In 1833 a German researcher named Constantin Lambert Gloger noticed that birds from warmer habitats had darker feathers than those from cooler climes. His observations soon became known as Gloger's rule; ornithologists later verified that tropical plumage indeed darkens closer to the equator. Mammals seem to fit the pattern as well. But why would latitude influence animal coloration? More than 180 years later a possible answer has emerged from a surprising place: flowers.

University of Pittsburgh biologists Matthew Koski and Tia-Lynn Ashman recently looked at 34 different populations of silverweed cinquefoil, a widespread plant native in temperate zones on both sides of the equator, and found that its flowers were darker near the tropics. In this case, “darker” meant they displayed larger “bull's-eyes”—dark circles surrounded by lighter petals that are invisible to the human eye but show up under ultraviolet (UV) light (below right).

The bull's-eyes may act as beacons to pollinating insects, which can perceive UV. But Koski and Ashman found there is more to the dark spots than that. In a laboratory experiment, they discovered that pollen from darker flowers was more likely to germinate when grown under harmful UV light than pollen from flowers that were lighter, with smaller bull's-eyes. The pigmentation is protective, according to the study published online in January in the journal Nature Plants: the larger the bull's-eye, the more UV light is absorbed, rather than being reflected onto the pollen. Absorption is more important for plants in lower latitudes, which face more intense UV rays. (Scientific American is part of Nature Publishing Group.)

Under UV light, silverweed cinquefoil flowers closer to the equator show larger areas of dark pigmentation than those farther away. Courtesy of Matthew Koski University of Pittsburgh

 

The role of bull's-eye size in UV protection does not necessarily discount other environmental factors correlated with latitude; for example, ornithologists have argued that Gloger's rule arises because darker pigmentation comes from a compound that protects feathers from bacteria in the wet, humid tropics. For mammals, researchers say that the overhead sunlight near the equator favors species with darker backs and lighter fronts because the combination offers camouflage in the shadowy rain forest.

Nevertheless, similar to rules, laws and theorems in chemistry or physics, general axioms exist for ecology that explain patterns. Because Koski's study established a link between UV radiation and the plants' reproductive potential, he thinks that UV protection will eventually emerge as a key mechanism behind pigmentation. “UV is universally damaging to DNA and protein structure in both plants and animals,” Koski says, and darker pigmentation—visible or not—may be a strategy across species to avoid damage from the sun's harmful rays.