As I have blogged about before, a big part of sexual preference is biological. Research shows that some people are hardwired to prefer the same sex. This is true in animals as well.

What scientists haven’t had much luck at yet is finding out why this is. There have been studies that have implicated the X chromosome (although not convincingly). And others that have pointed to brain structure. But none of these studies really gets at what is going on in these folks’ brains that makes them prefer relationships with the same sex.

A new study out suggests that, at least in mice, the neurotransmitter serotonin may play a big role. Scientists created a strain of mice that lacked most of a certain kind of serotonin receptors in the brain (central serotonin receptors). These mice were healthy and happy. And the males were not at all fussy about whom they hooked up with.

When scientists put wild type* male mice in a cage with other males, they mostly ignored the other mice. The male mice lacking their central serotonin receptors reacted differently. They got busy with the male mice almost every time.

These mice aren’t homosexual though. Given a choice of a male or female, they didn’t really care; they went after both at about the same rate. The genetically altered mice were more bisexual than homosexual.

The researchers did lots of other experiments as well that showed that these mice were not oversexed or lacking anything in particular. They just liked the boys as much as the girls.

What this study tells us is that in mice, serotonin plays a big role in sexual preference through these particular brain neurons. What it doesn’t tell us is if the same thing is true in people. After all, picking a mate is very different in mice as compared to people.

But there are hints that serotonin works differently in the brains of bisexual and homosexual men. For example, certain selective serotonin reuptake inhibitors (or SSRIs) have different effects in bisexual and homosexual men compared to heterosexual men. Still, this isn’t yet enough to finger serotonin use as the main driver of sexual preference in people.

What it does do though is provide scientists some direction for their research. Instead of wading through all 20,000+ genes, they can start out focusing on those that deal with serotonin. This will greatly simplify the research and if serotonin does play a role, then scientists will find the genetic variations involved sooner rather than later.

And frankly, given the slow progress thus far, focusing on serotonin genes won’t set the field back too far. It is probably worth taking the research in this direction.

* Wild type just means a mouse (or any living thing) that hasn’t been tampered with. In this case, it is a run of the mill lab mouse.

A more in depth look at the story from blogger Ed Yong at Not Exactly Rocket Science.

37.7749295 -122.4194155

Banana Slugs are hermaphrodites, which means they are an animal that has both female and male reproductive parts

Forty and Fabulous is the theme for Pride this year. San Francisco’s Pride Celebration and Parade has been going strong since 1970. Pride recognizes and celebrates the diversity that has put San Francisco on the map. NightLife at the California Academy of Sciences kicked off the Pride weekend with some help from Juanita MORE, Verasphere, SF Cheer and Jane Tollini. In prepping for the festivities and specifically Jane Tollini’s talk – “How Animals Get Down”, I had to research a specific Academy animal and its claim to fame.

Below are a few of the tidbits I’ve learned about in my tenure at the Academy which prove equally intriguing:

The most popular docent cart at NightLife is the baculum cart. Baculums are penis bones that are found in most mammals. They range in sizes depending on the mammal and aid in sexual intercourse. One of the largest Baculums belong to the walrus and Alaskan native Americans would polish and carve these baculums for clubs or knife handles. They are known is there culture as Oosik.

Banana Slugs are hermaphrodites, which means they are an animal that has both female and male reproductive parts. In some cases of mating, a male reproductive member will become stuck; if this happens it is chewed off and discarded; thus making the slug a female.

There is a species of Cichlids called the Cross Dressing Cichlids. In this species there are three sexes – a dominant male, a non-dominant male and a female. The beta male resembles the female and when the alpha male is protecting his female harem the beta male will mate with the females. It is also interesting to note males can change between dominant and non-dominant.

Clownfish live in small groups in a single anemone. They are one of only a few species of fish that are immune to the anemone’s toxicity. Within a group of Clownfish there is a breeding pair that cohabitate with smaller non-breeding males. When the female dies, the dominant male changes sex and becomes the female. This strategy is defined as sequential hermaphroditism.

Penguins like most birds, reptiles and amphibians reproduce by what’s known as a cloacal kiss. The cloaca is a single orifice that is used for many functions; namely, urinating, defecating and mating. Thus, both male and female penguins have cloacas. Mating is achieved by bringing the cloacas together to touch.

Often it is very difficult to sex a snake as all the reproductive parts are internal. Biologists will use a special probe to determine the sex of a snake in captivity. Males have two members that protrude during mating calling hemepenes. Other than during mating, it is hard to tell the difference between a male and female. I learned this the hard way when I went to pull out a Ball Python for an outreach program. It was curled around its cage mate and I realized too late that they were mating when I interrupted!

