The ASSET Program / Cornell

An image produced by a scanning electron microscope shows two Tetrahymena cells in the act of mating.

Biologists have known for decades that there are up to seven sexes of the single-celled organism known as Tetrahymena thermophila — but they didn't know exactly how those different sexes "did it." Until now.

When it's time for Tetrahymena to mate, two organisms of different mating types recognize each other and get together to swap DNA. The results of the hookup are totally random. One critter may be mating type No. 1, the other may be mating type No. 3, and the two resulting progeny may turn out to be, um, mating type No. 6. How do they do that?

In the journal PLOS Biology, researchers report that the hooked-up organisms almost literally roll the genetic dice to determine what the sex of the progeny will be.

The researchers say the key to Tetrahymena's sexual proclivities lies in its double genome: Every cell has a "somatic" genome that manages its everyday life, plus a "germline" genome that serves a function similar to that of the ovaries or testes in humans. The germline genome contains incomplete gene pairs for each of six or seven sexes, depending on the cell line. (In this case, the cells came in six sexual flavors.)

Random sex
When two microbes hook up, the progeny's newly created somatic genome latches onto one of those incomplete gene pairs, producing one complete sex-specific gene pair. The other sexy bits from the germline genome are wiped out. The random rearrangement leaves the resulting cells with exactly one complete sex-specific gene pair — and one mating type.

"It's completely random, as if they had a roulette wheel with six numbers, and wherever the marble ends up is what they get," senior researcher Eduardo Orias, a research professor emeritus at the University of California at Santa Barbara, explained in a news release. "By chance they may have the same mating type as the parents — but it's only by chance. It's a fascinating system."

Most of the time, Tetrahymena reproduces asexually, simply by having a parent cell divide into two progeny cells. But the organisms tend to pair up sexually when food is scarce, apparently as part of an evolutionary mechanism that takes advantage of genetic diversity. Sex-specific proteins on the surface of the cells serve as a signal that mating is likely to result in more diverse progeny. That's how two cells of the same mating type avoid pairing up with each other.

This type of mating process doesn't by itself increase the Tetrahymena population: Two cells hook up, and after recombining DNA, two cells separate again. "This is sex without reproduction," Orias said during a telephone interview. After mating, the recombined genetic information is passed down from parents to progeny through asexual reproduction — until it's time for the next hookup.

What it means for humans
Although the process sounds totally alien to us two-sex types, the lessons from Tetrahymena could have implications for human health.

"Tetrahymena has about as many genes as the human genome," Orias said in the news release. "For thousands of those genes, you can recognize the sequence similarity to corresponding genes in the human genome with the same biological function. That's what makes it a valuable organism to investigate important biological questions."

For example, Tetrahymena may reveal new tricks relating to the methods that cells use to recognize friend vs. foe. That could have implications for studying human immune response. Also, the way that the organisms rearrange their DNA may point to new strategies for fighting cancer, which often results from the faulty rearrangement of genetic material.

"The hope is that at some point, there may be useful applications for medicine," Orias told NBC News.

More about microbial marvels:

• No sex for 40 million years? No problem!
• Single-celled giant upends early evolution
• Strange organism has unique roots in tree of life

In addition to Orias, the authors of "Selecting One of Several Mating Types Through Gene Segment Joining and Deletion in Tetrahymena Thermophila" include Marcella D. Cervantes, Eileen P. Hamilton, Jie Xiong, Michael J. Lawson, Dongxia Yuan, Michalis Hadjitomas and Wei Miao.

Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. To keep up with Cosmic Log as well as NBCNews.com's other 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.

Millionaire space tourist Dennis Tito's plan to send two astronauts on a 501-day flight that zooms past Mars and swings back to Earth would set plenty of precedents on the final frontier — but the most intriguing precedent might have to do with the astronauts that are to be sent: one man and one woman, preferably a married couple beyond childbearing years. We're talking about sex in space, folks.

And if that's not intriguing enough, consider this: There are already a couple of candidates for the job.

"We'll certainly throw our hat in the ring," said Taber MacCallum, who's a member of the development team for the 2018 mission that Tito has in mind.

