The mountain lion, 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.