Humans exert a heavy pressure on nature, from exploitation of resources to habitat destruction. It turns out that we're having a subtler effect, too: We're driving other mammals to conduct their activities at night in an effort to avoid humans.
A study published Science found that, on average, mammals across the world have become 1.36 times more nocturnal than they've previously been in the past. Translated into percentages, animals that used to split their activities evenly between day and night have increased their nighttime activities by 68 percent.
"While we expected to find a trend towards increased wildlife nocturnality around people, we were surprised by the consistency of the results around the world," University of California Berkeley Ph.D. candidate and lead author Kaitlyn Gaynor said in a statement. "Animals responded strongly to all types of human disturbance, regardless of whether people actually posed a direct threat, suggesting that our presence alone is enough to disrupt their natural patterns of behavior."
Working the night shift
Gaynor and colleagues first noticed this shift in research they conducted in Tanzania, Nepal and Canada when they observed that animals in each country were more active at night when they were around people.
To determine just how widespread and what kind of an effect this was having on the animals, Gaynor's team conducted a meta-analysis, which is a review of previously published studies and reports that contained information in the 24-hour activity patterns of large mammals. These compiled observations of animal behavior were collected using "remotely triggered cameras, GPS and radio collars and direct observation," among other methods.
A coyote blends into the wilderness near the Arizona-Sonora Desert Museum. (Photo: Anne Reeves/flickr)
Additionally, Gaynor explains in a piece for The Conversation about the study that the researchers relied on studies that tracked a variety of behaviors, like "deer activity in and out of the hunting season, grizzly bear activity in areas with and without hiking and elephant activity inside protected areas and outside among rural settlement."
Using this data, Gaynor and her team determined the degree to which each species performed nocturnal activities and factored in whether there was a high or low amount of human disturbance.
Ultimately, 62 species across six continents were tracked in the study, and the results consistently showed an uptick in animal activity at night.
"While we expected to find a trend toward increased wildlife nocturnality around people, we were surprised by the consistency of the results around the world," Gaynor wrote for The Conversation. "Eighty-three percent of the case studies we examined showed some increase in nocturnal activity in response to disturbance. Our finding was consistent across species, continents and habitat types. Antelope on the savanna of Zimbabwe, tapir in the Ecuadorian rainforests, bobcats in the American southwest deserts — all seemed to be doing what they could to shift their activity to the cover of darkness."
These shifts to nocturnality were consistent regardless of the type of disturbance. So whether it was hiking or hunting, roads or farming, the very presence of humans — threatening or not — resulted in changes in the animals' behaviors.
"People may think our outdoor recreation leaves no trace, but our mere presence can have lasting consequences," Gaynor wrote.
Consequences of the night
The effects of this shift need more research before they can be fully understood.
One instance for consideration might be the sun bear, an animal that functions almost entirely during the day. According to Gaynor, in undisturbed areas, less than 20 percent of sun bears' activity occurred at night . In disturbed areas, that percentage skyrocketed to 90 percent.
"Such diurnally adapted animals, may not be as successful at finding food, avoiding predators or communicating in the darkness, which could even reduce their survival or reproduction," Gaynor wrote.
Some signs of nocturnal shifts are already noticeable. Coyotes in California's Santa Cruz's mountains are shifting to hunting prey that are more active at night, a change that could have ripple effects on the food web and the overall ecosystem as predator-prey relationships and competition for food shift.
"On the positive side, the fact that wildlife is adapting to avoid humans temporally could be viewed as a path for coexistence of humans and wild animals on an increasingly crowded planet," Justin Brashares, a professor at UC Berkley and the senior academic on the study, said in the statement. "However, animal activity patterns reflect millions of years of adaptation — it's hard to believe we can simply squeeze nature into the dark half of each day and expect it to function and thrive."
The team's study was partially funded by the National Science Foundation.