MacCallum and his wife, Jane Poynter, were crew members together in Biosphere 2, the controversial two-year-long experiment in long-term environmental containment. They went on to become co-founders of Paragon Space Development Corp., a company specializing in life-support systems for spacecraft. Their expertise in life support is why they're involved in Tito's "Mission for America," which was officially unveiled on Wednesday at the National Press Club in Washington. But it just so happens that they also fit the profile for the trip: Poynter is about 50, and MacCallum will turn 49 on July 20, the anniversary of the Apollo 11 moon landing.

The couple won't be the only candidates in the running. "When we tell people we're proposing to send a man and a woman on a mission to Mars, as a married couple, people line up. ... That chord gets struck over and over again," MacCallum said.

Paragon

Taber MacCallum and his wife, Jane Poynter, are part of the planning team for a mission to Mars in 2018. They're also potential candidates to take the trip.

MacCallum explained that Tito wants the crew on humanity's first trip to Mars to be representative of humanity, and because the current concept for the trip calls for two spacefliers, that means a man and a woman. A married couple would be ideal, MacCallum said, because of the "whole issue of companionship." MacCallum didn't refer specifically to sex, but that would presumably be part of the companionship package.

"When you're out that far, and the Earth is a tiny, blue pinpoint, you're going to need someone you can hug," Tito told Space.com. During Wednesday's briefing, Tito told reporters that he envisioned Dr. Phil giving the couple "marital advice" during the trip.

In addition to their experience with life-support systems (and with each other), MacCallum and Poynter can draw upon their experience with life in isolation during the Biosphere 2 experiment in Arizona, which lasted from 1991 to 1993. The isolation inside a two-room spacecraft for 501 days will be even deeper. Even though the Biosphere 2 crew was separated from the outside world, "we could walk out at any time," MacCallum pointed out.

That's not the only challenge: Even with radiation shielding in place, the round trip to Mars is likely to involve exposure levels higher than NASA's limits, MacCallum said. (That's why the astronauts should be beyond their childbearing years and willing to accept an increased risk of cancer.)

Then there's the exposure to the health effects of long-term weightlessness, including bone loss and muscle loss. The astronauts who fly past Mars will surpass Soviet cosmonaut Valery Polyakov's 437-day record for continuous time in microgravity, set in 1994-1995 aboard the much roomier Mir space station. 

"We're definitely pushing boundaries," MacCallum said. "It's definitely going to be hard and challenging. But we can rely on elegance and simplicity."

When, where and how?
The details of the mission plan have come to light just in the past few days, but MacCallum said that Tito has been mulling over the idea for years. Tito started out as an engineer at NASA's Jet Propulsion Laboratory, helping to design trajectories for the Mariner missions to Mars in the 1970s. Then he put his math genius to work in the investment world, building California-based Wilshire Associates into a multibillion-dollar powerhouse. In 2001, he spent around $20 million of his fortune for a seat on the Russian Soyuz spacecraft traveling to and from to the International Space Station.

After his eight-day space tour, Tito got back to business. But he also started working out a trajectory that could send a spaceship directly from Earth to Mars for a fly-by within 100 miles (160 kilometers) of the Red Planet's surface, and then back to the home planet 501 days after launch. Once the spaceship was on its way, only minor course corrections would be needed. There'd be no need for undocking or redocking ... no landing ... no do-or-die engine burn for the return from Mars.

There's one big catch, though: The trip will have to be started when the planets were aligned just right. One opportunity will come in 2016. Then there's another one in 2018. After that, the next chance won't come around until 2031.

Planning for a launch in January 2018 looked particularly attractive, and not just because that could plausibly provide enough time to put the mission together. That's also a time frame when solar activity is expected to be at a minimum, reducing the level of radiation exposure. So Tito assembled a team from Paragon as well as NASA's Ames Research Center and other space ventures to flesh out the mission plan.

The plan calls for launching the two astronauts in a crew capsule with a transfer rocket stage. If the launch vehicle is powerful enough — say, the size of SpaceX's Falcon Heavy — the upper stage and the crew capsule could be launched in one go. If the rocket doesn't have that much oomph, the capsule and the upper stage could be launched separately and then linked up in Earth orbit for the push onward to Mars.

