05/30/2026
A male mountain lion in the Santa Monica Mountains killed his mate. Then he killed two of their sons. Then one of his surviving sons killed a brother. Four members of one family, dead at each other's hands, inside a patch of mountains surrounded by freeways that none of them could cross.
His designation was P-1. He was the first mountain lion collared by the National Park Service in the Santa Monica Mountains study, and he was the dominant male in the range. His mate was P-2, the first female collared in the study. Together they produced at least six offspring, including P-5, P-6, P-7, P-8, and P-9. They were the founding family of the most closely monitored mountain lion population in the world.
In August 2005, P-1 killed P-2. Researchers believe the killing happened either during a fight over food or because P-2 was defending her young from P-1. A male mountain lion killing a female he has bred with has almost no precedent in populations with normal dispersal. In a healthy population, the male holds territory, the female raises kittens, and the young disperse into adjacent ranges when they mature. The male does not need to kill his mate because the social structure does not force them into sustained, close-quarters contact after breeding.
The Santa Monica Mountains do not allow dispersal. The 101 Freeway seals the population to the north. Urban development blocks every other direction. The mountains hold roughly 255 square miles of habitat. Researcher Paul Beier calculated in 1996 that an area smaller than 425 square miles was unlikely to support a mountain lion population without immigration. The Santa Monica Mountains are forty percent below that threshold.
After P-2's death, her offspring dispersed within the range. They had nowhere to go outside it. P-5 and P-7, both males, were killed by P-1 the following year. His own sons. A father killing his male offspring is documented in mountain lion populations, but it is typically a territorial response to a young male attempting to establish a home range in the father's territory. In a normal landscape, the young male would leave. He would cross into the next drainage, the next mountain range, the next county. He would find country where his father was not. In the Santa Monica Mountains, his father was everywhere, because everywhere was the same 255 square miles.
P-8 was killed by P-9. Brother killing brother.
Of seven documented intraspecific killings in the Santa Monica Mountains population between 2002 and 2014, five were carried out by an adult male against his own offspring, his brother, or a previous mate. Seth Riley, the NPS wildlife ecologist leading the study, described the pattern in the journal Current Biology in 2014. All of those things really do not seem to make much sense in an evolutionary perspective, he said. The whole goal is to pass your genes on to other generations.
The behavior makes no evolutionary sense because it is not evolutionary behavior. It is a stress response to artificial confinement. The freeways created a closed system. The closed system prevented dispersal. Without dispersal, related animals that should be living in separate territories are forced into permanent overlap. The overlap produces conflict. The conflict produces killing. The killing removes the genetic diversity that the population needs to survive. The system feeds on itself.
Riley's team genotyped forty-two mountain lions using fifty-four microsatellite loci and found that genetic diversity in the Santa Monica Mountains population was lower than any mountain lion population in North America except the endangered Florida panther, a completely isolated population in southern Florida where inbreeding depression had already caused reproductive failure. Multiple instances of father-daughter inbreeding were documented in the Santa Monica Mountains. The population's effective breeding size was calculated at 2.7. That is not 2.7 thousand. It is 2.7 animals. The estimated total adult population was five to ten individuals.
During the entire study period, only one male mountain lion successfully crossed the 101 Freeway into the Santa Monica Mountains from the outside. He arrived in 2009, mated, and single-handedly increased the genetic diversity of the entire population. One immigrant. One crossing. One animal doing what seventy-five percent of young mountain lions do naturally in every other population on the continent but what is nearly impossible in a mountain range sealed by a ten-lane freeway.
To the researchers' knowledge, not a single young mountain lion successfully dispersed out of the Santa Monica Mountains during the study. In a normal population, all young males and half of young females disperse. In the Santa Monica Mountains, they stayed. They stayed because they could not leave. They bred with their fathers. They fought with their brothers. They died at the hands of their own family. Intraspecific strife was the leading cause of death in the population, accounting for forty-one percent of all known mortalities in radio-collared lions.
Benson et al. (2019) modeled the population trajectory and predicted extinction within fifty years if connectivity was not restored. The median time to extinction was 15.1 years for the Santa Monica Mountains population and 11.7 years for the nearby Santa Ana Mountains population. The clock on that prediction started in 2019.
The Wallis Annenberg Wildlife Crossing at Liberty Canyon broke ground in 2022. It is a vegetated overpass spanning the 101 Freeway at the same location where P-64, the Culvert Cat we covered on this page, used a storm drain to cross forty-one times before the Woolsey Fire burned his paws and killed him. When the crossing opens, it will be the first structure designed to give the Santa Monica Mountains mountain lions what P-1's family never had: a way out.
P-1 was the largest male mountain lion documented in the Santa Monica Mountains until P-45 was collared in 2015. P-45 was confirmed through genetic testing to have been born north of the 101 Freeway, meaning he had crossed it successfully. He was one of two breeding-age males in the mountains. If he and the other male both died, there would be no breeding males in the population. The entire reproductive future of the Santa Monica Mountains mountain lions would depend on whether another male could cross a freeway.
A mountain lion population that produces fathers who kill their mates and sons, brothers who kill brothers, and fathers who breed with daughters is not a population exhibiting natural behavior. It is a population exhibiting the terminal symptoms of a habitat that has been sealed shut. The freeways did not kill these lions directly. The freeways removed the one thing that prevents a mountain lion population from destroying itself: the ability to leave.
Source: Riley, S.P.D. et al. (2014). "Individual Behaviors Dominate the Dynamics of an Urban Mountain Lion Population Isolated by Roads." Current Biology 24(17). / Benson et al. (2019). Ecological Applications. / California Department of Fish and Wildlife, Petition Evaluation Report, January 2020. / National Park Service, Santa Monica Mountains.
