All around the globe, human activity is driving mammals to become more active at night, according to new research. Likely driven by a fear of people, many animals shift their schedules toward nocturnality to avoid running into us.
“We were shocked at how consistent this was all over the world, from savannas in Africa to forests in Europe,” said Kaitlyn Gaynor, a wildlife ecology PhD candidate at the University of California, Berkeley, and lead author of the new paper.
The new meta-analysis, published today in Science, considered 76 studies and included 62 different mammal species across six continents. Those studies monitored animals using technology like motion-activated cameras and satellite data along with direct human observation to record their activities. They also included information on how much the animals came into contact with humans, and considered variables like hunting, hiking, construction, urban development, and farming.
Compared to mammals that had low human disturbance and split their time between day and night 50:50, highly disturbed mammals became on average 1.36 times more nocturnal—that equates to a 32:68 split, Gaynor said. Additionally, animals altered their behavior the same way whether humans were hunting them or just hiking on by. The researchers said this means they’re most likely acting out of fear when moving to a more nocturnal lifestyle.
“It seems that animals may be playing it safe,” Gaynor told Gizmodo. “They may find people scary, because we’re large and loud and unpredictable.”
She added that it’s very hard to know what effect these changes have on different individuals and populations—from group to group, it could be positive, negative, or simply neutral.
For example, since coyotes hunt at night, they might see an influx of prey, like squirrels, that are typically day-dwellers, but are slowly becoming more nocturnal to avoid contact with human beings. That situation is great for the coyotes but terrible for the squirrels. “Many prey species avoid risks from people, but run straight into the arms of a predator,” Gaynor said.
“Even if you give an advantage to one part of the food web, you take away an advantage somewhere else,” John Barentine, the director of conservation at the International Dark-Sky Association, told Gizmodo. “You’ll quickly throw things out of balance.”
Gaynor also brought up that newly nocturnal animals may find it harder to raise their young if they spend a lot of energy evading predators lurking in the dark.
Still, certain species may have better survival rates at night because they do successfully avoid human interaction. Deer and coyotes that live in urban and suburban areas are very often killed by cars, but “it’s easier for them to cross roads in the middle of the night when traffic volume is much lower,” Stanley Gehrt, a wildlife biologist at Ohio State University, told Gizmodo. “Their survival rates are much higher in those areas.”
Plus, the fact that many mammals adjust their schedules to live alongside humans instead of moving to new locations altogether is encouraging to Gaynor. As Earth’s population scrambles higher, finding ways for wild animals and people to share habitats is important. Further research could pinpoint mammals that may be most vulnerable in their new nighttime hours, perhaps guiding local policy and conservation efforts to protect them, Gaynor said.
“It’s easy to see this as just another story about how we’re deterring nature,” she said. “But a positive of this is that it demonstrates a mechanism where people and animals can coexist. Finding ways to share space with wildlife while the planet becomes more and more crowded is critical.”