Inspiration Mars

An artist's conception shows how the spacecraft for the Inspiration Mars Foundation's "Mission for America" might be configured — with a crew capsule, an inflatable module similar to the ones built by Bigelow Aerospace, and an attached upper stage that could provide radiation shielding. The actual design has not yet been set.

"We only need to attach the upper stage. There's no need to get rid of it," MacCallum said. In the right configuration, that upper stage could even provide some of the required shielding from solar radiation and heating, he said.

The crew's 600 cubic feet of living space would include a capsule for launch and re-entry, with a well-shielded sleeping quarters that could provide a safe haven if solar storms erupted. There would be a habitat module — perhaps an inflatable module like the one that Bigelow Aerospace has been working on for NASA's use. The main idea is to keep the crew compartment as simple as possible while providing all the necessary amenities for a 501-day trip. "It's a '55 Chevy," MacCallum said.

To test the feasibility of the plan, Tito and his colleagues looked at the specifications for the Falcon Heavy as well as a modified version of SpaceX's Dragon capsule. But MacCallum emphasized that the team was not committed to using SpaceX hardware. He said the idea was getting a "great response" from a variety of aerospace companies. "If this mission is going to happen, they want 'their vehicle' to do it," McCallum said.

How much? And why?
MacCallum characterized the mission as "purely philanthropic," with the aim of inspiring future scientists and engineers as well as bridging the gap in NASA's plans for exploration beyond Earth orbit. NASA's current timetable calls for astronauts to go no farther than the International Space Station until 2021 at the earliest. Even though the Mars-and-back mission wouldn't make any stops, the trip could produce useful scientific data — and an adventure as grand as the Apollo moonshots of the '60s and '70s.

"I think we really need what Apollo did for America, but we didn't realize it while we were doing Apollo," MacCallum said.

Toward that end, Tito set up the Inspiration Mars Foundation. "He has committed to funding the first two years of this development, and he is committed to finding the rest of the money," MacCallum said. "Dennis is already getting tremendous interest in this mission from people of means."

The foundation is also looking into media deals and sponsorships. "Farmers Insurance cut a $700 million deal for the naming rights for a stadium," MacCallum noted. "Wow ... that's a not-insubstantial part of the money that we're talking about."

How much money are we talking about? MacCallum quoted Tito as saying "it's a fraction of what Curiosity cost," with reference to NASA's $2.5 billion robotic mission to Mars. Other reports have put the cost in the range of $1 billion or so — which is far less than the projected price tag for the crewed missions NASA plans to send to Mars in the 2030s.

MacCallum emphasized that Tito's "Mission for America" was meant to support America's space agency, not compete with it. "This mission is only even remotely contemplatable because of all the work that NASA has done on the International Space Station," he said. And NASA is getting something in return: MacCallum said Inspiration Mars is paying NASA for access to thermal protection technologies developed by the space agency.

Even if MacCallum and Poynter aren't picked to go on the flight, it sounds as if they'll be having the adventure of their lives over the next five years. "I feel so thrilled every day to be working with these people," MacCallum said. "It's just fabulous."

More about sex, Mars and spaceflight:

• Outer-space sex carries complications
• How a TV show could create a Mars colony
• Astronauts could survive Mars radiation

Is Dennis Tito's idea crazy? Check out this follow-up posting for a reality check.

Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. To keep up with Cosmic Log as well as NBCNews.com's other 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.

Patric Vaelli / Logsdon Labs

Bdelloid rotifers like the one shown in this photomicrograph are thought to be the champions of sexless animals. They've apparently gone without it for 40 million years.

For the scientists who study sex and its mysterious origins, animal species that skip sex when they spawn were at first puzzling, and then exciting. Now it turns out that many of these supposedly sexless species can swing both ways. 

The latest peccadillo involves a type of ant that scientists thought had survived sans sexual reproduction for millions of years — until they discovered that the seemingly abstemious arthropods were covertly copulating. 

From an evolutionary point of view, sex is a costly business. Nevertheless, most species mate to multiply. For researchers grappling with the question of why sex exists, asexual species provide a clue, one small nudge in the direction of an answer. "We study how normal things work by studying mutant version of those things," John Logsdon, a biologist at the University of Iowa, told me. "In this case, how they basically get around the rule, because the rule seems to be sex."

