A review of Gloger's rule, an ecogeographical rule of colour: definitions, interpretations and evidence

Abstract

Gloger's rule is an ecogeographical rule that links animal colouration with climatic variation. This rule is named after C.W.L. Gloger who was one of the first to summarise the associations between climatic variation and animal colouration, noting in particular that birds and mammals seemed more pigmented in tropical regions. The term 'Gloger's rule' was coined by B. Rensch in 1929 and included different patterns of variation from those described by Gloger. Rensch defined the rule in two ways: a simple version stating that endothermic animals are predicted to be darker in warmer and humid areas due to the increased deposition of melanin pigments; and a complex version that includes the differential effects of humidity and temperature on both main types of melanin pigments -eu-and phaeo-melanin. The blackish eu-melanins are predicted to increase with humidity, and decrease only at extreme low temperatures, while the brown-yellowish phaeomelanins prevail in dry and warm regions and decrease rapidly with lower temperatures. A survey of the literature indicates that there is considerable variation/confusion in the way Gloger's rule is understood (based on 271 studies that define the rule). Whereas the complex version is hardly mentioned, only a quarter of the definitions are consistent with the simple version of Gloger's rule (darker where warm and wet), and most definitions mention only the effects of humidity (darker where wet). A smaller subset of studies define the rule based on other correlated climatic and environmental variables such as vegetation, latitude, altitude, solar radiation, etc., and a few even contradict the original definition (darker where cold). Based on the literature survey, I synthesised the qualitative (N = 124 studies) and quantitative (meta-analytically, N = 38 studies, 241 effects) evidence testing the simple version of Gloger's rule (I found no tests of the complex version). Both lines of evidence supported the predicted effects of humidity (and closely linked variables) on colour variation, but not the effects of temperature. Moreover, humidity effects are not restricted to birds and mammals, as the data indicate that these effects also apply to insects. This suggests that the simple version of Gloger's rule as originally defined may not be valid, and possibly that the rule should be re-formulated in terms of humidity effects only. I suggest, however, that more data are needed before such a reformulation, due to potential publication biases. In conclusion, I recommend that authors cite Rensch when referring to Gloger's rule and that they make clear which version they are referring to. Future research should concentrate on rigorously testing the validity and generality of both versions of Gloger's rule and establishing the mechanism(s) responsible for the patterns it describes. Since humidity seems to be the core climatic variable behind Gloger's rule, I suggest that the two most plausible mechanisms are camouflage and protection against parasites/pathogens, the latter possibly through p...

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“…Our results are in line with the expectations of Gloger's rule (Delhey, ) which predicts paler coloration on higher latitudes and in drier and colder environments, and are likely to present a general pattern in nature considering especially species with color polymorphism or seasonal color variation (Mills et al, ). For example, in eight seasonal coat color‐changing mammal species the probability of having white coat color is determined by the days of snow cover, which results in a latitudinal cline in seasonal coat color change (Mills et al, ).…”

Falcon genomics in the context of conservation, speciation, and human culture

“…For example, falcons size distributions appear to be influenced by Bergmann's rule (Meiri & Dayan, ), and broadly dispersed falcon species also often display variation in plumage color, which has been linked to Gloger's rule (Moseikin & Ellis, ; Olsen, ; White, Sonsthagen, et al, ), another commonly observed biogeographic principal that states that birds in more humid environments display darker plumage color (Delhey, ). The evolutionary forces that drive Bergmann's rule are disputed (McNab, ) and Gloger's rule currently lacks a robust evolutionary explanation, although linkage between pigmentation and pathogen resistance has been hypothesized (Delhey, ). Intraspecific genomic studies on broadly distributed falcons have the potential to provide new insights into these processes and other similar ecogeographic patterns by revealing specific loci that are under selection during these processes.…”

“…Such studies may be of broad interest as they have the potential to reveal the evolutionary mechanisms that drive many commonly observed biogeographic patterns. For example, falcons size distributions appear to be influenced by Bergmann's rule (Meiri & Dayan, 2003), and broadly dispersed falcon species also often display variation in plumage color, which has been linked to Gloger's rule (Moseikin & Ellis, 2004;Olsen, 1982;White, Sonsthagen, et al, 2013), another commonly observed biogeographic principal that states that birds in more humid environments display darker plumage color (Delhey, 2019).…”

“…The evolutionary forces that drive Bergmann's rule are disputed (McNab, 2010) and Gloger's rule currently lacks a robust evolutionary explanation, although linkage between pigmentation and pathogen resistance has been hypothesized (Delhey, 2019). Intraspecific genomic studies on broadly distributed falcons have the potential to provide new insights into these processes and other similar ecogeographic patterns by revealing specific loci that are under selection during these processes.…”

Darker ants dominate the canopy: Testing macroecological hypotheses for patterns in colour along a microclimatic gradient

“…Third, Gloger's rule predicts an increase in melanization (and hence darker colours) in warmer and more humid environments (Rensch, 1929), the opposite of the pattern predicted by the TMH and the MDH. While the mechanistic underpinnings of the TMH and the MDH are well defined (Clusella Trullas et al, 2007;Kalmus, 1941), the proposed mechanisms driving Gloger's rule are varied but, include camouflage, protection against parasites and pathogens, and protection against UV-B radiation (or photo-protection) (Delhey, 2019). Increased melanization correlates with greater UV-B radiation for Drosophila, ants and plants (Bastide, Yassin, Johanning, & Pool, 2014;Bishop et al, 2016;Koski & Ashman, 2015).…”