Top stories: The limits of human performance, sloth evolution, and the magic finger ratio

Study of marathon runners reveals a ‘hard limit’ on human endurance

Athletes who can run the equivalent of 117 marathons in just months might seem unstoppable. The biggest obstacle, it turns out, is their own bodies. A new study quantifies for the first time an unsurpassable “ceiling” for endurance activities such as long-distance running and biking—and it also finds that pregnancy’s metabolic toll resembles that of an ultramarathon.

Ancient molecules reveal surprising details on origins of ‘bizarre’ sloths

From elephant-size animals that browsed North American grasslands to moose-size swimmers that plied the Pacific coast of South America, sloths have roamed Earth for more than 50 million years. Yet scientists know little about how the dozens of known species are related to each other. Now, two new analyses of ancient sloth DNA and proteins—some of which are more than 100,000 years old—are rewriting the sloth family tree.

Talk to the hand. Scientists try to debunk idea that finger length can reveal personality and health

More than 1400 papers in just over 20 years have linked the relative lengths of the second and fourth fingers of the hand (known as 2D:4D) to a variety of attributes ranging from intelligence to cancer risk. Researchers who believe in the ratio’s predictive power say it reflects a fetus’s exposure to testosterone and other hormones that guide development, including that of the brain. But the notion has also riled up plenty of critics, who argue that researchers relying on the 2D:4D comparison have been seduced by a simplistic, faulty measure.

The transparent teeth of this dragonfish evolved for one lethal purpose

Five hundred meters below the ocean’s surface off the coast of California lives a creepy looking sea monster with a huge jaw and sharp rows of teeth. Even creepier, these teeth are transparent. Now, scientists think they know what makes them this way.

Researchers strapped video cameras on 16 cats and let them do their thing. Here’s what they found

Ever since video cameras became ultraportable, scientists have strapped them onto animals from sheep to sharks to see how they view the world around them. But relatively little has been done with cats, perhaps because they’re so hard to work with. Now, a new study equipped 16 cats with small cameras and followed them for up to 4 years as they prowled their neighborhoods.