That's what makes the ants interesting. When scientists started scooping up Amazonian fungus-growing ants in Mexico, Argentina and other parts of South America, they believed that the all-female colonies of ant clones stayed strictly sex-free. But in a fresh set of samplings in new locations, the same ant species was found propagating sexually with the usual mingling of genomes of both genders. 

What seems to have happened, the researchers who found the ants surmise, is that isolated ant colonies lost the ability to reproduce sexually, due to a genetic switch that was turned off over time. Once this change occurred in a colony, there was no going back, Christian Rabeling and his colleagues propose in a paper published last week in the Proceedings of the National Academy of Sciences.

The ants are the latest addition to a small list of species that have a sometimes sexy, sometimes sexless life. These invertebrates' genomes are a rich source of insights for scientists who are still puzzling over one of nature's most basic mysteries: why sex exists. 

Worms do it, snails do it ... or don't
In July, scientists studying the sex lives of the nematode C. elegans offered up one reason as to why sex exists. The worm propagates in two ways: Sometimes it mates with another worm, and sometimes it fertilizes itself. Scientists compared the offspring of two worms with the self-fertilized spawn of a single parent, and found that genetically diverse offspring were less likely to be infected by parasites than offspring from a single parent. This aligns well with a big idea called the Red Queen hypothesis, which claims that sex, as a behavior that allowed a mix-and-match of genomes, stuck around to help species win out over co-evolving parasites, Indiana University's Curtis Lively and his colleagues write in the July 8 issue of Science.  

Dodging parasites is probably just one chapter of the story. "My suspicion is that we're not going to come up with a universal solution to sex," Maurine Neiman, a biologist at the University of Iowa, told me. She expects the answer is going to be messy and complicated ... just like sex itself. 

Maurine Neiman / University of Iowa

New Zealand's Potamopyrgus antipodarum, a freshwater snail.

Neiman treks down to lakes in New Zealand every other summer to harvest a species of freshwater snail, which, like C. elegans, is sometimes asexual and sometimes sexual. For as yet unknown reasons, the snail's fate — to start a sexual or asexual lineage — is decided before the snails are born. Yet some lakes have both kinds of snails living in them. "They set the stage very nicely for comparing sexual and asexual genomes," Neiman told me. "You can compare populations that have lots and lots of sex with those that don't have any at all." 

Like the nematodes, the snails have a natural parasite, and Neiman is looking into how the snails' sexual behavior relates to their ability (or lack of ability) to survive being infected. There's another odd secret that is hidden in the snail genome: Those that are built to stay single sometimes have many, many copies of their DNA packed into the same space where most species just have two copies. 

Bart Zijlstra

Timema tahoe, a stick insect with a sexless past.

Some species stay sexless
A few species appear to have stuck with sexlessness, what scientists call "ancient asexuality." Tanya Schwander, a geneticist at the University of Groningen, recently showed that two species of stick insects have stayed asexual for more than 1 million generations.

Schwander and her colleagues wrote about the stick insects in the June 12 issue of Current Biology, and they're continuing to investigate how they managed to do this without going extinct.

While 1 million generations may seem like a long time, the stars of sexlessness are still the bdelloid rotifers — single-celled singletons who appear to have kept sex-free for 40 million years. Their unusual genomes also come riddled with questions, but researchers suspect they're getting closer to the answers.

Whether by coincidence or causation, other extreme survival skills are coded into the rotifer genome — the superbugs can survive being blasted with radiation, and even bounce back to life after being dried out. Scientists such as the University of Iowa's Logsdon reason that the rotifers' exceptional talent for fixing errors in their DNA caused by radiation could explain how they fix unwanted changes that crop up in genomes that don't mix it up every so often.

"It's not a smoking gun, but we smell a connection," Logsdon says. "What the connection is, is still an open question." 

More about animal sex and sexlessness: 

• No sex for 40 million years? No problem!
• Experts learn creature's no-sex secret
• Skewed sex ratio curbs courtship
• Of mice and men: 10 sex lessons from the wild
• Gallery: 10 peeks at sex in the wild
• Hook-ups in the wild: Do animals enjoy sex?
• How humans evolved big brains ... and barbless penises

Nidhi Subbaraman writes about science and technology at msnbc.com. Find her on Twitter or Google+, and join our conversation on the Cosmic Log Facebook page.

