Gloger's Rule

Origins and basic pattern

In Das Abändern der Vögel durch Einfluss des Klimas (1833), Gloger recorded that even within a single bird species, populations from humid, warm regions had darker plumage. The observation was later extended to other animals, including mammal fur and insect coloration. Darker coloration is largely determined by the amount of melanin pigment, and eumelanin, which produces brown to black tones, is thought to be particularly linked with humid environments. In reviewing geographic variation in birds, Zink and Remsen (1986) treated the rule as one of the classic ecogeographic rules, alongside Bergmann's rule on body size and Allen's rule on appendages.

Functional explanations

Several hypotheses have been proposed. The first is the feather-degrading bacteria hypothesis: Burtt and Ichida (2004) and Goldstein et al. (2004) showed that heavily melanized feathers better resist bacteria that thrive in humid environments. The second is the camouflage hypothesis, under which darker colors are less conspicuous in dark, dense habitats such as humid forests. The third involves ultraviolet protection and thermoregulation. Jablonski and Chaplin (2000) explained variation in human skin color in relation to ultraviolet intensity, and Clusella-Trullas, van Wyk and Spotila (2007) reviewed thermal melanism, in which dark coloration helps ectotherms absorb heat. Caro (2005) broadly reviewed the adaptive significance of mammalian coloration.

Global tests and definitional debate

In a comparative analysis of primate coat color, Kamilar and Bradley (2011) found that species in humid, warm regions are darker, supporting Gloger's rule. Analyzing the colors of thousands of passerine species, Delhey et al. (2019) showed that two effects operate together: higher rainfall is associated with darker coloration, while higher temperature is associated with lighter coloration. Delhey (2017, 2019) noted that definitions of the rule differ among authors and that it needs to be made clear whether humidity, temperature or latitude is the reference. Roulin (2014) argued that melanin-based color polymorphisms can shift with climate change, and Cuthill et al. (2017) placed the topic within the broader field of animal coloration research.

FAQ

What is Gloger's rule in simple terms?

Birds and mammals living in hot, humid regions tend to have darker feathers or fur, while those in dry or cool regions tend to be paler.

Why are animals in humid places darker?

Proposed explanations include melanin-rich feathers resisting bacteria, darker colors providing camouflage in dim forests, and roles in ultraviolet protection and thermoregulation.

Sources reviewed: 2026-10-26

References

1. Gloger, C. W. L. (1833). Das Abändern der Vögel durch Einfluss des Klimas. Breslau: August Schulz.
2. Zink, R. M., & Remsen, J. V. (1986). Evolutionary processes and patterns of geographic variation in birds. Current Ornithology, 4, 1–69.
3. Burtt, E. H., & Ichida, J. M. (2004). Gloger's rule, feather-degrading bacteria, and color variation among song sparrows. Condor, 106(3), 681–686.
4. Goldstein, G., et al. (2004). Bacterial degradation of black and white feathers. Auk, 121(3), 656–659.
5. Jablonski, N. G., & Chaplin, G. (2000). The evolution of human skin coloration. Journal of Human Evolution, 39(1), 57–106.
6. Caro, T. (2005). The adaptive significance of coloration in mammals. BioScience, 55(2), 125–136.
7. Clusella-Trullas, S., van Wyk, J. H., & Spotila, J. R. (2007). Thermal melanism in ectotherms. Journal of Thermal Biology, 32(5), 235–245.
8. Roulin, A. (2014). Melanin-based colour polymorphism responding to climate change. Global Change Biology, 20(11), 3344–3350.
9. Kamilar, J. M., & Bradley, B. J. (2011). Interspecific variation in primate coat colour supports Gloger's rule. Journal of Biogeography, 38(12), 2270–2277.
10. Delhey, K. (2017). Gloger's rule. Current Biology, 27(14), R689–R691.
11. Delhey, K., Dale, J., Valcu, M., & Kempenaers, B. (2019). Reconciling ecogeographical rules: Rainfall and temperature predict global colour variation in the largest bird radiation. Ecology Letters, 22(5), 726–736.
12. Delhey, K. (2019). A review of Gloger's rule, an ecogeographical rule of colour: Definitions, interpretations and evidence. Biological Reviews, 94(4), 1294–1316.
13. Cuthill, I. C., et al. (2017). The biology of color. Science, 357(6350), eaan0221.