Have you ever wondered why you often don’t see animals larger than a squirrel while on your Saturday morning nature walk or mountain-bike ride?

There’s a reason for that, scientists say: Us.

Around the world, larger mammals are becoming more active at night to avoid disturbance by humans—whether that’s roads and other development, energy exploration, or even recreation, according to a study to be published June 15 in the journal Science.

“As the human footprint is expanding across the planet, if animals are trying to avoid us, there are fewer and fewer places for them to go,” says Kaitlyn Gaynor, the study’s lead author and a doctoral candidate in the Department of Environmental Science, Policy, and Management at the University of California-Berkeley. “So they avoid us in time.”

The researchers found that, on average, larger mammals became 1.36 times more nocturnal in response to human disturbances. For instance, if an animal normally splits its active time equally between day and night, it will shift and spend 68 percent of its time active at night, on average, according to the findings.

The authors uncovered other surprises. You might think a mammal would shy away more from a hunter than, say, a hiker, camper, drill pad, or any other kind of intrusion. Not so. “Surprisingly, non-lethal human activities generated similar shifts in wildlife [daily] patterns as lethal activities, suggesting that animals perceive and respond to humans as threats even when they pose no direct risk,” the authors write. “You might expect that animals will respond more strongly to these more permanent fixtures,” like homes and other structures, Gaynor says. “But they respond really strongly to people.”

The findings are the latest in a growing body of research that has found even seemingly low-impact activities harm wildlife more than nature-lovers might hope. “When people say, ‘My backpacking in a wilderness area has a much lighter ecological footprint than ATVs or oil and gas development in those same areas,’ my response is, ‘Yeah, that’s probably true,’” says Justin Brashares, a professor at Berkeley in whose lab Gaynor works. But the new study shows that it doesn’t take an oil derrick to affect wildlife. Humans are often not a benign presence, says Brashares. “We have to accept that our presence alone alters the behavior of wildlife.”

To come to this conclusion, Gaynor and her colleagues did what’s called a meta-analysis. They gathered 76 published studies that looked at 62 species of mammals on every continent but Antarctica. Each study looked at how active wildlife was at night in areas with both high and low human disturbance. Then, using those statistics, the researchers looked for patterns.

The new study shows that it doesn’t take an oil derrick to affect wildlife. Humans are often not a benign presence.

They focused only on mammals one kilogram (2.2 pounds) and heavier, from the opossum to the African elephant. Larger mammals have greater need for space than small ones, and thus there’s more potential for conflict. But they also tend to have more ability to change their behavior.

Most of the effects the researchers found when the wildlife responded to human disturbance—83 percent—corresponded with an increase toward nighttime activity. Not every change was large. Overall, though, the findings show “significant, widespread increase in nocturnality among mammals living alongside people,” the authors write.

Rick Knight, a professor emeritus at Colorado State University’s Department of Human Dimensions of Natural Resources, who has for decades studied what happens where human communities meet ecosystems, applauds the authors’ efforts. Many studies have examined how animals avoid humans in space but not in time, says Knight. “I think they have chosen a topic that for far too long has been overlooked,” he says.

The shift Gayor and her team observed could have far-ranging consequences for mammals’ fitness, the food web, and even their evolution, according to the study.

Pushing mammals toward the night could mean that animals that are better adapted to a shift—physically, behaviorally—survive. Over time, and worldwide, this could affect evolution, as certain traits grow more pronounced to benefit life in the dark.

The shift also may cause “mismatches.” Many species have evolved traits that let them operate best during the day. If animals spend more of their waking hours at night, they may suffer, becoming easier prey or poorer hunters, the authors write. All of this could add up to poorer reproduction and survival, which in turn could disrupt entire populations.

As animals shift the period when they’re active, the age-old patterns of who-eats-whom may be disrupted. Food that’s a staple of a daytime diet may be harder to find at night, even as other food walks onto the scene. Such changes may ripple through the food web “and (transform) entire ecological communities,” the authors write.

Changes are already happening. In Alberta’s Rocky Mountains, yet-to-be-published work by Sandra Frey, a graduate student at the University of Victoria’s School of Environmental Studies, has found that wolves in the busy Kananaskis Country, west of Calgary, shift more toward nighttime activity than wolves in Willmore Wilderness Park, a roadless, little-trammeled park about a day’s drive north. Even within the Willmore, wolves show altered behavior, with animals near the park’s borders becoming more active at night than those deeper in the park, says Frey.

In California’s Santa Cruz mountains, coyotes that live in preserves adjacent to human activity have shifted to eating a diet of night prey, researcher Justine Smith at Berkeley and her colleagues found last year. In doing so, the adaptable predators are encroaching on gray foxes, who by nature hunt mostly at night, and who often are killed by coyotes they encounter.

In still other cases, the patterns emerging are more chaotic and less predictable. Researchers haven’t yet teased out why. In Alberta, Frey found that the American marten, a carnivore which is mostly nocturnal in the wilderness, has become much more active in the daytime in the busier Kananaskis Country. The Canadian Rockies hold a huge number of carnivores—wolves, cougars, wolverine, lynx, coyote, marten, and smaller weasels. Over time, these predators have figured out how to get along in time and space, Frey says. But if some animals move more to the nighttime, that could produce “cascading shifts” that jumbles all sorts of behaviors, she says. “It gets complicated, it gets nuanced, and it gets really fascinating.” Where it ends, or how much it matters, is, for now, an open question.

Not all the change is necessarily negative. Shifting to a more nocturnal existence may allow larger mammals to survive in the presence of humans, as the world grows ever more crowded, the authors suggest. The shift might reduce attacks on people by larger predators, and could reduce disease transmission such as rabies between wildlife and humans. “You can see these results as just another way we are messing things up,” says Gaynor. “The flip side is, this is a mechanism for co-existence.”

She and her colleagues call for more research, not only of the size of the effect that humans are having, but also on whether a similar change may be happening in organisms other than mammals.

They also suggest using this new information when planning for conservation—giving animals time and not just space. Approaches might include more "temporal zoning," which means keeping humans out of wildlife habitat when the animals are especially active or vulnerable. Such restrictions are already used in many places. Canada’s Banff National Park has several seasonal closures, including a two-month hiking restriction on a popular trail to allow bears to feed on berries. Officials also close more than 10 miles of the Bow Valley Parkway through the park in the spring, to give the animals the space they need.

Mammals are thought to have originally evolved to be nocturnal. Some of the traits many still have—whiskers, an acute sense of smell, decent night vision—are believed to be vestiges of that time when they snuffled around after sunset. Research published last year has bolstered the theory that our nocturnal forebears were able to emerge, blinking into sunlight, only upon the demise of the ravenous, day-living dinosaurs, and become the new kings of the land.

This theory perhaps holds a hope for mammals’ ability to persevere. It holds perhaps a very different message for the mammal that now scatters them.