Dreamstime.com

Researchers find that males spend less time in elaborate courtship displays such this peacock's feathers when males far out number females. Instead, they get sneaky to get a mate.

When a woman walks into a male-crowded bar she's unlikely to be showered with courtly attention — that is if findings about mating in the animal kingdom translate to the human realm.

"She might just be watching them fight it out and then have one particularly possessive one making sure others aren't getting access to her," Laura Weir, a postdoctoral fellow at Simon Fraser University in British Columbia, Canada, told me today. 

In other words, as the dudes duke it out with each other, one little weasel will sneak over and trap her in a corner and try to keep her all to himself?

"Exactly," she said, although she stressed her reluctance to take the analogy too far. The data, she noted, is compiled from the mating behaviors of the birds and bees … and alligators, fish, frogs, lizards and lobsters, too. 

Operational sex ratio
Her research focuses on the influence of the so-called operational sex ratio on competition for mates. Operational sex ratio is the ratio of males and females ready to get it on at any one time and place. 

In the animal kingdom, like at a bar, the ratio can often be heavily male biased. For example, in a lot of species, males are often first to arrive to the mating ground so they can establish territories.

The females will come afterward, and depending on the pace of female arrivals over  time, "you can have very biased sex ratios during some times of the mating season," Weir said.

When the first female arrives, the general thought is the males will get aggressive toward each other, fighting with each other to take out the competition. 

"We found that there is this increase in aggression to a point, but then they stop using aggression as a tactic to get females and they change to other tactics like sneaking in or scrambling around looking for females," Weir said.

Dying courtship
All this aggression and sneaking around comes at the expense of courtship, which is a costly, time-consuming investment. Think birds such as peacocks with their fabulous displays of feathers or sparrows constantly updating their playlists.  

Instead of investing in the displays and songs to attract the very best mate, the males put their energy into just trying to find a mate, any mate, which involves more covert sneaking around.

On the flip side, males in these situations tend to be more guarded of the mates they secure.

"If they've mated with her, they want to ensure they are the only one who's done it," Weir said. "And so, rather than go off and fight with other males or try to court another female, they'll just cling to the female that they've already mated with."

Overall, Weir and her colleagues note, the findings illustrate a considerable flexibility in the mating structure within species, which is likely related to the end goal of life: reproduction. 

At least, that's the next question they hope to tackle in their research. 

"If the goal of the biological world is to leave more offspring, are these changes in behavior beneficial to the males when they are actually competing for mates and competing to fertilize eggs?" asked Weir.

Weir is a lead author of a paper describing this research in the February issue of The American Naturalist. 

More stories on the science of animal sex:

• Same sex behavior nearly universal in animals
• Hook-ups in the wild: Do animals enjoy sex?
• Urine spray signals sex, violence to crayfish
• New tunes, not oldies, lure the feathered ladies
• Of mice and men: Tens sex lessons from the wild
• Despite flash, males are simple creatures
• Polygamous mice are better breeders

In a case of life imitating "Futurama," Maryland's gay-marriage debate has somehow morphed into worries about robot-human marriages.

The rant against robosexuals came during Robert Broadus' testimony against the gay-marriage legislation currently before Maryland legislature. "If you pass this bill, you will set the groundwork, that one day when artificial intelligence is that advanced, we will be considering whether or not people can marry their androids. ... If you say that any two people who love each other can get married, then you set that precedent," said Broadus, who heads Protect Marriage Maryland.

To make his case, Broadus referred to Lieutenant Commander Data's ability to feel emotion and shed a tear in "Star Trek: Generations," a science-fiction movie. "You laugh, but it's true," Broadus said.

For better or worse, sex with robots is already a reality, although the robots in question are really just glorified blow-up dolls. Researchers have long discussed whether there might come a day (maybe 2050?) when true love could exist between humans and artificially intelligent machines. In recent years, however, the idea of building robots that look and think like humans isn't as, um, sexy as it once was.

