Squirrels closely mimic bird warning calls, and help spread the alarm throughout the forest that hawks, owls or other predators are nearby. iStockphoto hide caption

itoggle caption iStockphoto

An impala strikes a pose under a forest canopy in Zimbabwe. Morkel Erasmus/Getty Images/Gallo Images hide caption

itoggle caption Morkel Erasmus/Getty Images/Gallo Images
Gary Waters/Ikon Images/Getty Images

People who don't get enough sleep show higher levels of inflammation, say scientists who study colds. Smoking, chronic stress and lack of exercise can make you more susceptible to the viruses, too. Frederic Cirou/PhotoAlto/Corbis hide caption

itoggle caption Frederic Cirou/PhotoAlto/Corbis

Shown a realistic human target — not just a silhouette like this one — shooters were more likely to pull the trigger if the target was black, according to an analysis of 42 studies. "Even if you think that you're not prejudiced," says researcher Yara Mekawi, "that doesn't necessarily mean that that's true in terms of split-second decisions that you might make in the real world." Joshua Lott/Getty Images hide caption

itoggle caption Joshua Lott/Getty Images

Male and female tungara frogs. Among these frogs, the guy with the best call usually wins the gal — except when you throw a third-choice loser into the mix. Alexander T. Baugh/Encyclopedia of Life hide caption

itoggle caption Alexander T. Baugh/Encyclopedia of Life

Male treehoppers make their abdomens thrum like tuning forks to transmit very particular vibrating signals that travel down their legs and along leaf stems to other bugs — male and female. Courtesy of Robert Oelman hide caption

itoggle caption Courtesy of Robert Oelman

Heroin sold in the U.S., like this dose confiscated in Alabama last fall, is often cut with other drugs. Tamika Moore/AL.com/Landov hide caption

itoggle caption Tamika Moore/AL.com/Landov

Michael Arnott, of Cambridge, Mass., says he used to have trouble staying awake on long drives. Sleep specialists discovered he has obstructive sleep apnea, though not for the most common reasons — he isn't overweight, and doesn't smoke or take sedatives. M. Scott Brauer for NPR hide caption

itoggle caption M. Scott Brauer for NPR
Katherine Streeter for NPR

Bacillus subtilis may look like pasta under the microscope, but the bacteria are common in the gut of humans. Could the microbes be contributing to our belly fat? Too soon to tell, scientists say. BSIP/UIG via Getty Images hide caption

itoggle caption BSIP/UIG via Getty Images

African forest elephants stampede in the Central African Republic jungle. Courtesy of Cornell Lab or Ornithology hide caption

itoggle caption Courtesy of Cornell Lab or Ornithology