The manipulation of animal hormones is an ancient technology. Farmers have castrated male animals for thousands of years to make them more docile. Testicle removal not only prevents the production of sex hormones that stimulate aggressive behavior, but also turns out to favor the production of fat tissue over muscle. This is why steers and capons have long been preferred as meat animals over bulls and cocks. The modern preference for lean meat has led some producers to raise uncastrated animals, or to replace certain hormones in castrates. Several natural and synthetic hormones, including estrogen and testosterone, produce leaner, more muscular cattle more rapidly and on less feed. There is ongoing research into a variety of growth factors and other drugs that would help producers fine-tune the growth and proportions of fat to lean in cattle and other meat animals.