I have wondered many times how house lizards lose their tails. Today I found the reason: they lose them in fights over territory with other lizards. This was brought home to me forcefully when I was made to nearly jump out of my chair in shock when two lizards fell from an adjacent wall, grappling with each other furiously on their way down (like Spider-Man and Doc Ock). One of the lizards had the other one's tail firmly in its mouth when they landed. Somehow I managed to scare them off and probably saved the weaker lizard in the process.

On the other hand, it could have been an attempted rape that I witnessed.