down, because there’s always that flurry of filthy, flip-flopped feet. And in every other direction: Man ass.

“Ass cleavage is really in right now,” said Antonio Jeffery, a national denim specialist at Diesel Jeans in Union Square. Ass cleavage, like regular cleavage, used to be strictly for women. Even the least careful observers of fashion will recall that a few years back, the rises on women’s jeans plummeted with the stock market; at one point, pants got so low that Christine Aguilera was literally prancing in assless chaps. This summer, it’s the men who are artfully displaying the tops of their bottoms, as dudes, gay and straight, squeeze themselves into ever-lower-riding jeans from Paper, Prada and Levi’s. Even the Gap’s in on the action, selling its “1969 extra low boot fit (burnished sky)” denim.

Man ass is suddenly everywhere, from the chichi shopathons of Soho to the hipster suburbia of Williamsburg. There’s so much semi-nudity in New York right now, you’d think you were living in Rio. Just last Friday night, on the Brooklyn-bound L train, an Asian dude posed, scruffy and tan: Between his too-short olive tee and his too-too-low gray Diesel jeans, the buttresses of his pelvic muscles flared architecturally. Try to ignore his pubes. And then, when he exited at the second stop into Williamsburg, his leather shoulder bag shifted just so, revealing the Metallica keychain dangling conspicuously out his back pocket, above which: a full inch of ass crack-at least.

This is becoming the norm-and, according to denim expert Mr. Jeffery (who’d been flown in from Portland, Ore., by Diesel just for the day, surely on urgent denim business), the waists of men’s jeans have actually been sinking like Venice for some time now.

“The rises have progressively dropped lower over the past five or 10 years. We’ve seen the rise go from the belly button to the hips, to right below the hips,” he said. “It has definitely picked up a lot of steam over the past few years.”

Even dads want to get their asses out there. In Clinton Hill, some punk rocker loaded his toddler into their bright red Volvo station wagon; bending over to install the kid in the car seat, a moon sliver of butt snuck out above his studded silver-and-black belt.

“You got two basic style schools of ass crack: You’ve got the intentional and the unintentional,” said Josh Tager, 35, style editor for PlanetOut, a gay and lesbian media company.

Mr. Tager at least respects the unintentional ass-crackers: the hard-at-work plumber, the bent-over grandpa, the generally absent-minded. “It’s not that they’re unaware of sartorial convention; it’s just that they don’t care,” he said in their defense. “People snicker behind their backs, and they point at their cracks-and do they pull up their pants? No. And I think this makes them bona fide rebels.”

The intentional ass-crackers, on the other hand, Mr. Tager finds to be “just a little too precious,” like the fad of men who, in the mid-90′s, went around wearing “nail polish in various shades of blue, green and black.”

The difference between intentional and unintentional ass crack is something that Mr. Tager can just feel in his gut. “It’s like what Justice Potter Stewart said, when he was trying to figure out obscenity: ‘I know it when I see it.’ You see some guy walking down the streets, and he’s showing some ass crack: Is this guy actually just going about his day and he doesn’t even care? Or is this some guy who’s trying to make it look like he just doesn’t care, letting his low, low-riding pants hang there? And you know he took out a ruler at home and made sure he was draping them just so. Probably poked another hole in his belt to make it hang just right.”

There is simply no imperative anymore for anyone to pull up his pants. We should have seen it coming, what with the old-man wool pants-tight in the thighs but saggy in the behind-favored by a certain sort of indie rocker. Or the jeans baggy enough to perturb mothers and disclose the full length of those Calvin Klein boxer briefs, or those novelty bloomers jumping with red-hot chili peppers. All that flourishing of underwear turns out to have been a sort of dry run, like firing blanks in a military training exercise, for the exposure of bare asses.

But there is a significant structural difference between the super-ultra-low-low-rise jeans and grandpa trousers (which are allowed to malfunction) and baggy jeans (which are encouraged to malfunction). The low-riding pants are actually, intentionally made without enough fabric in their seats to cover a man’s hindquarters. Tug all you like: Your ass will never be covered.

Kevin Hicks, 30, and Vincent Cheung, 21, were shopping at the Crate & Barrel on Houston. Both men were wearing low-rise jeans, and Mr. Hicks had on a brown V-neck T-shirt that discreetly covered his butt crack. When asked what he thought about intentionally showing a little something in back, Mr. Hicks insisted it’s “tasteless. It’s trashy-like you’re trying too hard.”

Despite his hard-line position on ass cleavage, Mr. Hicks-who owns 20 pairs of jeans-will sometimes purchase Bebe women’s denim, because they “fit a little more sexier.”

Apparently, certain occasions call for women’s jeans more than others. For example: “If I’m feeling more like a flower,” said Mr. Hicks.

At the Urban Outfitters on Broadway, Dan Flores, 24, lifted up his striped shirt to reveal his red-patterned, girlfriend-endorsed boxers sprouting way up out of his unbelted and decidedly low-rise Diesel jeans. Whatever crack he shows is strictly inadvertent. “I’m a skinny kid; it’s hard to get jeans to stay up,” he said.

“There is male butt crack, though,” Mr. Flores conceded. “I think I’ve seen every butt crack in New York. It’s a skater thing. I see a lot of skater kids showing off their underwear.”

The disappearing pants seem to be a part of a much larger wave of disconcerting male fads. Lately, it’s as if men will accept whatever fashion trends are imposed upon them as happily and willingly as Vichy collaborationists. The short-cropped jackets that make grown men look like monkeys in search of an organ grinder. The striped shirts, which make frat boys’ closets look like the laundry room at an especially colorful gay prison. The paisley shirts. The striped shirts with paisley on the inside. The weirdly hieroglyphic denims with washes and creases, burns and rust, wears and tears. The motherly tote bags (which, by the way, seriously obscured many of our attempts to reconnoiter crack). The Herman Munster square-toe shoes. Capri pants, for clam-digging with the Kennedys. Capri pants worn with flip-flops and a short-cropped jacket, for a look that practically begs for a pie in the face. Shiny shirts. Shiny ties. Shiny shirts and matching shiny ties-gee thanks, Regis.

