The mountain lion (Felis concolor), also called the cougar, is the largest member of the cat family living in Yellowstone.  Mountain lions can weigh up to 200 pounds, although lions in Yellowstone are thought to range between 140 and 160 pounds for males and around 100 pounds for females.  Two to three kittens may be born at any time of year, although most arrive in summer and fall.  For reasons that are not clear, only about 50 percent of kittens survive their first year.  The current population of lions in Yellowstone is estimated to be 18-24 animals and is thought to be increasing.

Mountain lions are rather secretive, consequently, most visitors are unaware of their existence in Yellowstone.  Lions probably live throughout the park in summer.  In winter, difficulty of movement and lack of available prey causes most lions to move to lower elevations.  Lions are territorial and will kill other lions.  The dominant animals reside in the northern range areas of the park where prey is available year-round.  Mountain lions prey chiefly upon elk and deer, although their diet probably varies based upon opportunity, porcupines provide an important supplement to the lion's diet.

Mountain lions were significantly reduced by predator control measures during the early 1900s.  It is reported that 121 lions were removed from the park between the years 1904 and 1925.  At that time, the remaining population was estimated  to be 12 individuals.  Mountain lions apparently existed at very low numbers between 1925 and 1940.  Reports of lions in Yellowstone have increased steadily from 1 each year between 1930 and 1939 to about 16 each year between 1980 and 1988.  However, increases in visitor travel in Yellowstone and improvements in record keeping during this period probably contributed to this trend.  

In 1987, the first study of mountain lion ecology was initiated in Yellowstone National Park.  The research documented population dynamics of mountain lions in the northern Yellowstone ecosystem inside and outside the park boundary, determined home ranges and habitat requirements, and assessed the role of lions as a predator in the ecosystem.  In recent years in other areas of the West, mountain lions have occasionally attacked humans.  No documented lion/human confrontations have occurred in Yellowstone.