Highways let us humans zip from one place to another—at least when the traffic's not horrendous. But for wild animals, highways are like wide concrete fences that feature metal boxes of death barreling down at 70 mph. In the Santa Monica Mountains, a population of wild mountain lions has become penned in by highways and housing developments, turning inbred—and aggressive.


The problem has been gradually growing for decades, as a roads and houses have sprung up around the mountains. But a just-released 10-year study from the National Park Service paints an especially sobering picture of males killing sons and mating with daughters. Since mountain lions can't leave the area to mate with others, genetic diversity in the population is startlingly low—that's a another way of saying inbred.

The main culprit here is 101 to the north, which pins the mountain lion population against the coast. Interstate 405 to the east and housing developments all around also restrict the movements of the usually wide-ranging creatures.

National Park Service


The study is "dramatically documenting impacts of isolation by highways," Paul Beier, a biologist at the Northern Arizona University, told Science. Mountain lions aren't the only creatures fenced in by highways, of course, but they are some of the largest and most visible.

Is there a solution? If so, it's probably animal highway crossings. A proposed 13 x 13 foot tunnel under the highway near the mountains would cost at least $10 million, which isn't chump change. With millions already poured into preserving the nature of the Santa Monica Mountains, it's certainly worth considering whether a little extra is worth it to allow mountain lions to range more naturally.

For now, the California Department of Transportation put up fencing to encourage the lions to pass under the highway rather than over it—after the earlier death of a young mountain lion by car. The occasional male has escaped across the highway, but even they would still need females to mate with on the other side.



The whole episode illustrates the many ways something as simple as a highway can impact geography. We build highways as arteries for human connectivity, but ironically, they also carve up the natural landscape into isolated and artificial islands. [Current Biology via City Lab]

Top image: National Park Service