The mountain lions of southern California roam through a human-created gauntlet. There are speeding cars on freeways, sterile subdivisions with fields of cement and pavement, and more than 20 million people with limited tolerance for sharing space with other big predators. Now add another feline headache: Bright lights.
While moonlight and even the dispersed glow of a city doesn’t appear to faze these animals, two decades of tracking data reveal that they avoid brightly lit areas, even during the daytime. The discovery points to another way urban sprawl can scramble the movement of animals.
The insights could extend well beyond the confines of southern California, to other big cats such as leopards, jaguars and lynx that bump up against the network of roads and cities spreading across the planet. And it illuminates (pun intended) another obstacle to aiding southern California’s at-risk mountain lions.
“Our research has shown that even when structures exist to allow mountain lion passage under freeways, the light and noise can deter mountain lions from use of these safe crossing structures,” said Winston Vickers, a wildlife veterinarian at the University of California, Davis who worked on the new study.
The explosion of artificial light in modern cities is known to mess with a variety of natural processes. It can distract migrating birds, dim worms who glow to attract a mate, even fool trees into budding earlier in the spring. Researchers at UC Davis wanted to get a clearer picture of how this non-stop illumination affected a wide-ranging predator living at the edges of a heavily populated area.
To do that, they turned to data from GPS trackers attached to 102 adult and sub-adult mountain lions gathered between 2001 and 2022 by people at the university and the California Department of Fish and Wildlife. That added up to nearly 150,000 location points on more than 20,000 square kilometers of land extending from the southern half of Los Angeles to San Diego and the border with Mexico.
Then they looked at how the animals’ movements were influenced by a variety of factors: human infrastructure such as roads and towns; natural features like forests and streams; and varying light conditions, including moonlight, and the intensity of the overall urban glow and individual hotspots such as streetlights—a phenomena detected by a satellite.
The patterns revealed that mountain lions went out of their way to avoid these shining hotspots, even in the daytime when artificial lighting wouldn’t be a factor. While bright lights are connected to other human-made structures, the presence of light appeared to be a bigger factor e even than the proximity to roads, the scientists reported recently in Philosophical Transactions of the Royal Society B.
“It’s not just the outsized human footprint that is squashing lion habitat, but the extended glow from that footprint, too,” said Fraser Shilling, director of the UC Davis Road Ecology Center, who worked on the study.
By contrast, the presence of the broader urban glow appeared to slightly increase the likelihood of a mountain lion turning up during the night.
The researchers chalk this up to the animal’s hunting habits. Mountain lions are ambush hunters, hiding and waiting for unsuspecting prey such as deer to wander within reach of their claws. A bit of extra light might help them spot an approaching deer more easily. A study in Latin America, they noted, found that increased urban glow correlated with more mountain lion sightings.
The new findings suggest that when it comes to mountain lions in relatively urban areas, artificial light is both a problem to be solved and a potential tool. While bright lights cause cats to shun important habitat such as road crossings, they could also be used to discourage the animals from preying on livestock, which can also lead to the cat being killed, noted Vickers.
There is one other potential long-term twist. The study revealed significant variation between animals. Some cats were more light-averse than others. It’s possible that as city lights grow brighter, the mountain lions of southern California might evolve to become more willing to put themselves into the spotlight.
Barrientos, et. al. “Nearby night lighting, rather than sky glow, is associated with habitat selection by a top predator in human-dominated landscapes.” Philosophical Transactions of the Royal Society B: Biological Sciences. Oct. 30, 2023.
Photo: National Park Service