Testosterone and some of its other forms, like dihydrotestosterone, are strongly related to libido and sex drive in the pursuit and ability to mate. However, the hormone estrogen is also strongly associated with libido and mating behavior—so much so that, for people that either chemically or for some other reason have very low estrogen, libido can severely suffer. So, it's a coordinated dance of estrogen and testosterone in both males and females that leads to libido or sex drive.