An ecogeographical rule concerns itself with the variation of traits (mainly morphological) of an organism over geographical gradients. In 1833 Constantin Wilhelm Lambert Gloger (1803 – 1863), a German ornithologist and zoologist, proposed that within a species of warm-blooded animals, darker or more heavily pigmented forms occur in humid environments and more lightly pigmented forms are found in arid areas with the darkest individuals within a species generally found closer to the equator. This ecogeographical observation is Gloger’s Rule.
Melanin is a “catch-all” term for the natural pigments found in most organisms. Melanin is composed of many different smaller molecules. Two of these smaller components of melanin are eumelanins and pheomelanins. Eumelanins are brown and black and are tougher and harder to degrade than the red to pink pheomelanins. In birds with color variations between geographic areas, the melanin in feathers of lighter specimens contain more pheomelanins while eumelanins predominate in feathers of darker individuals.
In a 2004 paper in “The Condor” Edward H Burtt, Jr. and Jann M Ichida studied song sparrows – lighter song sparrows from the arid Southwest and darker song sparrows from the humid Pacific Northwest. Bacterial degradation of feathers is a severe problem in humid areas and a lesser problem in more arid areas. Burtt and Ichida showed that the presence of melanin, particularly eumelanins, increased song sparrow feather resistance to abrasion and decreased bacterial degradation of their feathers. Other researchers suggest that melanins may also be involved in thermoregulation (darker feathers absorb more heat), camouflage, and drying of feathers.
There is also research to suggest that some flowers may follow Gloger’s Rule.
This song sparrow (Melospiza melodia) was photographed at Baum Lake (Shasta County CA).