American men have come to vanity late and practice it with the zeal of the newly converted. And, frankly, it seems to be driving them a little bit nuts. Dale Winston, a manager at Urban Outfitters, said that he too has to head over to the women’s section to find a rise low enough.

“A lot of guys do buy women’s jeans. I buy them,” he said. “Everyone I know buys them.”

It’s bad enough news that men are putting on jeans so tight-so sexy-that they’ve given up the utility of their own pockets. Women’s pants pockets have always seemed as nonfunctional as wings on flightless birds, whereas men filled theirs with sticks of gum, Swiss Army knives, Kleenex, inhalers, hash pipes, pencils and pens, packs of smokes, paper clips, business cards, condoms, loose coins, wadded-up dollar bills, wallets the size of encyclopedias. Batman, pocketless, wore a utility belt to tote around his Batarangs and grappling gun; the rest of us will be forced to carry man-purses.

What next? Will men become late adopters of every female fashion trend of the last decade? Will great herds of dudes stampede over to Bliss and the Red Door for Brazilians? Will Western civilization relinquish its standards altogether about what actually constitutes clothing? Will men amble about in Xtina-like chaps? Will the banana hammock become acceptable not just at the beach, but at the office?

Besides, these too-low-in-the-ass, too-tight-everywhere-else jeans should come with warning labels stitched into their crotches. That tingly numbness in your ass and thighs? Well, the Canadian Medical Association thought it worrying enough to give it a Latin name: meralgia paresthetica. Plus, every Saturday night, as men stuff and cram their balls into hip huggers, they’re almost certainly smothering millions of perfectly viable, perfectly innocent little fellas. Maybe that’s not such a bad thing. After all, according to no less reputable a source than condom.com, Ben Franklin, in his baggy britches, fathered 53 bastard kids (plus three legitimate ones). Those skin-tights that Mick Jagger spent decades slithering around in may have saved him oodles in child support, because, despite what we presume was a great deal of rutting, his progeny total … only seven.

Johnny Rocco, 31, owner of an eponymous boutique in Boerum Hill, often wears Seven jeans for women, which better complement his body type. (“The jeans for boys are made for straight bodies. I have curves! I’m not a skateboarder boy! I come in, and I come back out.”) On this particular Sunday, X-ray vision would have shown that underneath his low-rise cut-offs, he was just wearing-take a big step back-a “microfiber” jock strap. It’s “breathable for summer,” he explained. “Nothing is worse than having tight elastic around your skin.”

(Men, do you love your low-rise jeans but don’t want to spend all day at work in your jock strap? Are you too prudish to go commando? Try 2(x)ist No Show briefs: low-cut for low-riding jeans or pants! One hundred percent cotton! Plus, they’re guaranteed to make your dick look 12 times its normal size.)

Back to Johnny Rocco: Why flash crack? “It’s naughty. I was being naughty! It’s more like flirting with your body instead of words. It’s the truth!”

But what is the truth, really? “You look at these guys-straight or gay, I think in this instance it doesn’t really matter-who so carefully, so deliberately reveal a measured amount of ass crack, and the reality is, I think, they’re acting out of their deepest fears of like being alone and isolated in this world,” said PlanetOut’s Mr. Tager. “They’re using their ass cracks as a lure for attention and love.”

Surely, when women started baring their cracks a few years back, it was with hopes for attention (don’t know about love). It was about power: Who couldn’t help but stare at the smooth, pillowy flesh revealed at so many bars, as women leaned over to get a bartender’s attention? In the past, women had found power by dressing like men: Remember Diane Keaton, who in her ties and vests and hats made it sexy and exciting and forward-leaning for a woman to dress like Charlie Chaplin? Remember all those women swathed in flannels and carpenter pants in the 90′s, leveling the playing field? But now we have Hillary front-running for the White House and Condi steering the ship at Foggy Bottom. Women have real power, more than ever. Maybe this threat has finally got men copying women.

This is an evolution (or devolution) beyond metrosexuality, which simply validates in men a) narcissism and b) good old-fashioned consumerism-i.e., another excuse to spend massively, stupidly large amounts of money, this time on hair cuts and shirts rather than car stereos and television sets. The low-rise jeans speak to a male co-opting of a very particular form of female vanity: the idea that we need to suffer to look good. Not the aggressive, grunting, controlled burn of bodybuilding or sports playing, but the more passive, erotic, masochistic pain of the corset, the girdle, feet bound by lotus shoes or stilettos.

Of course, these tight pants are, in their own weird way, meant to be super-fucking macho. It’s like manscaping: Do men trim their pubes out of consideration for whomever they might convince to venture down there? More likely, manscapers have seen the shorn scrotes of porn stars and, inspired, buy beard trimmers in the hopes of making their dicks look giant. Men might be showing off ass cleavage, but they’re always more concerned with whatever optical illusion they can conjure up about the size of their package.

But then, this ass cleavage may be all for nothing. Aviva Yael, the fashion director and buyer for the Vice store, was sorry that the trend had found its way into the mainstream.

“Only normal people are wearing low-rise,” she said. “All the people who are into fashion are wearing the skin-tight high-rise.”

Ms. Yael said Kaddish for the low-rise jeans: “Trust me, it’s over. All the trendy New York kids are wearing Swedish denim.”

-Additional reporting by Max Abelson, Raquel Hecker, Leon Neyfakh, Sara Levin and Adrian Quinlan