Image: Gene experiment
Shah Lab / UCSF
The female mouse at right is missing the hormone-reactive gene Cckar, and was seen rejecting the sexual advances of the male mouse at left. Researchers believe that hormones acting on CCkar and other genes may play a role in sex-specific behaviors.
updated 2/2/2012 1:20:43 PM ET 2012-02-02T18:20:43

Men are from Mars and women are from Venus, but how did they get there? Our gender differences might be a function of how our brains react to hormones, a new study on mice suggests.

The study showed that when different sex hormones turned genes in the brain on or off, the mice showed different parenting behaviors.

Though the research was performed in mice, these sex hormones show similar effects in mammals, and many of the genes the scientists discovered are found in humans, the researchers say. It's possible that hormones are having an effect on brain genes in humans, too, though the behaviors controlled might be different.

"Testosterone and estrogen control sexually dimorphic behaviors in vertebrate species. We've known for a long time that they control behaviors at a very large-scale level," study researcher Nirao Shah of the University of California at San Francisco told LiveScience. "What we've discovered are [some of] the gene networks that are regulated by these sex hormones."

Hormonal brain
The researchers focused on the hypothalamus, a specific region of the adult mouse brain that is involved in mating and parenting behaviors, such as courtship, nursing and defending pups. They looked to see what genes were being turned on and off in this region by the presence of sex hormones, such as testosterone, estrogen and progesterone.

They found 16 genes that showed clear sex differences. The researchers then studied four sets of mice that were missing one of these hormone-reactive genes (Brs3, Cckar, Irs4 and Sytl4) to see how their behaviors might change. The mice all looked normal, but when the researchers studied specific mating and parenting behaviors, they found that the mice all had slightly different quirks. [The Animal Kingdom's Most Devoted Dads]

"In each one of these mutants we found deficits in one or another behavior, but everything else appeared untouched, it was normal and looked like the controls did," Shah said. "The larger implication of this is that you can take a complex social interaction, like mating or fighting, and you can sort of break it down into genetically controlled elements of that behavior."

Controlled behaviors
These genes seemed to control male sexual behavior, male aggression, maternal behavior and female sexual behavior in the mice. For example, the female mice missing the gene Cckar had lowered sex drives — about two to three times lower than normal females. Another gene, Irs4, controlled maternal aggression ; female mice missing Irs4 were less aggressive to invaders in their nest and didn't chase after escaping pups as quickly.

Male mice missing the Tytl4 gene had subtle changes in the pattern of their mating behaviors — they performed their courtship displays in a different order than normal mice, though they still behaved like males and were able to impregnate females. The other gene they discovered, Brsr3, also controlled male mating behaviors: Males missing the gene initiated sex faster with females and were faster to start fights with other males. [10 Surprising Sex Statistics]

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The researchers are looking at the rest of the 16 genes they discovered to determine if they are also regulating certain behaviors. "I suspect there are many more genes like this that are going to be discovered," Shah said.

Gendered diseases
Knowing that genes controlled by sex hormones can have impacts on behavior, Shah says in humans this could help explain why some mental disorders, such as ADHD or autism, affect one sex over the other. "With the genes we've identified that are involved in setting up male and female sex dimorphism in the mouse brain," Shah said, "we can see how the brains could be differentially vulnerable to disease conditions."

Scientists can't be sure yet how these specific genes will translate to humans, but Shah suspects there may be hormone-gene interactions in the human brain. Hormone-controlled brain genes could play a role in behavior in humans, but we really don't know what behaivors might be controlled, since human sex and mouse sex are so different.

"There are efforts under way to look at gene expression in the human brain, but we don't know what that's going to look like yet," Shah said. "At least with animals we can say, yes, we can explain at least some of the variation and we can at least now begin to think about genetic explanations for some differences in social behaviors."

The study was published Thursday in the journal Cell.

You can follow LiveScience staff writer Jennifer Welsh on Twitter @microbelover. Follow LiveScience for the latest in science news and discoveries on Twitter @livescienceand on Facebook.

© 2012 All rights reserved.

Explainer: 10 peeks at sex in the wild

  • Image: scroll fragment
    Jerome Maison  /  AP file / Warner Independent Pictures

    Emperor penguins endure some the harshest conditions on the planet — the Antarctic winter — to satisfy their primal urge for sex. The annual ritual begins with a days-long, up to 75-mile slog to their inshore breeding grounds. Once there, an elaborate courtship of calls and poses reunites old mates and enables young lovers to form lasting bonds.

    Copulation itself occurs under the cover of the dark of the polar night. Then the true sign of their devotion begins: Males huddle together to incubate the eggs as the females waddle back out to sea to feast, fattening up to provide for their newly hatched young. Once she returns, males depart in the first of a tradeoff that may, eventually, allow for healthy offspring.

    Click on the "Next" label for nine more peeps at sex in the wild.

