Sexual dichromatism is a form of sexual dimorphism that refers to a difference in coloration between sexes within a species. While sexual dichromatism is common among birds, and to a lesser extent among lizards and fish, it is rare among mammals, with the exception of primates. Both male and female primates sometimes exhibit changes in skin color related to dominance and reproduction, but the term sexual dichromatism is typically reserved for differences in pelage, or fur, color. Among primates, the most striking examples of sexual dichromatism are found in black lemurs, black-and-gold howlers, saki monkeys, and several species of gibbon. In part because of the broad diversity of the species exhibiting sexual dichromatism, attempts to identify its functional significance have met with only limited success.