"The robotics field is away from that approach and geared toward specific applications," said Anne Foerst, a computer science professor at St. Bonaventure University in New York who has been dubbed "the robot theologian." (And yes, she does hold a degree in theology as well as computer science and philosophy.)

Tony Gutierrez / AP file

Self-titled "sculptor roboticist" David Hanson, right, poses with his creation, Hertz. Hertz is an animated robot head that Hanson modeled after his girlfriend. For the record, Hanson and Hertz are not in a relationship. Click on the image to learn more.

In the old days, roboticists could build cute-eyed machines like Cog, Kismet and Leonardo to test the boundaries between biological and mechanical beings. MIT's Leonardo, an Ewok-like robot, could even pass some of the psychological tests aimed at studying social cognition in children.

That kind of research "is pretty much dead right now, which is very sad,"  Foerst told me.

"You basically get applications, and applications which in my opinion are much less interesting," she said. "Those machines do not raise any questions of personhood, because they don't have anywhere to go. ... The reason why is that the funding has changed."

For better or worse, it seems that the folks funding the research are more interested in real-world results rather than the prospects for robot romance. Who knows? Maybe the academic debate over sex and marriage with robots will be revived in 2050, or more likely 2150. But in any case, Broadus needn't worry about robosexuals anytime soon.

"Gay relationships obviously consist of mutual give and take," Foerst observed. "They're equal partners, and that's completely different from robotics. To apply human-android research to gay marriage is, in my opinion ridiculous."

Foerst is apparently not the only person who feels that way. Lezgetreal's Bridgette P. LaVictoire reports that Maryland state Sen. James Brochin changed his mind and decided to vote for the gay-marriage legislation after hearing the "appalling and disgusting" testimony against it.

Did Broadus' "Star Trek" maneuver backfire? Feel free to weigh in ... after you watch this "Futurama" video clip about Proposition Infinity on robosexual marriages.

More on robot-human relations:

• 'I do' goes high-tech with robot priest
• Robot can read and learn like a human
• Sneaky robots taught the art of deception
• Soon, the 'new guy' at work may be a robot
• Robo-teacher smiles and scolds in schoolroom

Join the Cosmic Log community by clicking the "like" button on our Facebook page or by following msnbc.com science editor Alan Boyle as b0yle on Twitter. To learn more about Alan Boyle's book on Pluto and the search for planets, check out the website for "The Case for Pluto."

Felix Kaestle / AP

A blue tit is reflected in a wing mirror of a car that is covered with raindrops in Friedrichshafen, southern Germany in this file photo. Female blue tits who mate with experienced males have slower ticking biological clocks, a new study says.

Biological clocks tick more slowly for female blue tit birds that consistently choose mates whose first reproductive success came in their first year, according to a new study.

The finding suggests that males, who help build the nest and feed mom and her chicks, create an environment that influences how the female interacts with the world.

"The thought was that males didn't matter," Josh Auld, a study co-author at the National Evolutionary Synthesis Center in Durham, North Carolina, said in a news release.

It turns out that they do.

Auld and his fellow researcher, Anne Charmantier of the French National Center for Scientific Research, don't know exactly why certain male blue tits fail to reproduce in their first year, nor do they know what cues a female uses to pick different males. But the birds mate once a year, often with a different partner, and the females with mates that started reproducing early have slower-ticking biological clocks.

The age of the male in a given mating year doesn't matter; the key factor is his history. Males with early experiences are somehow superior.

"These males that are able to reproduce early may in some way ameliorate the decline and fitness of the females," Auld told me today.

French data
To determine the male factor in female fertility, the researchers took advantage of a long-term data set from the yellow and blue forest birds on the French island of Corsica. The birds were outfitted with identification tags on their ankles, allowing researchers to track who mated with whom, how many eggs they laid, and when and how the fledging birds fared over time.

Auld and Charmantier analyzed data collected between 1979 and 2007 for nearly 600 female and 600 male birds. They found that the positive effect of experienced males is greatest a bit later in the birds' 6-year or so lifespan, Auld noted.

"Going through parenting once means the second round might go a little bit better … so when that 4- or 5-year-old female chooses even a 4-year-old male, and he started reproducing when he was 1, that's when we see that decline was lower," he said.

