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Discovered: Guppies strive to be the most attractive member of their friend group; the world's most popular painkiller greatly raises the risk of a heart attack; sea slugs have "disposable" penises; skulls reveal violence inflicted upon Stone Age women.

Guppies shun attractive peers in favor of uglier friends. It looks like schools of fish can be just as shallow and competitive as schools of popularity-obsessed teenagers. Sounding like something out of a Mean Girls voice over, a new study conducted by University of Padua researchers finds that male guppies keep their less attractive friends closer when potential mates are nearby. In doing so, they hope to stand out as the most attractive member of the group. Lead researcher Clelia Gasparini is hesitant about generalizing from guppies to humans, but she admits "If you ask me as a person, sure ... I saw this tactic working pretty well with humans. If you want to impress someone, do you think you will look more attractive in comparison with Mr. Bean or George Clooney?" [New York Daily News]

The most widely taken painkiller raises heart attack risk by 40 percent. If you're a U.S. resident, it's unlikely that you've been prescribed the painkiller diclofenac (sold as Voltaren, Cambia, Cataflam, and Zipsor). But the drug is the most widely taken painkiller. And that's a troubling fact, in light of a new study led by the Institute for Clinical Evaluative Sciences' David Henry. He and his colleagues find that the non-steroidal anti-inflammatory drug raises the risk of heart attack by as much as 40 percent. "Clearly thousands of people die as a result of using [diclofenac]," Henry tells NPR's Richard Knox. "But these are invisible victims. And therefore, there's no advocacy lobby group on their behalf." [NPR]

Sea slugs have a detachable penis. King Missile's '90s stoner anthem about mutant genitalia turns out to be not totally fanciful. Japanese researchers have found that Chromodoris reticulata sea slugs actually do have a "detachable penis." But once their member comes unstuck, it's disposed of forever. But don't worry! A new one grows back in its place. The species are "simultaneous hermaphrodites," which is the fancy science term for when animals have both male and female genitalia. After copulating, the sea slugs dispose of their penises and regrow them 24-hours later. If the whole mating process sounds confusing to you, you're not alone. Marine invertebrate expert Bernard Picton says, "They do have very, very complicated biology - and a lot have awfully complicated things in terms of reproduction." [BBC News]

Dented skulls reveal Stone Age violence toward women. A new study suggests that Stone Age men weren't the only ones getting beaten in violent conflicts between warring farmers. Newly exhumed skulls from Scandinavia show that during the historical period spanning between 6,000 and 3,700 years ago, 1 in 6 women received head injuries big enough to leave a mark. That's about the same rate of violent head trauma observed in male fossils from the same era. This doesn't necessarily mean that women were attacked as often as men — it could simply suggest that they weren't as good at defending themselves, having been raised to rear children instead of fighting in battle. This study, led by University of Edinburgh archaeologist Linda Fibiger, is "the first that really looks at a really large sample size," and "draws from a larger region," according to University of Mainz anthropological researcher Christian Meyer.  [Scientific American]

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