Mammals adopt nightlife to avoid contact with humans, study finds

Human activity is forcing mammals to become more active during the night, research published Thursday in the journal Science has found.

A review of 76 studies of 62 mammal species from six continents found that "human disturbances" — settlement, hunting or even just recreational activity — have increased nighttime activity for these animals by more than one-third. While the effects of human activity on animal habitat is well documented, until now very little was known about how humans impact the hours that animals keep.

Lead author Kaitlyn Gaynor, a wildlife ecologist at the University of California, Berkeley, said she and her colleagues embarked on the meta study after observing patterns of increasingly nocturnal animal behaviour in their own field research.

Her co-author, Justin Brashares, for example, had found that antelope in Tanzania were shifting their activity to nighttime in areas outside the protection of national parks where there is hunting and human settlement.

When they completed their broad analysis of mostly recent scientific literature on the topic, they found that disruption by humans has increased nocturnal behaviour by a factor of 1.36, or 36 per cent.

"It's a very striking pattern, and we don't yet understand the consequences of this really dramatic shift for individual animal populations," Gaynor said.

"But many of the animals included in our study have evolved adaptations to living in the daylight over millions of years and … might not be as successful at finding food or avoiding their predators or communicating with others in the darkness, which could potentially even reduce their ability to reproduce and provide."

Technology lets us 'spy' on animals

It wasn't until relatively recently that humans had the technology "to spy on animals at night," Gaynor said.

Now scientists use things like remote camera traps that are triggered by infrared technology when animals approach, and radio collars with accelerometers that tell researchers what individual animals are doing and how they're moving around.

"But I do suspect it's been happening for probably as long as humans and wildlife have been living in the same spaces," she said.

The study looked at a variety of mammals ranging from the common opossum, which weighs just 1.13 kilograms, to the African elephant, which tips the scale at around 3,500 kilograms. 

It found that mammals of all body sizes showed a strong response to human activity, although this trend was slightly greater among the larger species. That's likely because larger mammals are more likely to be hunted, and because they have greater space needs that force them into more contact with humans, the study says.

Not necessarily bad news

But it's important to note that this newly nocturnal behaviour isn't necessarily a bad thing, said Gaynor.

"A lot of animals do possess traits that allow for some plasticity in their activity patterns, and some of these animals do have the ability to get by at night," she said.  As long as animals are able to meet their needs at night, they may be able to thrive in human-dominated landscapes this way, avoiding close encounters with people — which could be dangerous for both parties.

"You could see this as another example of humans messing up the natural world, but you can also see temporal partitioning — this dividing up of the day — as a mechanism in which people and animals can co-exist in what's an increasingly crowded world."

Albrecht Schulte-Hostedde, a professor at Laurentian University in Sudbury, Ont., and Canada Research Chair on evolutionary ecology, said it's worth considering that for the mammals featured in the study who now live in urban environments, part of their increased nighttime activity could be tied to the proliferation of artificial light in city environments.

"It wouldn't surprise me if there were circadian disruptions with these shifts in behaviour."

David Sampson, an assistant professor of anthropology at the University of Toronto, Mississauga, said in an email to CBC that it isn't surprising at all that animals around the world are adapting to human activity.

"In fact," he said, "this is a great example of evolution in action." Case in point, several species of lemurs have evolved a pattern of shorter periods of activity during both the day and night, he said.

Gaynor said the findings could help successfully target conservation efforts for sensitive species. That could mean "restricting human activity at certain times of day to leave some daylight for wildlife."

Even something like restricting hiking hours in a protected area could make a difference.

But more research is needed, she said. "We're just beginning to scratch the surface of what these behavioural systems mean for ecosystems."