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Why Are Some People Gay? - Part 2


By Kim Schuske

Salt Lake City, UT –

When it comes to same sex attraction, humans are not alone. University of California Professor Marlene Zuk says hundreds of animal species engage in same sex activity. Zuk was studying behavior in a colony of Hawaiian sea birds when she realized many of the long-term couples were of the same sex.

"About a third of the pairs of that colony of albatross are actually two females," says Zuk. "So they'll get together they exhibit the same behaviors with each other that the males and females will exhibit with each other, and the two females will collaborate to raise one chick together."

One of the female albatross pairs has been together for seventeen years. Other animals have same sex encounters that are much briefer, including our closest living primate relatives the Bonobo.

"Same sex behavior is part of the social fabric of their group," explains Zuk. "For instance they use sex as a way to resolve social tension."

Zuk says for a long time scientists ignored these same sex interactions because they didn't know what to make of sexual behavior that doesn't lead to offspring.

"Animals turn out to be a lot more flexible in their behavior then people had necessarily thought," she says.

To really dissect a behavior and determine which cells and genes are involved, scientists often turn towards simpler animals. University of Utah researchers Jamie White and Professor Erik Jorgensen study round worms, also called nematodes. The worms are small, 1 mm long about as long as a dime is thick. So White spends most of his day looking through a microscope.

"Worms do everything that we do," says White. "They eat, they sleep, they have sex and they poop, and they reproduce. And at a certain level that is all behavior has to accomplish for any organism."

White and Jorgensen wanted to find out if sexual attraction in worms is wired in the brain. It's a fairly easy experiment to do because the researchers can use genetic tricks to make part of the worm male and part female.

"So each cell in the worm knows whether it is a boy or girl cell," explains White. "So what you can do is genetically manipulate worms so that the nervous system is one sex and the body of the worm is another sex, so you can have a worm that have the brain of a boy and the body of a girl and visa versa. And a key finding is that worms behave like their brains would tell them to behave not like their body."

A transgender worm with the brain of a male but the body of a female is attracted to other female worms, even though it doesn't have the correct body parts to mate. White says sexual attraction is built into the worm brain as the embryo develops. For now they don't know which genes are involved. But they do in fruit flies almost a dozen genes have been found. David Featherstone, associate professor at the University of Illinois found one gene called genderblind. He says there are odors in male flies that other males don't like. But, when the genderblind gene is damaged, male flies become attracted to that smell.

"A normally aversive cue, something that turns them off, becomes an attractant," says Featherstone. "And so in this case, these male flies were attracted to other male flies. And that was interesting because it's not like the brain switched sex. It's not like they had a sensory deficit. They could still smell and taste perfectly fine, they could still hear fine, see fine. But they simply changed how they viewed a certain cue."

Undoubtedly the biology of same sex attraction in animals is different than in humans. For example, Featherstone says that while the genderblind gene is expressed in the brains of mice and humans, there is no evidence it's involved in sexual attraction. But according to University of Utah psychologist Lisa Diamond, studies of male identical twins suggest genes do play a role in humans.

"If you have two twins and one of them says, I experience same sex attraction, the chances that the other twin also claims to have same sex attraction is significantly higher then would be expected by chance. And it's about 30-percent," says Diamond. "Now what's interesting about that is remember, twins are 100-percent genetically identical. Now the important thing to realize is that 30-percent is greater than zero, and it is significantly greater than the population prevalence of same sex attractions, but it obviously cannot explain the whole phenomenon, because if it did it would be 100-percent."

Diamond says what the twin studies make clear is that genes influence same sex attractions in some people, but they are not the whole story. She adds that no single gay gene has been identified for men or women.

So besides genetics, are there other potential biological causes of same sex attraction in humans? Simon Levay, neuroscientist turned author says the environment in the womb as a baby develops may be involved.

"There are a lot of biological mechanisms that operate before birth that are not genetically controlled," says Levay.

In mammals, sex hormones are important for differences in brain development between males and females. Levay says multiple studies show the size and activity of some brain regions in gay men are more similar to women than straight men. A theory gaining steam among many in the scientific community is that these brain differences are caused by exposure to unusual levels of hormones.

"The differentiation of this part of the brain and the brain in general as more male or more female really seems to depend mostly on the levels of sex hormones and specifically testosterone that the brain is exposed to while its developing," says Levay.

However, hormones are not expected to be the whole story either. Studies on women with a condition where they were exposed to high levels of male sex hormones in the womb had conflicting results. In some studies a high percentage of the women identified as gay, but in others there was no difference.

In the end, psychologist Lisa Diamond says there will likely be different paths leading to same sex attraction in humans. In some people it may be genetic, in others it could be something that happened in the womb, and in some there might be multiple factors. Copyright 2010, kuer

For questions or comments, contact reporter Kim Schuske at:

The first movie below is of Unaltered Fruit Flies and the second is of the Altered Fruit Flies. Movies courtesy of David Featherstone, associate professor at the University of Illinois.

Courtesy of David Featherstone, associate professor at the University of Illinois

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