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Science and sex: Everything you wanted to know about 'doing it'

Medical experts offer advice about breastfeeding. WBAL's Lisa Robinson reports.

"How We Do It" may sound like a sex manual, but it isn't: In fact, that's about the only topic you won't find addressed in anthropologist Robert Martin's book-length survey of human reproduction and what we can learn from the animal world.

There's still news you can use, however. For example, how long should mothers breastfeed their babies? The standard advice from the World Health Organization is six months to two years. But Martin, who is the curator of biological anthropology at Chicago's Field Museum, cites evidence suggesting that three years is a more natural length of time.

"Exclusive breastfeeding is probably six months to a year, then for the last two years or so, breast milk is combined with supplementary food," Martin told NBC News.

That estimate is based on comparative studies of other primates, adjusted for the human body size. It so happens that anthropological studies of tooth enamel, going back to 5,000-year-old remains, arrive at a similar estimate. "The earlier you go back, the closer you come to something like three years," Martin said.

Studies suggest that brain development is better in babies who are breast-fed, probably because of nutritional factors contained in human milk. Martin's point isn't so much that you're a bad mother if you can't breastfeed for three years. "My point is that we should find out what's in human milk that is essential," he said. "If we're going to use artificial milk, we've got to get the formula right."

Basic Books

Robert Martin's book, "How We Do It: The Evolution and Future of Human Reproduction," looks at the myths and realities surrounding reproductive research.

Other chapters delve into the facts and fictions surrounding sex. True or false?

Humans do it faster: True, to an extent. A large-scale study found that human copulation lasts five minutes on average, although it may rarely last as long as 45 minutes. That's much shorter than the 12-hour mating roundsseen in marsupial mice, or the 15-minute couplings for orangutans, but longer than the chimpanzees' eight-second trysts. The males of some species have a bone in their penis, presumably to aid with prolonged mating. (Martin advises doing a Web search for "mountain man toothpick" to find examples.)

Humans are naturally promiscuous:False, at least in comparison with chimps and bonobos, our closest modern-day evolutionary relatives. The evidence for that is in our reproductive system: Chimps' sperm is much stickier than humans', so much so that it forms a "plug" inside the female tract. Scientists believe the plug is part of a strategy known as sperm competition, aimed at preventing other males' sperm from wriggling their way to fertilization. Another tip-off is the relative size of a male chimp's testes: They're bigger than humans, and that's linked to sperm competition. Humans (as well as gorillas, which also lean toward monogamy) lack the genetic machinery for sperm competition. And as for the bonobos ... we all know they sleep around, right?

The rhythm method works: False ... or at least not as true as some people might think. When it comes to contraception, you can't always trust the "egg timer." Researchers found that sperm cells can be stored for days in the womb, probably hidden in crypts in the womb's neck. This means that intercourse leading to conception can occur 10 days or more before ovulation occurs.

Sperm counts are declining: Signs point to "true" ... and that's a worrisome development. Studies from Israel and France, published last year, suggest that average sperm counts have dropped 30 to 40 percent over the past couple of decades. "It's quite obvious that this is going to lead to more cases of infertility," Martin said. The prime suspects include BPA, a chemical found in food packaging and other plastics. Studies have also implicated dairy products, soy products, sauna visits, TV viewing and even trends in male underwear (or the lack thereof).

There's nothing unique about the way we 'do it': Mostly but not completely true. Martin says one of the goals of his book is to "demolish myths of human uniqueness that don't stand up to observation." But when it comes to childbirth and child development, our big brains require special handling. A baby's head has to go through a complex rotation just to fit through the mother's pelvis — and at birth, a human baby's brain is only a quarter of its adult size. In comparison, a newborn chimp's brain is half the adult size. "Our extended period of childhood is really unique," Martin said. "The primary reason for this is that our brains are so poorly developed at birth."

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Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by "liking" the NBC News Science Facebook page, following @b0yle on Twitter and adding the Cosmic Log pageto your Google+ presence. To keep up with NBCNews.com's stories about science and space, sign up for the Tech & Science newsletter, delivered to your email in-box every weekday. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.