IRA FLATOW: This is Science Friday. I’m Ira Flatow. Later in the hour, we’re going to be talking about hidden gems you’ll want to check out on your science-themed vacation this summer– stuff like geological wonders, strange museums, atomic era oddities. So give us a call with your recommendations. I know you’ve been to lots of places for a geeky road trip.

Give us a call at 844-724-8255. That’s 844-SCI-TALK. Or tweet us @scifri. We’ll get to that a little bit later.

But first, another story about a mysterious and intriguing place. Archaeologists excavating an Aztec temple site in what is now Mexico City have uncovered a tower filled with hundreds of human skulls. This terrifying tower was described in the journals of invading Spanish soldiers in the early 16th century.

Joining me now to talk about the find and other selected short subjects in science is Annalee Newitz, tech culture editor at Ars Technica. She is at KQED in San Francisco. Welcome back.

ANNALEE NEWITZ: Hey, thanks for having me.

IRA FLATOW: Tower of skulls sounds like a horror movie or a ride in Disney World or something like that.

ANNALEE NEWITZ: This is one of those great examples of actual archeology proving that something that sounded like the crazy ravings of Spanish invaders was actually true. So, as you were saying, we have written accounts from Spanish soldiers who were coming into the Aztec capital of Tenochtitlan. And they’re with Cortez’s invading forces.

And they record seeing a tower made from skulls. And they’re just terrified by it. And for hundreds of years, since this tower and, of course, all of Tenochtitlan were basically buried under Mexico City, we haven’t known for sure if this was just a wild exaggeration. But this ongoing excavation in the heart of Mexico City has revealed that there was, indeed, not just a tower of skulls, but some other structures that were covered and skulls, too.

And the tower would have been about six meters in diameter. It’s round. And it wasn’t full of skulls. It was sort of decorated with skulls. Skulls were sort of sunk into the structure of the tower.

IRA FLATOW: [? Right. ?] And they were people of– men, women, children, everybody? And no one more than the other?

ANNALEE NEWITZ: That’s what it seems like. And that’s one of the exciting things about this excavation is that now scientists have gotten enough skulls out that they can start analyzing, you know, a little bit about who these people were that wound up with their heads in a tower.

And this is one thing that the Spanish got wrong, because these were soldiers coming in and they saw this tower. And they expected that it was to frighten them away and that it was probably the skulls of dead soldiers, and that would make them men. But instead, it’s actually from cross-sections of the population– women, children, men.

And what this tower was was it was part of a temple to a god who is associated with human sacrifice. And so instead of it being intended to frighten away soldiers, it was really to provoke awe in the citizens of the city and to be a kind of spiritual reminder, because human sacrifice, in the Aztec religion, was part of the cycle of life. You couldn’t really live as a civilization without sacrificing.

And so this was a spiritual place. It was not– it wasn’t like a Game of Thrones thing where we put the head on a spike. It was really a religious kind of icon. And so the Spanish kind of got it right. And they kind of got it wrong.

IRA FLATOW: Yeah, that’s fascinating. We’ll have to follow that story. Let let’s move on to a new topic. There’s an old stereotype that attributes cognitive lapses or memory problems and other mental effects to women’s menstrual cycles. And now there’s this new study that looks at that.

ANNALEE NEWITZ: This is, in fact, one of the only studies out there that actually takes a cultural myth, like the idea that women are kind of out of their minds during their periods, and takes it seriously scientifically. And there have been a few other studies that have tried to tackle this, but none in a terribly scientific manner.

And so what these researchers did is they took a group of 68 women in Switzerland and Germany, and they created an experiment that had reproducibility built into it, which is a basic tenant of science. So they looked at these women during two of their menstrual cycles, which for some crazy reason no one had ever thought to do. They’d only looked at women during one menstrual cycle. So, of course, they could never know for sure if what they were seeing had anything to do with that– had with the menstrual cycle, because they weren’t able to repeat it.

So they took women through these two cycles. And they did a variety of cognitive tests on the women during their two menstrual cycles. And they tested ability to have working memory, visual memory, concentrate on two or more things at once– so, basically, multitasking– and they also measured cognitive bias, which just looks at whether prejudices kind of overwhelm your ability to make a rational judgment. And what they found was that when they took into account both measurements, during both menstrual cycles, there was absolutely no relationship between cognitive performance and where you were in a menstrual cycle.

So some women did not seem to perform quite up to typical cognitive levels during their first menstrual cycle. But during the second one, they did. So that clearly shows that it wasn’t related to the menstrual cycle, it was something else. Because when they tried to reproduce the experiment, it didn’t have the same– cognitive deficits didn’t come up.

So the researchers really want to continue in this area with this research. They really feel like there needs to be more work done on women who have, you know, hormone deficiencies and other problems. So these were all women who had typical hormone levels, so just women who have average types of menstrual cycles.

IRA FLATOW: Right.

ANNALEE NEWITZ: So feel free to go memorize philosophy while you’re having your period. It’s all good. You’re going to be just as up for it as you would be normally.

IRA FLATOW: I’ll keep that in mind.

[LAUGHTER]

ANNALEE NEWITZ: You and your friends can keep that in mind.

[LAUGHTER]

IRA FLATOW: You have a story about the emergence of frog diversity. This is very interesting. Give us the background on that.

ANNALEE NEWITZ: So people who are fans of dinosaurs may know that about 66 million years ago, a giant rock smashed into the planet and kind of wiped out most of the dinosaurs. But what that rock also did during the subsequent mass extinction was kind of pave the way for a lot of other life forms to really take off. So one of those life forms was, of course, a little furry shrew known as a mammal that’s our ancestor. We were all descended from this little shrew.

And this other group that really benefited from the mass extinction was frogs. And it turns out that three lineages of frogs wound up being the ancestors of 88% of frogs alive today. So those three lineages that made it through the giant rock hitting the planet have basically blossomed into this huge number of frogs all across the planet.

IRA FLATOW: I guess this was kind of surprising, kind of interesting. Wow. All those frogs coming from those few frogs.

ANNALEE NEWITZ: Exactly. And it’s fun to think about what made them good survivors, because we don’t think about frogs as being particularly robust. But they actually managed to find a new ecosystem niche, because flowering trees became incredibly common after this mass extinction.

Before that, it was mostly pine trees, conifers. And frogs love flowering trees. They loved those big flat leaves that fall on the forest floor. They can hide in them. They can lay eggs in them.

And it became– basically, Earth became the perfect frog environment. So the dinosaurs, you know, not so great for them, except for the ones that evolved into birds. But frogs were just loving it.

IRA FLATOW: [? Tweet ?] perfect Earth environment for you on that one. Thank you, Annalee.

[LAUGHTER]

ANNALEE NEWITZ: Thanks for having me.

IRA FLATOW: Always a pleasure. Annalee Newitz, tech culture editor for Ars Technica in San Francisco.

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