Infidelity is rife throughout the animal kingdom, but much remains unknown about why levels of such promiscuity can vary widely across and within species. Now, scientists find whether female sparrows cheat on their mates can depend on the altitude. The researchers analyzed 24 populations of 12 species of sparrows, including the white-crowned sparrow (pictured), ranging across more than 3000 meters of elevation. They discovered the higher the sparrows nest, the more faithful females can be, the researchers report in January in The American Naturalist. The investigators suggest that higher altitudes are more physically demanding, so females stay faithful to keep mates around to provide fatherly aid that can help their young survive.