Initially, I assumed fact checking Gloger’s Rule would be a simple, straightforward task.
That naive assumption was quickly dispelled as the reference materials I consulted plunged me headlong into a quagmire of preference, prejudice, propaganda and political correctness.
Separating fact from fiction is a serious challenge when it comes to Gloger’s Rule.
The first challenge regarding the zoological and ornithological writings of Wilhelm Lambert Gloger [1803- 1863] is that they are written in German.
The second problem is that Gloger never formulated the biological “rule” that bears his name.
In 1833 Gloger wrote about his observations on the regularities that occur between the pigmentation of feathers and furs and the relationship of these colors to the geographic areas in which the animals lived.
From this article, scholars have formulated what is now know as Gloger’s Rule, which states that the darkest forms of a species or subspecies and race occur in the hot, humid areas of their range.
Black Skin: Structure and Function – 1993
William Montagna, Giuseppe Prota, John A. Kenney Jr.
The third problem is that “scholars” continually redraft Gloger’s Rule [to suit their own purposes].
Additionally, some “scholars” have elevated the rule to the status of Gloger’s Law.
Therefore, this is no precise [and consistent] definition of Gloger’s Rule.
the closer a warm-blooded animal is to the equator, the darker its coat.
Echoes of the Old Darkland – Charles S. Finch III, M.D.
within a species of endotherms more heavily pigmented forms tend to be found in more humid environments, e.g. near the equator.
dark pigments increase in races of animals living in warm and humid habitats.
Gloger’s rule states that animals should be dark in warm and wet regions.
Feathers tend to be darkly colored in habitats where relative humidity is high and pale where it is low.
Gloger’s Rule correlates darkness (low reflectance) with humid environments. http://www.mun.ca/biology/scarr/Gloger%27s_Rule_in_Humans.htm
Individuals of many species of insects, birds, and mammals are darkly pigmented in humid climates and lightly coloured in dry ones.
The contention that for a given species those individuals which inhabit the colder and drier areas within its area of distribution are lighter in colour than those living in the warmer and wetter areas.
many species of insects, birds, and mammals are darkly pigmented in humid climates and lightly coloured in dry ones.
the skin of a warm-blooded animal species tends to become darker in color towards warmer climates at lower altitudes or lower latitudes, and to become lighter in color towards colder climates at higher altitudes or higher latitudes.
within the same species of endotherms, more heavily pigmented forms tend to be found near the equator and lighter forms away from the equator.
heavily pigmented animals are to be found mostly in hot climates where there is intense sunshine. Conversely, those in cold climates closer to the poles commonly have light pigmentation.
Rensch (1935) interpreted Gloger’s rule to mean that within a polytypic species of warm-blooded vertebrates, those members that live in cool, moist regions have less dark pigmentation than those in hot, moist regions.
Black Skin: Structure and Function – W. Montagna, G. Prota, J. A. Kenney Jr.
Picking through these scholarly contradictions and inconsistencies is not an easy task.
However, having reviewed the above definitions, it is evident:
a) There is no agreed scientific definition of Gloger’s Law.
b) There is no agreed definition of Gloger’s Rule.
Therefore, it’s more appropriate to re-designate the underlying concept: Gloger’s Rule-of-Thumb.
But what precisely is the underlying concept?
From my perspective the “pigmentation of warm-blooded animals” is the underlying thread in these definitions.
However, it is incorrect to polarise pigmentation into “black” and “white” because animal pigmentation includes all the colours of the rainbow. Therefore, it is more appropriate to focus upon a range of pigmentation that increases from “light” levels of pigmentation through to “heavy” levels of pigmentation.
But what precisely is pigmented?
There are a range of options covered in the scholarly definitions: coat, feathers, hair, skin…
Unfortunately, none of these terms holistically defines the “outer layer” of all warm-blooded animals i.e. the “outer layer” by which each individual animal can be visually identified.
Therefore, there seems little option but to run with the term “outer layer”.
But what precisely affects the “outer layer”?
Again the “scholars” have provided a range of conflicting options: heat, dampness, dryness, humidity, altitude, latitude and sunshine. The majority of these options [heat, latitude, altitude, sunshine] can be interpreted as synonyms for sunlight. Therefore, within this context, I will select sunlight as the main agent that affects the pigmentation of the “outer layer”.
Now I can construct slightly more precise definitions:
Prolonged exposure to strong sunlight results in heavy levels of pigmentation in the “outer layer” of warm-blooded animals.
Prolonged exposure to weak sunlight results in light levels of pigmentation in the “outer layer” of warm-blooded animals.
However, these definitions are still far from perfect.
The phase “prolonged exposure” [to sunlight] is designed to exclude warm-blooded animals that are aquatic [or predominantly aquatic], such as: dolphins and whales.
Unfortunately, this is still open to interpretation in the case of, say: penguins, seals, otters…
Is Gloger’s Rule-of-Thumb valid?
There are species that appear to [broadly] conform to Gloger’s Rule-of-Thumb but the sources [usually] attribute the pigmentation variations to rainfall.
Long-tailed weasels – but the webpage associates the variations with rainfall.
Tree Shrews – but the webpage associates the variations with rainfall.
Song Sparrows – but the webpages associates the variations with rainfall.
Brown Falcons – but only “maybe” conforms.
However, the concept does not appear to have been well tested.
Furthermore, the limited testing appears to have yielded mixed results.
Although this hypothesis was formulated nearly 200 years ago, it has not been well tested for many animal groups. In addition, existing studies of birds and mammals at both the across- and within- species levels have yielded mixed results.
Interspecific variation in primate coat colour supports Gloger’s rule.
Jason M. Kamilar and Brenda J. Bradley
Overall, it looks like the “scholars” are onto a good thing propagating contradictions and confusion. This opens up opportunities for research, field trips and [most importantly] funding.
Personally, I find all this scholarly shenanigans very disreputable.
From my perspective animals have to adapt to there surroundings as a matter of survival.
1) The chances of survival for predators [and their prey] will be greatly enhanced if their “outer layer” adapts to provide camouflage in the animal’s environmental surroundings.
2) Animals with defence mechanisms [especially poisons and venom] enhance their survival chances by adapting their “outer layer” to blatantly advertise their defensive capabilities.
3) Animals deploy many additional strategies to improve their chances of survival: body mass, reproductive rates, social groupings, communication, defensive lairs, instincts, tools…
However, from my perspective, where it gets really interesting is when the adaptation results in the animal losing most of its protective “outer layer”…