AUDIE CORNISH, HOST:

It's ALL THINGS CONSIDERED from NPR News. I'm Audie Cornish.

ROBERT SIEGEL, HOST:

And I'm Robert Siegel. It sounds like something out of a science fiction novel: a virus that can manipulate another creature into having more sex. NPR's Lulu Miller brings us this story of some reptiles, crickets and rats and the humans who made a scientific breakthrough.

LULU MILLER, BYLINE: This non-fantasy story about a literal love virus begins with a dragon.

DR. SHELLY ADAMO: It just happened that my student had bearded dragons as a pet, and she had a few of them. So, we brought them into the lab.

MILLER: This is Shelly Adamo, a neuroscientist at Dalhousie University in Halifax, who was bringing bearded dragons - which are, OK, just little lizards with a forked tongue and frill around their chin - into her lab because she needed predators - predators to scare the creatures she usually studies.

(SOUNDBITE OF CRICKETS)

ADAMO: Which are crickets. We work with crickets.

MILLER: She was trying to measure their stress response.

ADAMO: So, anyway, it just turned out - we had no idea - that one or all three of the bearded dragons was infected with this cricket virus and it gave it to our crickets.

MILLER: A couple weeks after the dragons came in, her whole colony of crickets was infected.

ADAMO: This spread very quickly in the colony, over the matter of, you know, three to four weeks. It just spread like wildfire and...

MILLER: And how many crickets did you have?

ADAMO: Oh, we probably had a couple thousand.

MILLER: All dead. But what was weird was that in the days leading up to their death, the crickets weren't acting sick.

ADAMO: Because even crickets behave a little differently when they get sick. So, as you know, when you get sick, you tend to lose your appetite, you become lethargic, you may not be very sociable. You want to just curl up in your bed and get better. And crickets kind of do the same thing. They also tend to move less. They have a number of changes in their behavior when they get sick. And one of the changes they have is that they tend not to be very interested in mating with other crickets.

MILLER: But these crickets seemed...

(SOUNDBITE OF MUSIC)

MILLER: ...very interested in mating with other crickets.

(SOUNDBITE OF SONG)

MARVIN GAYE: (Singing) ...trying, baby...

MILLER: You know what? Let me just translate this into cricket.

(SOUNDBITE OF CRICKETS)

MILLER: This is a recording of the actual cricket courtship sound, which basically means let's get it on, let's get it on, let's get it on...

(SOUNDBITE OF SONG)

GAYE: (Singing) ...sugar...

MILLER: And Shelly could swear she was hearing it more and more in the days before the crickets died. Let's get it on, let's get it on...

UNIDENTIFIED PEOPLE: Let's get it on, let's get it on, let's get it on, let's get it on, let's get it on...

ADAMO: And that's what started our study because it seemed unusual to me.

MILLER: So, she set up an experiment to see if the infected crickets were indeed mating more than the uninfected crickets.

ADAMO: So, what you do is take males and females into an arena...

MILLER: ...and you measure how long it takes for the male to start singing his courtship song.

ADAMO: That's right, exactly.

MILLER: And usually, it takes him about 10 minutes. But the infected males?

(SOUNDBITE OF SONG)

GAYE: (Singing) Aw, baby...

MILLER: ...in just three...

(SOUNDBITE OF CRICKETS)

(SOUNDBITE OF SONG)

GAYE: (Singing) Let's get it on.

(LAUGHTER)

ADAMO: Yes. So, in other words, the virally infected males are more eager to mate.

MILLER: In other, other words? Adamo had stumbled across a virus that seemed to make crickets mate more.

ADAMO: And I had never seen anything that increased sexual behavior in a cricket. Because they're usually pretty avid most of the time, so this was quite surprising.

MILLER: Now, why would it be good for a virus to get its cricket host to start mating more? Well, in evolution, success is getting your genes to spread and live on after you. So, from a virus's point of view, what better way to get your genes into a brand new host than...

(SOUNDBITE OF CRICKETS)

MILLER: And once Adamo looked into it, she found that there are other viruses and parasites that manipulate their hosts into having more sex.

ADAMO: So, one is a moth, which is hit by a virus, which causes it to spurt pheromones, increasing their sexual desirability.

MILLER: Similar affects have also been shown in beetles, ladybugs, little mosquito-like creatures called midges, and most recently...

ADAMO: Rats.

MILLER: ...mammals.

SHANTALLA HARIDAS: So, the albino rats, they're white in color.

MILLER: This is the scientist who first observed the effect, Shantalla Haridas(ph).

HARIDAS: At the Nanyang Technological University in Singapore.

MILLER: And when she infected her white lab rats with the parasite toxoplasma gondii, she found that the males got this surge of testosterone.

HARIDAS: They infected animals. Their testosterone levels are higher.

MILLER: Which in turn made the females mate with them more.

HARIDAS: Which is great for the parasite because it's just increasing its spread.

MILLER: Increasing its spread. I mean, it's kind of genius.

HARIDAS: It's remarkable.

MILLER: So, let me just step through that one more time. If you are a male rat and you get this parasite, you become more attractive to females, which makes the females flock to you, have more sex with you so that you can transmit the parasite to more and more and more hosts. So, Dr. Adamo, to just embody the question that may be dawning in people's mind, like, is there a way this could make it up the food chain?

ADAMO: Like do we have any evidence in humans that sexual behavior is manipulated by infectious agents?

MILLER: Yeah.

ADAMO: No. We don't actually.

MILLER: But now that she knows about what's possible in the animal kingdom, one thing did stand out to her while she was reading about how sexually transmitted infections work in humans.

ADAMO: Unlike most diseases, sexually transmitted infections, people who have them usually have no symptoms.

MILLER: For example, if you have syphilis, you can often have no or minimal symptoms. Same goes for chlamydia, gonorrhea. People can go years without symptoms.

ADAMO: And usually when you have a raging bacterial infection, you show signs of it.

MILLER: And so Adamo can't help but wonder if having no symptoms could be to the advantage of the microbe inside a person. That is, could the lack of symptoms in the host be the result of a very clever evolutionary adaptation in the parasite?

ADAMO: If you're a sexually transmitted organism, there's going to be selection on you, on any mechanism that you may come across that is going to encourage your host to continue to mate. Because if they do not, your transmission is going to be very low.

MILLER: Of course, this is just a hypothesis. But Adamo says the place to start figuring out how and if sexually transmitted organisms could be influencing our behavior is to look at simpler creatures like her crickets. So, are you going to keep studying this?

ADAMO: No. Because the virus is so virulent and spreads so quickly through the colony, I just would be afraid to have them anywhere near my now-healthy crickets. So, I'm afraid, although I'm very interested in parasitic manipulation of behavior, I won't be studying it with this virus.

(SOUNDBITE OF MUSIC)

MILLER: But, to the grad students of the world interested in what mysteries of free will are waiting to be unlocked with STD-infected crickets?

ADAMO: As a matter of fact, we have some frozen. So, if anyone else is interested, I've got a supply.

(SOUNDBITE OF SONG)

SCREAMIN' JAY HAWKINS: (Singing) I put a spell on you...

MILLER: Lulu Miller, NPR News, Washington.

(SOUNDBITE OF SONG)

HAWKINS: (Singing) 'Cause you're mine. You better stop the things you do...

CORNISH: This is NPR News.

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