Broader effect?
Whether this positive effect of male experience on female fertility is broader than just the world of blue tit birds is, for now, unknown. Auld and Charmantier are currently studying swans, and they're not getting exactly the same result.

"But they are very different," Auld said of the swans. "They are long-lived. So it is a little early to say. Certainly we would like to know that, but I really don't know for sure how general these effects are," he said.

The study was funded by the National Science Foundation, and the findings were published online last week in the journal Oikos.

More stories on bird mating:

• Why lusty canaries change their tune
• Marker turns wimpy birds into chick magnets
• Size matters: Bird uses illusion to wow a mate
• New tunes, not oldies, lure the feathered ladies

John Roach is a contributing writer for msnbc.com. Connect with the Cosmic Log community by hitting the "like" button on the Cosmic Log Facebook page or following msnbc.com's science editor, Alan Boyle, on Twitter (@b0yle).

Josh Wickham / Univ. of Florida / IFAS file

Researchers found that a high-mercury diet had an effect on the mating behavior of white ibises confined in a net-covered aviary at the University of Florida. They said the degree of homosexual pairing increased along with the birds' mercury exposure.

A years-long study at the University of Florida suggests that mercury pollution can alter the hormones of white ibises to make males more likely to mate with other males.

"We knew that mercury can disrupt hormones -- what is most disturbing about this study is the low levels of mercury at which we saw effects on hormones and mating behavior," Peter Frederick, a wildlife ecology and conservation professor who led the study, said in a news release this week. "This suggests that wildlife may be commonly affected."

The study was published online on Wednesday in the Proceedings of the Royal Society B.

Frederick and his co-author, Nilmini Jayasena, were hasty to warn against drawing any inferences about the roots of human homosexuality. They didn't set up the experiment to find out what makes birds gay -- rather, they were trying to figure out why ibises in the Everglades went through a stretch of poor breeding in the early 1990s, followed by a baby boom in the late 1990s.

Scientists knew that improvements in the birds' watery habitat was one factor behind the increased breeding, but they suspected that mercury concentrations played a role as well. During the downswing in breeding, low-level mercury contamination made its way increasingly into the Everglades via municipal and medical waste incineration -- but that waste became more regulated at about the same time as the start of the baby boom.

To find out if there was a connection between mercury contamination and a low birth rate, the university set up a 13,000-square foot net-covered aviary in 2005. They brought in 160 ibises, and divided the birds into four groups with equal numbers of males and females. Each group was fed a different diet -- low, medium or high mercury, or no mercury at all. The highest level of mercury was no higher than what the birds would have consumed if they had been in the wild during the early 1990s.

In 2006, about 55 percent of the high-mercury-diet males were nesting with other males. Frederick said the degree of male-male pairing was proportional to the degree of mercury in the diet. That played a role in breeding differences, as well: In comparison with the control group, high-mercury males were less likely to be approached by females during courtship. All of the mercury-consuming males were less prone to perform the ritual head bows and bobs that are part of the ibises' mating ritual.

Frederick and Jayasena, who was Frederick's doctoral student and is now based at the University of Peradeniya in Sri Lanka, reported that the high-mercury-diet females produced 35 percent fewer fledglings than the females in the control group.

The study lasted for three breeding seasons. After the experiment, the birds were put on a cleansing, mercury-free diet for several months and then released back into the wild.

Does all this mean homosexuality is linked to mercury pollution? For the ibises, maybe. For humans, almost certainly not. Frederick pointed out that there have been a number of long-term studies on the effects of mercury on humans, and none of those studies has noted a change in sexual behavior. Speaking more generally, the researchers noted that sexual preference is a much more complex phenomenon for humans than it is for birds.

In a report on Nature's website, a German expert on animal physiology cautioned that the results might not even be applicable to other bird species. Heinz Köhler of the University of Tübingen told Nature that this might be something that's just between ibises. "Their behavior may be less fragile and more robust to methylmercury," he said.

More perspectives on the research:

• Nature: Mercury causes homosexuality in male ibises
• BBC: Mercury 'turns' wetland birds homosexual
• New Scientist: Mercury poisoning makes male birds homosexual
• Miami Herald: Mercury makes ibises gay, study says

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