Human activity is driving many mammals worldwide to be more active at night, when they are less likely to encounter humans, a new study reveals. Human presence can instill strong fear in wild animals, causing them to change their behavior to avoid contact with humans. Such risk avoidance can have profound effects on animal physiology and fitness, affecting demography and altering food chains. To more fully understand the impact of humans on the daily activity patterns of mammals, Kaitlyn Gaynor et al. conducted a large-scale analysis of 76 studies of 62 species from six continents. These studies monitored animal activity using tools such as GPS trackers and motion-activated cameras; the researchers used these data to compare the nighttime activity of each species during seasons of high and low human disturbance. "Human disturbances" included a range of activities, from hunting, to hiking, to agriculture. The authors found that, overall, higher levels of human disturbance caused nocturnal mammal activity to increase by a factor of 1.36. The authors note that, on the positive side, these shifts in animal behavior may help humans and animals coexist; however, these changes may also impose substantial fitness costs on individual animals, prompting them to engage in behavior to avoid predators at the cost of reproduction, for example. "Eventually, long-term disturbances may result in lower fitness, lower juvenile survival, or lower reproduction rates, with negative consequences at [the] population level," Ana Benítez-López writes in a related Perspective.