Gloger’s Rule in Plants: The Species and Ecosystem Levels

Abstract

Gloger’s rule, one of several zoological rules that identify patterns of adaptations at a global scale (rules that do have exceptions), posits that darker birds are found more often in humid environments than in arid ones, especially in the tropics (Gloger 1833). In many animal taxa, desert animals tend to be paler compared to other members of the same taxon that inhabit wetter environments. Being paler in desert habitats is considered to reflect the selection by both physiological factors such as reflecting sun irradiation, and by visual defense from predators or prey (e.g., Cloudsley-Thompson 1999; Stoner et al. 2003a, b). This rule was later demonstrated to also be true for certain mammalian groups including humans (Stoner et al. 2003a, b; Hancock et al. 2011). Gloger’s rule in animals is manifested at two levels, (1) at the species level (different populations of the same species have different patterns of pigmentation at different latitudes or habitats), and (2) at the species assembly level (different groups of taxa have different pigmentation than other taxa groups found in different habitats or latitudes). It is not easy to pinpoint all the reasons for the operation of Gloger’s rule in animals. It seems, however, that three factors, (1) crypsis via background matching, (2) resistance to keratin-degrading micro-organisms in hair or feathers rich in eumelanin, and (3) thermoregulation, are involved (Burtt and Ichida 2004; Kamilar and Bradley 2011).