Gloger's rule
It was named after the zoologist Constantin Wilhelm Lambert Gloger, who first remarked upon this phenomenon in 1833 in a review of covariation of climate and avian plumage color.
[1] Erwin Stresemann later noted that the idea had been expressed even earlier by Peter Simon Pallas in Zoographia Rosso-Asiatica (1811).
[2] Gloger found that birds in more humid habitats tended to be darker than their relatives from regions with higher aridity.
[3] One explanation of Gloger's rule in the case of birds appears to be the increased resistance of dark feathers to feather- or hair-degrading bacteria such as Bacillus licheniformis.
Among mammals, there is a marked tendency in equatorial and tropical regions to have a darker skin color than poleward relatives.
G.t."harterti" (= ruficolor ) from the warm and humid coastal plains
G.t."hilgerti" (= superflua ) from the colder and drier Atlas Mountains
G.t."deichleri" (= carolinae ) from the hot and very dry Sahara desert