The reason why mountain lions have long tails is because they need them for balance. Another wild cat species with a very long tail is the snow leopard. Both of these species of wild cat have a particular need to maintain their balance. I will quote a passage from the book Wild Cats of the World which I think explains what I have just stated in more detail.
“Compared with other large cats, the puma has unusually long hind legs, which are thought to be an adaptation for jumping. One account describes a female puma, sitting near her kill, leaping over a 2 meter high mesquite bush and catching a vulture in midair as it was about to land in the shrub….Showing little regard for the desert terrain, one Puma overtook and killed a javelina in a 200 meter chase. In California, pumas have been seen leaping boulders and dodging clumps of vegetation while chasing and eventually catching jackrabbits…Hunters’ stories also tell of the puma’s amazing ability to leap 20 feet straight up a cliff, and pumas chased by dogs have made downhill leaps 30 and 40 feet long.”
The gist of what I’m saying is that the puma needs good balance in order to travel very quickly over rough terrain both in catching prey and escaping predators (by which I mean the human hunter). The long tail provides a means to maintain balance as it swishes from side to side placing a weight on one side of the body as a counterbalance to the body’s movements.
Wild Cats of the World ISBN-13: 978-0-226-77999-7 and ISBN-10: 0-226-77999-8 – Buy this book. It is the best on the market.