Testing the simple and complex versions of Gloger’s rule in the Variable Antshrike (Thamnophilus caerulescens, Thamnophilidae)

Abstract

Gloger’s rule is a classic ecogeographical principle that, in its simplest version, predicts animals should be darker in warmer and wetter climates. In a rarely tested more complex version, it also predicts animals should be more rufous in warmer and drier climates. The Variable Antshrike (Thamnophilus caerulescens) is a widely distributed South American passerine that presents an impressive amount of plumage color variation and occupies a wide variety of climatic conditions. Moreover, genetic and vocal evidence indicate ongoing hybridization in south-central Bolivia among 3 populations with very distinct plumages. We collected color data from 232 specimens from throughout this species’ distribution to test the predictions of Gloger’s rule. We found a negative correlation between brightness and precipitation, consistent with the simple version of Gloger’s rule. In contrast, we found that birds were darker in cooler climates, contrary to the simple version of Gloger’s rule, but consistent with recent findings in other taxa. We found support for both predictions of the complex Gloger’s rule and suggest it might be driven by background matching. We conclude by concurring with a recent suggestion that the simple version of Gloger’s rule should be reformulated exclusively in terms of humidity.

Accounting for clinal variation and covariation in the assessment of taxonomic limits: why we should remember the ‘rules’

“…2019, Marcondes et al . 2020). They have generally been interpreted in the context of latitude or altitude; i.e.…”

Ecogeographical patterns in owl plumage colouration: Climate and vegetation cover predict global colour variation

“…So far, a large body of empirical work in birds has found interspecific (e.g., Dalrymple et al., 2018; Delhey et al., 2019; Zink & Remsen, 1986) and intraspecific latitudinal gradients in colour plumage (e.g., Hogstad et al., 2009; Marcondes et al., 2020; Romano et al., 2019; Roulin & Randin, 2015) that fit Gloger’s rule. Nevertheless, some studies have also yielded mixed support, revealing patterns in agreement with the thermal melanism hypothesis (Delhey, 2018; Friedman & Remeš, 2017; Galván et al., 2018).…”

“…So far, a large body of empirical work in birds has found interspecific (e.g., Dalrymple et al, 2018;Delhey et al, 2019;Zink & Remsen, 1986) and intraspecific latitudinal gradients in colour plumage (e.g., Hogstad et al, 2009;Marcondes et al, 2020;Romano et al, 2019;Roulin & Randin, 2015) that fit Gloger's rule.…”

“…Gloger’s rule points out that within populations of the same species, or, between different species, more heavily pigmented forms should be found in warm and humid regions, whereas the lighter ones should be more frequent in cold dry regions (see Delhey, 2017). In contrast, ‘the thermal melanism hypothesis’, also termed Bogert’s rule (Bogert, 1949), first postulated for ectotherms but also potentially functioning for endotherms (e.g., Amar et al., 2014; Marcondes et al., 2020, 2021; Rising et al., 2009), states that animals inhabiting cold regions would benefit from having darker colourations as this would allow them to absorb more solar radiation and to improve thermoregulation performance (Clusella Trullas et al., 2007; Pinkert & Zeuss, 2018). While the mechanism causing Bogert’s rule is based on the thermal properties of colouration, the mechanisms behind Gloger’s rule could be diverse, but are still to be elucidated (Delhey, 2019).…”

Plumage balances camouflage and thermoregulation in Horned Larks (Eremophila alpestris)

“…Thermoregulation is also closely tied to coloration (Walsberg 1983) and sometimes favors phenotypes that either conflict with background matching (Smith et al 2016) or that simultaneously enable thermoregulation and camouflage (Wacker et al 2016). Many species conform to 'Gloger's Rule,' a highly prevalent ecogeographic pattern that ascribes lighter colors to more xeric environments and darker colors to more mesic environments (Gloger 1833;Delhey 2019;Marcondes et al 2020). This widespread trend is thought to be the product of multiple selective pressures, including camouflage and thermoregulation (Burtt 2004;Delhey et al 2019).…”