The cubs stay with the female until they are about one and a half years old, by which time the male kittens are larger than their mother. In spite of her instructions in the rudiments of survival, the cubs have a difficult time when they are first on their own. Resident females usually produce litters every 18-24 months. In the southwestern foothills of Alberta, the rate at which kittens reached independence was close to two kittens per female per year over a seven-year period. In jurisdictions where cougar hunting with hounds is allowed, hunting is the most common cause of death.
Natural mortality occurs in many ways.
Because cougars frequently kill prey larger than themselves, they are continually exposed to the risk of serious injuries which eventually take their toll. Cougars have sustained broken backs, massive chest injuries, and perforated abdomens while trying to kill prey. When a female with a litter dies, the cubs usually die also, unless they are over 9-12 months old and can feed and defend themselves. The independent young subadult cougars are more prone to starvation than their elders. Male cougars occasionally kill kittens, young subadults, and other adult males in territorial disputes.