The study analyzed artificial nighttime light detected by the NASA-NOAA Suomi satellite and paired it with GPS data from 117 mountain lions and 486 mule deer in Utah, Arizona, Nevada and California, along with 1,562 sites where mountain lions cached a deer they killed.
“It’s a subject that has received very little scientific attention, but when we look at the rate light pollution is growing globally, it’s extraordinary. It’s impact can go well beyond the footprint,” said David Stoner, a research assistant professor at USU and co-author of the study.
Deer are attracted to artificially lit areas because it mimics their preferred grazing times at dawn and dusk, the study found. While researchers wondered whether the urban light deterred cougars, thus protecting the deer, it appears the predators take advantage of dark spots on the landscape to hunt their prey.
Or, as Stoner put it, there’s “a lot more going on just beyond the streetlamps than we ever realized.”
Deer are active during twilight hours in naturally lit landscapes for a range of reasons. “It’s cool and they can have difficulty overheating," Stoner said. "Then they tend to settle down in the middle of the night and the middle of the day.”
Cougars looking for a meal follow the same pattern, Stoner added, but in urban and artificially lit areas, those patterns flatten out.
“They’ll feed all night,” Stoner said.
“I think the other part of this that was surprising was that we were able to look at light separate from land use change,” Stoner said. “Light is correlated with roads and buildings, but we were able to statistically pull it apart from that and show light itself ... definitely has an effect on both species.”
Asked whether light pollution makes life harder or easier for deer, Stoner said it’s a mixed bag.
“It is a benefit because it allows them to forage longer, but there’s a cost to it,” Stoner said. “They’re in an urban environment where there are novel dangers.”
When it comes to mountain lions, however, Stoner said the topic needs more research.
“The advantage to the cougar is that the deer are aggregated in very predictable locations, more so than if they were in a natural community,” Stoner said. “That makes finding them easy.”
But cougars' eyes are better adapted to hunt in darker conditions, which may be why they seek dark areas in urban landscapes to make their kills.
“In native states, cats generally have very good night vision,” Stoner said. “Urban light impedes that, puts them at a disadvantage, so what they’re doing is hunting the deer in the relatively darkest areas.”
Those shadows include canyons, forested areas near neighborhoods or places where ridges and vegetation block urban light.
"Cougars are not avoiding town just because of traffic and human activity,” he said. “They aren’t necessarily deterred by that, they simply work around it.”
Kent Hersey, big game projects coordinator for the Utah Division of Wildlife Resources, said the light pollution study provides useful insight for wildlife managers about how animals interact with the urban environment. DWR shared six years' worth of deer movement data from across the state with the study’s authors.
“We’ve always tried, when we model these movements, to understand how humans influence both carnivores and ungulates,” Hersey said.
The division typically looks at variables like road density, distance from habitat to roads and housing density when trying to understand herds' behavior. Light data can now add another clue.
Hersey said that while human encounters with mountain lions remain rare, deer populations in Utah’s urban areas have become problematic in recent years.
“Whether that’s because animals are actually growing in these neighborhoods or if towns are expanding more into traditional ranges, we can’t say,” Hersey said. “With some of the light [making deer] able to avoid predation, does that factor into their use of urban areas? It certainly brings up that question.”
One thing that came as no surprise, however, is that cougars have adapted to Utah’s urban interface.
“The reality is, wherever there are deer, you’ll have cougars,” Hersey said. “They’ll follow. That’s how things work.”