Sperm are energetic little guys, that's for sure. But they aren't particularly hardy. All sorts of seemingly little things can throw them off their game, rendering them unfit for their egg-penetrating duty.
Here are 9 threats every guy should know about, from experts including Dr. Craig Niederberger, professor of urology at the University of Illinois College of Medicine.
Too much testosterone? File it under strange but true.
It takes plenty of testosterone to make sperm, but testosterone supplements can actually KO the body's sperm-making machinery, says Dr. Niederberger. That's because too much testosterone shuts down the pituitary, the gland that regulates the testicles' production of sperm.
Some scientists have even suggested using testosterone as a male contraceptive.
Weed Is Whack
The stuff in marijuana that makes you feel high - tetrahydrocannabinol, or THC - can lay low your sperm.
Seems THC is mimics testosterone, so smoking dope exposes sperm to a hormonal imbalance that can cause sperm counts to fall, according to Dr. Niederberger.
And pot isn't the only drug that can affect male hormones. Heavy use of narcotics like the morphine derivative hydromorphone (Dilaudid) can cause testosterone levels to plummet. When that happens, a guy's sperm production - and sex drive - goes along for the ride.
Tobacco Takes a Toll
Toxins in tobacco smoke can cause sperm to swim slowly, says Dr. Niederberger.
What's more, they can damage sperm cells' DNA cargo. And cigarettes can do their damage even if guys themselves don't smoke. The sons of mothers who smoke have poorer sperm quality than sons of non-smoking moms.
Hot, Hot, Hot
Heat puts the hurt on sperm and sperm production.
The testicles - where sperm cells are made - are "designed" to stay about seven degrees Fahrenheit cooler than the rest of the body. If the testicle temperature rises above that level, the sperm assembly line slows to a crawl.
There isn't a lot of research on how much is too much, but one 2007 study found that regularly taking a hot bath or jacuzzi can hurt fertility, albeit temporarily.
Other research has shown that resting a laptop on your lap can have the same damaging effect.
Of course, there are many babies born to men that take baths and use laptops, so the jury is still out.
Where's My Testicles?
The testicles are supposed to be in the scrotum, whose built-in cooling system helps keep the sperm assembly line humming along. But some guys have a condition known as cryptorchidism, in which the testicles have failed to "descend" from the body to the scrotum. And since heat kills sperm, says Dr. Niederberger, the longer the testicles remain outside their cool scrotal home, the more likely future sperm production is likely to be imperiled.
Anybody with cryptorchidism should have his testicles surgically brought into the scrotum as quickly as possible.
Testicles in a Twist
You might not be able to break or sprain a testicle, but testicles can be "twisted."
The intensely painful condition - most common among tall young men in the middle of the night - can cut off the blood supply to a testicle, says Dr. Niederberger. If that goes on too long, it can kill the testicle. It can even disrupt sperm production in the other, untwisted testicle.
So if you wake up with a scrotum that really smarts, don't be stupid. Head to the nearest emergency room.
Calling All Sperm
Semen exposed to the radio waves emitted by cell phones can result in damage to sperm, recent studies have shown. But does this phenomenon translate into the real world - when sperm cells are "protected" by skin and clothing?
Doctors don't know. But until the science gets sorted out, Dr. Niederberger says, it probably makes sense for guys to keep their cell phones someplace other than a pants pocket.
Radiation: Sperm Poison
Radiation, including that from X-rays can stop sperm production inside the testicles. The bigger the dose, the longer it takes sperm to recover. Very big doses can render men permanently sterile, but the occasional X-ray, experts say, isn't enough to put your little swimmers down for the count.
Flab Flays Sperm
Obesity has been linked to poor sperm, possibly by lowering levels of testosterone and raising levels of the female hormone estrogen (a little of which all men have). The fatter a guy is, says Dr. Niederberger, the worse his hormonal situation - and the greater his sperm troubles.
Eating a balanced diet seems to keep the sperm factory humming along. And two recent studies suggest that a nutritional supplement known as coenzyme Q can boost sperm production, says Dr. Niederberger.
In one of the studies, coenzyme Q improved sperm cells' wiggling. In the other, it boosted sperm counts and improved their motility (movement). The studies were small - but promising, says Dr. Niederberger.