Sexual reproduction has been around roughly a billion years, and we're learning more about it every year.
In this slideshow, explore the most interesting stories about the science of sexuality, gender and reproduction published by Discovery News in 2013.
We begin with good news to all the New Year's resolutionaries looking to exercise within the comfort of your own home: Sex can burn calories. In fact, it's better exercise than a walk, though not quite as effective as a jog.
A study published in the journal PLoS One found that men on average expended 101 calories, or 4.2 calories per minute, during a single sexual encounter. A session of similar duration on a treadmill would have burned 276 calories, or 9.2 calories per minute. Women burned an average of 69 calories during a session of sex, averaging 3.1 calories per minute. That compares with average of 213 calories, or 7.1 per minute, for the same amount of time spent on a treadmill.
Looking to put a little extra fuels in the fire while still getting a balanced breakfast? Sexcereal might be just what you're looking for.
Billed as "the world's first all-natural, GMO-free, gender-based breakfast cereal," Sexcereal promises increased libido with ingredients tailored separately for each gender. A his-and-her set of two boxes of cereal costs around $20.
When it comes to sea lampreys, parasites that feed on the blood of fish, it's go hot or go home.
Once thought to be simply ornamental, the anterior dorsal fins of sexually mature male lampreys heat up when females approach. The process is energy-intensive for the sea lamprey males, especially given that they are cold-blooded. After spawning, sea lampreys die.
Given that sea lampreys are an invasive parasite in the Great Lakes, scientist hope to find ways to develop population-control strategies based on the research.
Scientists have long theorized that ancient humans and Neanderthals interbred. In 2013, they found the evidence to back up that theory.
The remains of a human-Neanderthal hybrid dating back 30,000 to 40,000 years ago were unearthed in northern Italy. DNA evidence showed the individual had a mother who was a Neanderthal and a father who was a Homo sapiens.
To a sea slug, the family jewels aren't worth very much, as Discovery News' Jennifer Viegas reported.
When a sea slug (Chromodoris reticulata) finishes having sex, it discards its penis and grows a new one, only to have sex again the next day.
These slugs are what are known "simultaneous hermaphrodites," meaning they perform reproductive male function of donating sperm and female function of receiving it from the partner during sex.
Insects, like humans, don't have sex only for reproduction. In fact, their sexual relationships can be as complicated as ours as well, a study published in the journal Behavioral Ecology and Sociobiology finds.
Like humans, insects may engage in homosexual behavior. The reasons for this are varied. In some cases, males and females look so alike, that even male insects of the same species can't tell the difference. In others, a male who has mated with several females still has their sent, confusing other males looking to procreate. In still other cases, some males will engage in homosexual behavior to distract potential rivals from a female mate.
A sexual act between two insects that occurred 165 million years ago was discovered in northeastern China, as reported last month
Called "Forever Love," the fossil shows a pair of copulating froghoppers, tiny insects that jump around plants similar to frogs, mating in a belly-to-belly position (on the left) inserting his reproductive organ into the female.
You don't need to scientist to explain what's so great about sex, but allow the lowly, sexless starfish to give one reason: genetic diversity.
Certain starfish are hermaphrodites that reproduce asexually, which leaves these species vulernable. As in the cases of Parvulastra parvivipara and Parvulastra vivipara, both species have little genetic diversity and inhabit a limited territory, leving them primed for extinction, according to a study published in August in the journal Biology Letters.
Maybe it's not so much the size of a man's heart that makes him a good father as it is the size of his testicles, according to a study published in the journal Proceedings of the National Academy of Sciences.
Men who produce more sperm have larger testes, forcing an energy trade-off between mating and being a parent. Men with larger testes, who also had higher level of testosterone, were shown to have more of a hands-off style of parenting. Those with smaller testes and lower testosterone proved to be more emotionally invested in the upbringing of their children.
Thanks to bikini waxing, a scourge that has plagued mankind for thousands of years, pubic lice, is rapidly becoming a thing of the past.
Waxing has interrupted the breeding cycles of crab lice, insects that infest pubic hair. And because the lice have nowhere to take root, infestation rates have been plunging.