A condition known as melanism occurs in the species. The melanistic form is less common than the spotted form — six percent has been reported for its South American range — and is the result of a dominant allele. Jaguars with melanism appear entirely black, although their spots are still visible on close examination. Melanistic Jaguars are informally known as black panthers, but do not form a separate species. Rare albino individuals, sometimes called white panthers, occur among jaguars, as with the other big cats. The jaguar closely resembles the leopard, but is sturdier and heavier, and the two animals can be distinguished by their rosettes: the rosettes on a jaguar's coat are larger, fewer in number, usually darker, and have thicker lines and small spots in the middle that the leopard lacks. Jaguars also have rounder heads and shorter, stockier limbs compared to leopards.