THE sex lives of men can seem like an Aesop’s fable. As years go by and sexual desire fades, men may stop mating with an enthusiasm akin to rabbits and become more like red foxes, lonely animals who make do with one brief breeding season each year.

The moral: Even if you’re a happy rabbit now, don’t take your sex life for granted. You could still turn into a fox.

This tale is a warning, not a law of nature. Men can take steps to protect their sexual desires and abilities throughout their lives, says Jim Pfaus, a professor of psychology and neuroscience at Concordia University in Montreal who studies the biology of libido. With some regular sex drive maintenance, “Our generation might keep having intercourse until we drop,” he says. “I hope so.”

There’s no biological reason why a man can’t carry a strong libido to his retirement party or even to his nursing home, according to David Rowland, sex expert and professor of psychology at Valparaiso University. “It’s a myth that sex drive always diminishes as men get older,” he says. “A lot of men in their 60s and 70s line up at the pharmacist’s office for Viagra.”

Still, desire can crumble over the years, often under the weight of physical problems, emotional turmoil and, of course, troubled relationships.

One way to protect yourself: Have lots of good sex now (or when convenient). Not only will you be giving your partner, if you have one, extra incentive to stick around, you’ll be building a buffer around your libido. Frequent, stimulating sex primes the brain to want and expect more sex later in life, Pfaus says.

Research by the late Julian Davidson of Stanford University suggests that a properly primed libido can even withstand a shutdown of testosterone, the male hormone that helps fuel sex drive. He found that about one-third of men with extremely low testosterone levels (often due to testicular disease) still had frequent, satisfying sex. He surmised that these men had become so accustomed to sex that they could be aroused without help from hormones.

Regular, heart-pounding exercise is another good way to keep your sex drive humming. “When you increase your blood flow, you’ll have a much easier time getting aroused,” Pfaus says. You don’t have to jump straight from the treadmill to the bed, either. The high will remain for a few hours afterward.

In the long-run, regular exercise will help strengthen your lungs and heart, two of your most important sexual organs. Exercise can also help prevent obesity, a condition that could potentially mess with hormones -- and your sex drive. As Rowland explains it, fat cells in men tend to secrete estrogen, the female sex hormone. The consequences of extra estrogen in men aren’t clear, but the hormone could conceivably dampen a man’s sex drive as well as his ability to perform, he says.

When a man starts to lose desire, the problem may be emotional, not physical. An unsatisfying job, financial troubles, a death in the family and other upheavals can definitely steal a man’s mojo. “Stress is the antithesis of sex,” Pfaus says. Anything that can help relieve stress, whether it’s relaxation exercises or a trip to the gym, can help put a man’s mind back on more pleasant subjects.

For people who are married or in a relationship, sex drive also reflects the health of partnerships, for better or for worse. Resentment or power struggles can kill desire. And, after a few years or decades together, men and their partners tend to fall into ruts that drain the excitement out of sex.

Some men try to compensate through affairs. But in most cases, they need a change in routine -- not a change in partners, Pfaus says. (“Many men end up just doing the same boring stuff with their new partner,” he says.) Men can kick-start their sex drives without straying from their partners by having sex at different times and in different places -- unexpected venues, motels, the kitchen -- or by just holding hands or giving each other massages. The famed sex researchers Masters and Johnson found that a week of nonsexual touching exercises put a charge into tired relationships. “By day seven, the couples were acting like they were 16, even though they were 60,” Pfaus says.

Men often lose interest in sex with their partners because they spend too much time on solo projects, Rowland says. Men who masturbate frequently (many do it every day) may even lose their ability to reach orgasm through intercourse. Often, they can regain their desire -- and orgasms with their partner -- by simply taking a break from masturbation, he says.

While you’re busy enhancing your libido, you should also work to protect your ability to achieve and maintain erections. In other words: Don’t smoke, don’t drink too much, and do what you can to avoid clogged arteries and diabetes.

After all, a healthy sex drive won’t do you much good if you can’t use it.