A proverbial riddle asks why the chicken crossed the road. But the better riddle might be why the mountain lion didn’t, and what happened when it finally did. A Royal Society Open Science paper published this month, co-authored by two University of Wyoming researchers and their collaborators, finds that just one migrant mountain lion crossing into a generally isolated, inbred population — and successfully mating there — significantly enhances the genetic diversity of that population.
That diversity can stave off extinction, said Kyle Gustafson, post-doctoral researcher for UW’s Wildlife Genomics and Disease Ecology lab.
“In a general scientific view, most people don’t think individuals have very large effects on populations,” he said. “And this study really shows that when times get tough, individuals do matter and they can have very large impacts on populations.”
In California, mountain lions living in the Santa Ana Mountains are separated from the mountain lions living in the Eastern Peninsular Region by the 10-lane Interstate 15.
This highway acts as a barrier to movement between the two groups, which is dangerous for both populations, Gustafson said.
“In a general sense, low genetic diversity and inbreeding can cause physical abnormalities.”
While the Eastern Peninsular Region mountain lions have managed to maintain a large, more genetically diverse population, mountain lions on the other side of the highway, in the Santa Ana Mountains, have seen a decrease in their genetic diversity — and a subsequent decrease in the health of the population — brought on by rampant inbreeding.
In 15 years, while keeping tabs on 146 individual mountain lions, the researchers observed just seven individuals successfully cross the interstate, all of them male.
Females are less likely to be genetic dispersers, said Holly Ernest, professor in the Department of Veterinary Sciences, wildlife population geneticist and senior author of the paper.
“In general, a male’s home range might be 100 square miles, depending on where he lives,” she said. “And (a) female’s home range — the area that they move within — might be a quarter of that. So, a number of females rest their home ranges within a larger male’s home range.”
Of the seven mountain lions that crossed the highway without being killed by cars, just one was observed to have produced offspring in the Santa Ana Mountain population.
This mountain lion, dubbed M86, sired 11 offspring with Santa Ana females. And even though M86 mated with his own daughter in one of these instances, the introduction of one migrant mountain lion to the isolated and inbred Santa Ana mountain lions had immediate positive effects, according to the paper.
Gustafson and Ernest’s collaborators — Winston Vickers and Walter Boyce of the University of California-Davis — led a California-based team in capturing, tagging and monitoring mountain lions in both isolated populations. Whenever mountain lions were captured, samples of their blood were taken and sent to UW, for analysis at Ernest’s Wildlife Genomics and Disease Ecology lab.
“Those offspring — we were only able to find out through genetic work,” Ernest said. “So, using the DNA, (Gustafson) was able to reconstruct the family trees of these mountain lions and determine which of them are the sons and daughters of that M86 mountain lion that did cross the road.”
However, the positive effects were limited; since siring his 11th offspring, M86 was killed by a car and many of his offspring are now dead or in captivity, Gustafson said. More migration is desperately needed, he said, but becoming less and less likely to happen.
“Although mountain lions are moving across the interstate highway, there’s increasing development in the area and the chances of movement being successful are likely to decline,” he said. “And even if they get across, they have to mate to make a difference in the population.”
If the Santa Ana mountain lions go extinct, the rest of their ecosystem could be in danger, Gustafson said.
“Mountain lions are … apex predators,” he said. “They don’t have any predators that prey on them, but they can regulate wildlife communities by feeding on ungulates and other types of animals.”
The paper — titled “A single migrant enhances the genetic diversity of an inbred puma population” — follows up on other research Ernest published throughout the past two decades, establishing the low genetic diversity of the Californian mountain lion populations.
Possible solutions to the problem include relocating outside mountain lions to the Santa Ana Mountains region, but this is expensive and comes with no guarantees, said Gustafson. A relocated mountain lion could be hit by a car trying to return to the Eastern Peninsular Region or otherwise fail to mate.
Another solution could be wildlife corridors — land bridges connecting two sides of a highway and allowing animals safe passage across. This idea hints at a possible solution to Wyoming’s own animal-crossing problem.
“People are considering wildlife overpasses for I-80 with our ungulates — pronghorn, mule deer, elk — where that highway is causing a lot of mortality to those important ungulates,” Ernest said.
Ernest co-authored a separate paper — also published this month — for PLOS ONE, which suggests establishing a conservation network in California.