    - John Roach, contributor

  • Captive pandas require help to get it on

    Image: Pandas
    Wichai Taprieu  /  AP file

    In the wild, biologists say pandas get it on without too much of a hitch. But since the wild population in China's mountain forests hovers around just 1,600, there's a push to boost the panda population in zoos around the world. And that's where the problems lie. Many pandas lack interest in their arranged mates and some inexperienced males who give it the old college try fail to engage the proper body part. As an aid, some zoo keepers attempt to arouse the bears and teach them appropriate technique with specially made DVDs, a.k.a. panda porn. When that fails — and it often does — artificial insemination is considered a last resort. These tricks combined with a better understanding of what makes the panda libido tick are beginning to pay off. The captive population has boomed in recent years.

  • Male macaques groom for sex

    Image: Japanese Macaque monkeys
    Shuji Kajiyama  /  AP

    Male macaques groom females in exchange for sex, according to a study that examined the market underpinnings of monkey sex. According to the research, based on 20-months of observation in an Indonesian nature reserve, a female is three times more likely to mate with a male if he grooms her first. Supply and demand also comes into play: Males spend more time grooming when competition for female attention is greater. Scientists refer to this practice as a biological market.

  • Male antelopes play hard to get

    Image: Antelopes
    Jakob Bro-Jorgensen / Zoological Society of London

    For highly-desired male topi antelopes in Africa, the frenzied six-week-long mating season is exhausting. Any given female is receptive for about a day, thus she wants to mate as many times as possible, especially with the fittest males. This creates intense competition for high quality antelope sperm, allowing those that possess it to be picky. A study of the phenomenon found the choosy males deliberately select the least mated females and fend off aggressive females they've already mated with. The aggressive female in the center of this image is attacking the male on the left as another female eyes the scene.

  • Sumatran rhinos tussle before they tango

    Image: Newborn rhino
    Tom Uhlman  /  AP

    Mating for the typically solitary and territorial Sumatran rhinoceros is a drawn-out affair. Studies of the critically endangered species in captivity show that when a female becomes receptive to a male's approach, she'll exhibit increased urine spraying, tail raising and swinging, and vocalizations. Foreplay includes head and genital butting, which can be a bit too much when a female isn't quite ready to tango with a young and aggressive male. But when the tussle is successful, a male will mount, often riding his mate for up to an hour. The image here shows the results of a successful mating at the Cincinnati Zoo. Such captive breeding efforts are a silver lining for conservationists hoping to keep the Sumatran rhino alive — just 300 are thought to remain in the wild.

  • Burliest walrus bulls get the harem

    Image: Walrus
    Liz Labunski  /  AP

    With a cacophony of clicks, clacks, whistles, and bellows, male walruses swim around the chilly Arctic waters vying for the attention of ice-bound females. Males will also fight off other males that get too close, sending the loser packing. Once the fussing and fighting is done, female harems surround the burliest males in the water for an underwater romp. Though scientists know little about what actually happens under the cover of the waves, they do know walruses are endowed with the mammal world's largest penis bone, called a baculum, which extends up to 30 inches.

  • Dolphin mating is brief, but bountiful

    Image: drainage channel
    Noaa  /  NOAA

    For playful dolphins, mating seems just like another carefree and pleasurable way to pass away the day. Along with some typical male posturing for access to females, there's plenty of chasing, rubbing, nuzzling and stroking that constitutes as foreplay. The belly-to-belly copulation act itself lasts less than a minute, though is often repeated several times over the course of an hour.

  • Virgin female spiders risk all for a big mate

    Image: archaeological site in Masada

    For some male spiders, sex is the ultimate sacrifice: females eat them as part of the reproductive ritual. But among the East African blood-gorging jumping spider Evarcah culicivora, shown here, males possess the coital-infused cannibalistic urge. Nevertheless, female virgins opt to be deflowered by bigger males before settling in with a small guy for the long haul. Scientists suspect females gamble with their fate once in hopes of producing larger, fitter offspring, but decide not to double down after the flirtation with danger.

  • Sappy sex for beetles of all sizes

    Kensuke Okada

    As this picture shows, Japanese sap beetles come in big, medium, and small sizes. Generally, in the beetle world, mating success is only bestowed on males with the biggest bodies because they can beat out the competition with brute force. Big sap beetles successfully employ this strategy, but when they do, the medium guys take to the air with their extra long wings and survey for sites where big males are absent. But the little guys have the biggest testicles. This allows them to hang with the big guys and sneak sex behind their backs. Since the little beetles have such big testes, they produce more competitive sperm, upping their chances at siring offspring when they get a shot.

  • Cycads have 'hot' plant sex

    Irene Terry  /  Univ. of Utah

    Hot sex has ancient roots. The males in a group of plants called cycads, which have been around for at least 250 million years, get all hot and bothered as a means to compel pollen-covered insects deep in their cones to flee forth and find a female to pollinate, according to scientists. The plants begin the process by emitting a fragrance that lures little insects called thrips into their cones. After a few hours of the thrips feasting and rolling around in there, the cycads heat up as much as 25 degrees Fahrenheit, which turns their sweet smelling fragrance into a stench. The thrips flee, some landing on benign-smelling female cycads and thus completing the pollination cycle.


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