# Gloger's Rule or Historical Conjecture? Tests in Mammals

## TL;DR

This study tests Gloger's rule and 19th-century naturalists' ideas about animal coloration in mammals, finding strong support for darker coloration in humid regions but little evidence for brighter tropical coloration.

## Contribution

The study provides the first comprehensive global test of Gloger's rule and historical coloration conjectures in mammals using phylogenetic comparative methods.

## Key findings

- Mammals are darker in the tropics and in areas with high precipitation and evapotranspiration.
- There is little support for darker coloration in warmer areas or redder coloration in arid regions.
- 19th-century conjectures about brighter tropical coloration in mammals are not supported by the data.

## Abstract

Gloger's rule states that homeotherms are darker at lower latitudes; however, a number of 19th‐century naturalists also suggested that animals are more brightly coloured in the tropics than in temperate regions. Using phylogenetic comparative methods, we investigated and compared both ideas across a global sample of 2726 species of mammals, examining their head, torso, legs and tail regions. Coloration data were obtained from photographs and compared with a colour chart specifically devised for mammals; ecological data were extracted from pre‐existing, open‐source databases. All analyses were conducted using phylogenetic comparative generalised linear mixed models in a Bayesian framework. We found strong support for mammals being darker in the tropics and in areas of high precipitation and evapotranspiration, little support for them being darker in warmer areas, little support for them being redder in more arid regions (a more nuanced interpretation of Gloger's rule), and virtually no support for 19th‐century naturalists' conjecture regarding coloration, contrast, or patterning being more conspicuous in the tropics. These results were replicated at both class and order levels. Our findings provide clear evidence for eumelanic coloration to be more prevalent in more humid climates (one facet of Gloger's rule), operating at a class level, but indicate that 19th‐century observations about bright coloration in the tropics do not pertain to mammals. Our results confirm the importance of Gloger's rule across mammals as a whole and add to a growing tide that darker coloration is linked to humidity at a macroecological scale.

Ecogeographical rules for animal coloration include Gloger's rule, which states that homeotherms are darker at lower latitudes; however, 19th‐century naturalists observed that animals are more colourful in the tropics. We investigated these ideas across the head, torso, legs and tail regions of 2726 species of terrestrial mammals using phylogenetic comparative methods. Our findings provide strong evidence for one facet of Gloger's rule operating at a class level but indicate that 19th‐century observations do not pertain to mammals.