In 2011, the wildlife biologist Justin Brashares and his students set up a series of camera traps in and around Ruaha National Park in southern Tanzania. They were studying the effects of human activities on antelope reproduction, but their cameras soon revealed an odd and far more obvious pattern. While the antelope inside the park were active during the day, those outside the park, closer to human settlements, were active primarily at night—even though lions, which prey on antelope both inside and outside the park, typically hunt at night. The contrast in behavior was so stark that when Brashares and one of his students looked at a plot of the data, they laughed in disbelief. When faced with a choice between humans and lions, it appeared, antelope preferred to tangle with lions, and they were going nocturnal to do so.
Brashares, a professor at the University of California at Berkeley, wondered if other animal species were shifting their daily schedules in response to humans. Over the next few years, he and his students analyzed more than six dozen studies of 62 mammal species, ranging in size from opossums to elephants. The results, published on Thursday in the journal Science, suggest that the pattern Brashares documented in Tanzania is part of a much larger phenomenon: On every continent save Antarctica, humans are forcing other animals to work the night shift.
Kaitlyn Gaynor, the lead author of the study and a doctoral student of Brashares, says she and her co-authors expected that animals directly persecuted by humans would shift their daily schedules more dramatically than those who simply lived alongside people. But hiking and other such “non-lethal” activities appeared to trigger just as much nocturnal behavior as intensive hunting, with human activities increasing nocturnality by an average of more than 30 percent overall. “What shocked us was the consistency of the effect,” Gaynor says. “They perceive a threat, even when there is no threat.”
Fallow deer emerge onto a London street after things have quieted down. (Jamie Hall)
This may sound like a prudent strategy, and in the short term, it may benefit everyone. Species that can’t avoid sharing space with humans may still be able to avoid sharing time, reducing the risk of disease transmission or dangerous run-ins for both parties. In Nepal, for instance, tigers have reached at least a temporary détente with humans by shifting toward nocturnality in