As Californians mourn the death of celebrity mountain lion P-22, work continues on a massive wildlife crossing bridge in Los Angeles County which will connect two natural landscapes that have been bisected by one of the nation’s busiest highways.

Perhaps the lasting legacy of the cougar, who was captured and euthanized by California Fish and Wildlife officials after his health started rapidly declining, is the attention his story brought to the plight of animals who find themselves trapped on all sides by urban sprawl.

That’s why in September 2022, Gov. Gavin Newsom signed legislation to require the state to identify locations where animals face barriers that separate them from moving freely and prioritize building or converting existing infrastructure to allow them to cross more safely.

The decision was lauded by animal welfare groups, but researchers say special care will be needed to ensure that wildlife crossings make sense for the animals that will be using them.

A recently published study by researchers at UCLA found that some animals might actually be fearful of using wildlife crossings, so designers will need to take that into account when planning any new crossings.

“We know that species use them, but sometimes they have to be sort of designed for specific species because different species perceive open areas or the habitat on crossings differently,” said Dan Blumstein, a UCLA professor of ecology and evolutionary biology.

Blumstein’s team studied an underpass wildlife crossing in Alberta, Canada, to see how animals used it, if they did at all.

Videos from that wildlife crossing showed that some deer and elk near the wildlife crossing would shift from foraging for food to becoming fearful or fleeing entirely when cars passed. Animals that appeared to be more fearful, or vigilant, were less likely to use the crossing.

Other animals, including some rodents, would likely cross without giving it a second thought.

Ultimately, what they found was that there was no universal set of behaviors for animals, so there’s no one-size-fits-all solution for designing a wildlife crossing.

“At the end of the day, we found that those animals that were foraging seem to be less concerned by vehicular traffic than those that were more vigilant and that it sort of says that individuals are behaving differently on these wildlife crossings,” Blumstein said.

The researcher said that part of it comes down to whether or not the specific animal views the habitat structure as protective or obstructive. Animals that consider things like shrubs and bushes as safe places will utilize cover, but other animals might think of those same obstacles as dangerous.

“On these wildlife crossings, different species behave differently, and we need to think about designing crossings, as other people have already realized, we need to think about designing crossings to sort of be bespoke for the species you’re dealing with,” Blumstein said.

Even closely related species — in his example, wallabies and kangaroos — will interpret different landscape and vegetation in opposite ways. Wallabies value cover; kangaroos prefer open space.

While there won’t be any kangaroos crossing the 101 Freeway, the logic remains the same: animals perceive different things as safe or dangerous.

He added that the architects and researcher designing wildlife crossings need to have a good understanding about what species they are trying to get to cross and then designing a structure that’s “attractive to multiple species.”

As for the Wallis Annenberg Wildlife Crossing in Agoura Hills, Blumstein said the size of the project could allow for multiple “microhabitats” to fit the needs to multiple species.

Blumstein called it a bold idea to try and reconnect fragmented habitats, one that comes with a hefty price.

The Los Angeles County crossing will come at a price tag of more than $90 million, but not building it also has a cost.

“We know there are costs of isolation, we can see that with genetic mutations and inbred animals and in mountain lions in Los Angeles. And you know, some will use this and this will lead to gene flow and this is going to be a good thing,” Blumstein said. “We mess with mother nature at our own risk and the solutions are often expensive, but wildlife crossings and wildlife corridors has been very successful in other places, and I expect at some level, will be successful here.”