In Japan some call them herbivores, and on Saturday nights they come out to graze: a perfumed army of preening masculinity. Groomed and primped, hair teased to peacock-like perfection and bodies wrapped in tight-fitting clothes, their habitat is the crowded city where they live in fear of commitment, and the odd carnivorous female who preys on them.

For much of this decade, the older men who drove this country to the top of the economic league tables have looked on in bewilderment at the foppish antics of the generation below.

Japan's twenty- and thirtysomething males seem disinterested in careers and apathetic about the rituals of dating, sex and marriage. They spend almost as much on cosmetics and clothes as women, live with their mums and sit down on the toilet when they pee. Some have even been known to wear bras. "What is happening to the nation's manhood?" wonders social critic Takuro Morinaga.

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Now they have their answer: Japanese males are transforming into grass-eaters.

Coined by columnist Maki Fukasawa, the term soshoku-danshi (herbivorous male) has become one of those cultural buzzwords that hijacks the Japanese media every couple of years. With its implied disdain for vegetarians, the term has been popularised in a bestselling new book called The Herbivorous Ladylike Men (who) are Changing Japan by Megumi Ushikubo, president of Tokyo marketing firm Infinity. Her company claims that roughly two-thirds of all Japanese men aged 20-34 are now partial or total grass-eaters, and a very long way from the classic twin stereotypes of 20th-century Japanese masculinity: the fierce, unyielding warrior and the workaholic salary-man.

"I noticed these major changes taking place between my father's generation, the 58 to 63-year-olds who are retiring now, and the under-35s," she explains. "This is just a very different breed."

Ms Ushikubo believes that the post-war corporate samurai is increasingly a carnivorous dinosaur, whose legendary dedication to the company – at the expense of family – is as much a relic as dawn calisthenics on the factory floor.

"Grass-eaters" by contrast, are uncompetitive and uncommitted to work, a symptom of their epic disillusionment with Japan's troubled economy. "People who grew up in the bubble era (of the 1980s) really feel like they were let down. They worked so hard and it all came to nothing," says Ms Ushikubo. "So the men who came after them have changed."

Like many all-encompassing buzzwords, "herbivore male" can be laughably imprecise. Among his other qualities, the herbivore is close to his mum, has a liking for deserts and foreign travel and leans toward platonic relationships with the opposite sex. He will happily share a night with a woman without laying a hand on her and doesn't waste his money on prostitutes.

But the term resonates with a generation struggling to make sense of profound social disruption rooted in economic changes. Wealth disparities are corroding Japan's meritocracy and poverty is rising. A 2007 OECD report showed that relative poverty in Japan is the second worst in the developed world, after the United States.

Business magazine Weekly Diamond recently noted that more than 80 per cent of 35-year-olds in Japan live on an annual income of two million yen – a key poverty benchmark. "I don't think the lives my parents had is an option for us anymore," laments Kai Ishii, a 26-year-old broker in Shizuoka Prefecture. "I want to eventually get married and buy a house. I just don't know when I'll be able to do that, even if I'm still in a job."

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About one third of the Japanese workforce is now casual or part-time, and confidence in the future is at rock bottom. For many young men, the post-war dream of lifetime employment, home and family, with all the sacrifices it entailed, is fading. In response, some have turned their energies elsewhere, toward the once feminised sphere of consumption – or away from life altogether.

Millions remain at home as "parasite singles", meaning they live with, and off, their parents. The pressing need to find a partner has been alleviated by the ubiquity of porn, sex toys and virtual sex on bedroom computers – one reason, say analysts, why consumption of condoms has been falling for a decade. Even those who opt for conventional marriage find their old role of main breadwinner is no longer available: men and woman increasingly share the roles of work and home.

Many of these complex changes are also occurring elsewhere, and are not unwelcome, points out sociologist Yuko Kawanishi. "Japanese men had it good for a long time. They were macho and sexist, and neglected their wives, so it's good that they're discovering their feminine side, and learning to cooperate."

Ms Ushikubo also hails the rise of the ojyo-man, or ladylike men. "My generation expected that sort of traditional man to pay for everything, to get the good job and support us," the 41-year-old author recalls. "But that system put a lot of pressure on men. They don't know when they'll be fired, or restructured. The idea that they had to carry the burden by themselves is fading and I think we're seeing more equal relationships."

While sociologists debate its merits, the herbivore phenomenon has become popular media fodder. On one discussion show this year, a group of grass-eaters faced their older counterparts like opposing armies across a battlefield. "Men are turning into women," lamented critic Mr Morinaga.

The blurring of gender boundaries has been highlighted by stories appearing to demonstrate that once proud alpha-males are being symbolically castrated in the home. Toilet-maker Matsushita Electric Works reported a survey this year suggesting that more than 40 per cent of adult men in Japan sit on the toilet when they urinate – a figure that is rising year by year.

Nagging wives are also blamed for the rise of the Tenshi no Hizamakura, or Angel's Knee Pillow, a kneeling stool with an unfortunate resemblance to a church pew that brings men closer to the bowl when they pee. Designed to stop splashing around the bowl – women after all still do the vast bulk of household cleaning – the product's arrival prompted the following headline in one media outlet "Men brought to their knees by angry housewives".

Marketing experts like Ms Ushikubo, who has also written a book called The Consumption Power of Twenty-something Happy Parasites, have been quick to learn the lessons of the new herbivorous world.

Men are now leading purchasers of hair products, make-up, fashion accessories and manicures. A Tokyo-based company called WishRoom is even selling men's bras, some to middle-aged salary-men.

"They were the generation we had been told were 'manly' – they led Japan in the post-war period," WishRoom president Masayuki Tsuchiya told the Japan Times this month. He said the company had sold more than 5,000 of the bras to men who are probably reacting against the classic stereotype of stoic, silently enduring male. "They said wearing a bra just made them feel more calm, relaxed and revived."

True carnivores sigh in disgust, but could the grass-eaters be merely the latest flowering of an old tradition? Japanese culture has long had a strong element of androgyny: During the Tokugawa period (1603-1868), men played women and women dressed as men for the theatre, while erotic art celebrated bisexualism and transgender role-playing. The traditions live on in the Takarazuka Review, which features women performers in dress suits playing men, and in Kabuki theatre.

The common element between the Tokugawa era and today, says Osaka-based philosopher Masahiro Morioka, is peace. "Japan has been free from any form of conflict since the Second World War, and that has liberated men from the need to be manly."

Not that he or anyone else is advocating a return to war to give men back their symbolic cojones. "I think the changes among men are mostly healthy and are here to stay," says Ms Ushikubo. "Men are nicer to the women in their lives and happier with themselves." What can be bad about that?