HUMAN HABITS KEEP OTHER MAMMALS UP AT NIGHT
Human encroachment is pushing wild mammals all over the globe to increasingly become creatures of the night, moving their daytime activities toward the darker hours, a new study finds.
This day-to-night shift, described in the journal Science, could have a host of implications for the health and survival of these species—and the structure of their ecosystems as a whole. Roughly 75 percent of the world’s land surface has been impacted by humans, researchers say—and as animals have been trapped in these shrinking parcels of pristine land, they’ve had to adapt to living in the presence of cities or near human activity. Scientists have found that some birds have had to change the frequency of their songs around loud urban environments; others have found that black birds become more sedentary.
But lead author Kaitlyn Gaynor, a wildlife ecologist and Ph.D. candidate at the University of California, Berkeley, wondered if mammals were not just getting displaced in space, but also in time—if they were changing their routines to avoid humans, who primarily operate during the daylight. That question has been hard to answer until recently, scientists said. “The effect of human disturbance on animal temporal activities has been difficult to assess, particularly in secretive wildlife,” Ana Benitez-Lopez of Radboud University, who was not involved in the paper, wrote in a commentary. “In recent decades, the advent of technologies, such as satellite and GPS telemetry or camera traps, has made it popossible to monitor wildlife activity more accurately.”
Using these methods, researchers have now published a number of studies documenting changes in wildlife activity regionally, but Gaynor wanted to find a global pattern. So she and her colleagues put together a meta-analysis of 76 papers on 62 different species studies spanning six continents.
They focused on medium- and large-sized mammals, partly because these animals need a lot of space, have more potential to interact with humans, and are behaviorally very flexible. (Also, there was more data on their 24-hour activity patterns.)
The researchers compared the “nocturnality”— is, what share of an animal’s activity was conducted at night—of animals that lived in places with low and high human disturbance.
Gaynor found that animals that lived in areas with high human activity were indeed shifting to more nocturnal activity by a factor of 1.36. (For example, this meant that an animal that used to spend 50 percent of its active time at night would see that share rise to 68 percent.)
“We expected to find a trend towards increased wildlife nocturnality (across) species, but we were surprised by just how consistent the results were,” Gaynor said. The trend held across continents, habitats, types of animals, and even types of human activity.
Whether that human activity was lethal (such as hunting) or largely harmless didn’t seem to matter, Gaynor said.
“The response is of equal magnitude to activities that don’t actually pose a risk to animals, like hiking through the woods— activities that we think of leaving no trace,” she said.
The phenomenon was widespread—83 percent of the 141 case studies that they analyzed saw an increase in nocturnality. Larger mammals appeared to shift more strongly, the scientists wrote, “either because they are more likely to be hunted or as a result of an increased chance of human encounter.”
This shift could have a broad range of impacts that could ripple through an ecosystem, Benitez-Lopez said.
Among them: If apex predators can’t hunt as well at night as they can during the day, they may not effectively regulate the populations of prey species. A nighttime shift by one species could force them into competition with other animals who use the same resources but at different times. As some animals move into the nighttime, competitors might take over their daytime niches. Animals that are sensitive to human presence might start to lose out to those that are not. Seed sizes may have to evolve if the large mammals that usually disperse them
“We expected to find a trend towards increased wildlife nocturnality (across) species, but we were surprised by just how consistent the results
were.” —Kaitlyn Gaynor
during the daytime are no longer doing so.
For those animals that remain active mostly in the daytime, their stress levels might go up—which could have long-term physiological consequences that could affect their survival or reproduction rates but would be more difficult to observe.
“Holistic approaches that take into account behavioral, physiological, population, and evolutionary responses to human disturbance across taxa are urgently needed to fully understand the consequences of human encroachment for the persistence of wildlife populations,” Benitez-Lopez said.
It’s unclear whether the changes stop at the behavioral level—or whether having humans nearby is influencing deeper, more permanent changes. “That’s the next frontier in research,” Gaynor said. “We don’t really know whether these behavioral adaptations are accompanied by morphological or physiological adaptations in which animals are developing traits through natural selection that facilitate improved success at night.”
Ultimately, the scientists said, the findings could be used to create protected times of day for wildlife, just as we already create protected spaces. In some ways, that’s already done during certain times of the year for breeding seasons, the authors pointed out.
“Currently, spatial ecology informs commonly used land-planning tools, but new tools are needed that explicitly address temporal interactions,” the study authors wrote.