Martha (right), the last known passenger pigeon, died in 1914. Her preserved body is now on display at the Smithsonian's National Museum of Natural History in Washington, D.C.
Macaques are social animals, whether in a group enclosure like this one at the Gelsenkircen zoo in western Germany, or in the wild. But many research monkeys are still housed in separate cages.
Patrik Stollarz/AFP/Getty Images
The collision of two neutron stars, seen in an artist's rendering, created both gravitational waves and gamma rays. Researchers used those signals to locate the event with optical telescopes.
Robin Dienel/Carnegie Institution for Science