Red fox scavenging in London at nightJamie Hall

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 areas where people farm and forage; in the Santa Cruz Mountains of California, coyotes keep the peace with hikers and bikers by hunting more often at night.

But in the dark, these species face new risks. Because they can’t see as well or move as fast at night, it’s often harder for them to find food and water and defend themselves. And like the antelope in Ruaha National Park, whose avoidance of humans places them in the path of hungry lions, species that shift their schedules may face new predators or competitors long accustomed to the darkness. Even if such stresses don’t kill the animals outright, they can chisel away at populations by shortening lifespans or depressing reproduction—and they can alter relationships among species throughout the food web, with unpredictable consequences.

Wild Boars look for food in Barcelona, Spain. (Geslin Laurent)

Coexistence between humans and predators such as tigers and wolves has become something of a buzzword in conservation circles, especially in the developing world; conservation groups often promote coexistence in hopes of expanding wildlife habitat beyond dedicated reserves while still meeting basic human needs. Brashares, who has worked in Africa for many years, has supported coexistence as a conservation strategy, and still sees it as an important path forward. He emphasizes, however, that easing direct conflicts between humans and animals is only a first step. “We should be careful not to confuse co-occurrence or co-persistence with coexistence,” he says. “Just because humans are living with animals doesn’t mean that either is doing particularly well.”

Ironically, our earliest mammalian ancestors may have gambled on nocturnality, too. They are thought to be among the few to make it through the so-called “nocturnal bottleneck,” surviving the Mesozoic by ceding daytime to the dinosaurs. In their case, the risky shift to nocturnality paid off: They successfully avoided one set of super-predators—and gave rise to another.

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