The handicap principle proposes that females prefer males with handicaps (mating characters that reduce survival chances) because handicaps are indicators of heritable viability. According to the principle, honest signals of quality must be costly to produce so that low-quality individuals cannot fake them. Examples of honest signals include a peacock's tail, which is eye-catching but heavy, and a gazelle's stotting behavior before fleeing from predators, which requires energy but signals the gazelle's strength. The principle suggests signals of quality effectively lower an individual's fitness but are affordable only by those of high biological fitness.