On the other hand, melanism, or a tendency to the production of dark or even black forms, reveals itself.
melanism is rather more common in the case of the male sex than in the female sex.
Only one case of melanism was observed among all the specimens of both species examined.
melanism: an abnormal or unusual darkening: a suffusion with blackish.
melanism mel·a·nism (měl'ə-nĭz'əm)
Dark coloration of the skin, hair, fur, or feathers because of a high concentration of melanin.