Lions and tigers and bears are increasingly becoming night owls because of us, a new study says.
Scientists have long known that human activity disrupts nature. Besides becoming more vigilant and reducing time spent looking for food, many mammals may travel to remote areas or move around less to avoid contact with people.
The latest research found even activities like hiking and camping can scare animals and drive them to become more active at night.
"It suggests that animals might be playing it safe around people," said Kaitlyn Gaynor, an ecologist at the University of California, Berkeley, who led the study. "We may think that we leave no trace when we're just hiking in the woods, but our mere presence can have lasting consequences."
Gaynor and her colleagues analyzed 76 studies involving 62 species on six continents. Animals included lions in Tanzania, otters in Brazil, coyotes in California, wild boars in Poland and tigers in Nepal.
Researchers compared how much time those creatures spent active at night under different types of human disturbance such as hunting, hiking and farming. On average, the team found that human presence triggered an increase of about 20 percent in nighttime activity, even in animals that aren't night owls.
Results were published Thursday in the journal Science.
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The findings are novel because "no one else has compiled all this information and analyzed it in such a ... robust way," said Ana Benitez Lopez of Radboud University in the Netherlands, who reviewed the study.
Marlee Tucker, an ecologist at Goethe University Frankfurt in Germany who was not part of the research, was surprised that any kind of human activity is enough for mammals to see people as a threat.
"It's a little bit scary," she said. "Even if people think that we're not deliberately trying to impact animals, we probably are without knowing it."
Gaynor said animals that don't adapt well to the darkness will be affected. But she said that behavioral shift could also help other animals reduce direct encounters with people.
"Humans can do their thing during the day; wildlife can do their thing at night," she said. That way, people would be sharing the planet "with many other species that are just taking the night shift while we're sleeping."