I have often heard that homosexuality is not natural. Yet is has been observed, researched, and documented in many species – so has sex changes, several sexes in one species, hermaphrodites, harems, polygamy and sexual cannibalism. An interesting article was written up in the New York Times entitled, "Can Animals Be Gay?" If you are interested in learning more about the diversity of reproduction methods in the animal kingdom and why this diversity is needed; these two books are great:

Dr. Tatiana's Sex Advice to All Creation: The Definitive Guide to the Evolutionary Biology of Sex by Olivia Judson

Evolution's Rainbow: Diversity, Gender, and Sexuality in Nature and People by Joan Roughgarden

37.7699 -122.467174

Last blog I talked about some studies that link homosexuality and genes. The most powerful studies are those that compare identical twins to fraternal twins. These studies show that both twins in an identical pair are more likely to be gay than are both twins in a fraternal pair. Since identical twins have the same DNA, this suggests that something genetic is going on.

But no study showed that if one identical twin was gay, then the other one was always gay as well. We'd expect both twins in an identical twin pair to share a purely genetic trait 100% of the time. Because they don't, the environment definitely plays a role. But not like you might think.

By environment I don't mean certain family situations (although these sorts of factors probably contribute as well). What I am referring to are environmental factors that can affect brain development. Factors like viruses, hormones, or maybe even antibodies.

We know, for example, that the more older brothers a man has, the more likely he is to be gay. Even if he doesn't live with the older brothers.

This suggests that something biological is going on. It is as if the mother's body remembers how many sons she has had. One way this might happen is through her immune system.

Perhaps when a mother has a son, she makes antibodies to something having to do with carrying a male child. The more sons she has, the more antibodies she makes. At some point, she makes enough antibodies to affect brain development and the younger son is now gay.

Of course, not every youngest son is gay– he is just more likely to be homosexual. Other factors have been reported to increase the chances that someone is gay too. These include being left handed, having a counter-clockwise hair whorl and maybe even different finger lengths. All of these traits are associated with differences in brain development.

There are apparently many paths to a homosexual brain. This isn't surprising as human sexuality is much too complex to be due to a single gene or environmental factor. Most likely, it is the result of many factors all working together.

Some gay men may have inherited genes that made environmental factors more likely to affect their sexuality. And some gay men may have been exposed to multiple environmental effects that affected their sexuality despite their genes.

I think you can appreciate how these kinds of complex interactions can make finding "gay" genes incredibly complicated. And why it is hard to pinpoint the environmental effects that contribute to becoming homosexual as well.

Dr. Barry Starr is a Geneticist-in-Residence at The Tech Museum of Innovation in San Jose, CA.

latitude 37.3316, longitude -121.89

Gay Pride Parade in Brazil

For example, one study showed that if one identical twin was gay, then the other was 50% of the time. If the two brothers were fraternal twins, then the odds dropped down to 22%. And if one brother was adopted, the odds were only 11%.

Another more comprehensive study gave similar results with both twins in an identical twin pair being gay around 32% of the time as compared to 13% of the time for fraternal twins. Lots of other studies have been done that show the same trend even if they don't have the exact same numbers.

These results strongly suggest genes are involved because identical twins share the exact same DNA. If both twins in an identical twin pair have some trait in common more often than do fraternal twins, then odds are that genetics plays a role.

These twin studies show that being gay is at least partly genetic. So the new study is really just looking to explain explain the older results.

That said, I am torn about whether I want this study to succeed or not. If it is successful, that'll shut up all those people who maintain that sexual preference is purely the result of personal choice. In my opinion, quieting those folks is a good thing. But finding the gay gene(s) may have unintended consequences. Some people may see being gay as akin to a genetic disease like cystic fibrosis or sickle cell anemia. As something to be minimized through carrier screening, embryo selection, or even abortion.

Scary, but I have heard people say things like this. Just the other day someone asked a friend of mine if homosexuality was genetic. They weren't merely being curious. They were hopeful that there was a gay gene so that homosexuality could be eliminated from the human population at some point. Yikes!

And that was here in the San Francisco Bay Area. Who knows what they are saying in Wyoming or Texas. Do we want to find the gene(s) responsible for homosexuality? Or are some things better left unknown? Comment below.

Dr. Barry Starr is a Geneticist-in-Residence at The Tech Museum of Innovation in San Jose, CA.

latitude 37.3316, longitude -121.89