Rodents show darker and redder coloration in warm and rainy environments

Aim: Gloger’s rule predicts that warmer and wetter areas favor more
pigmented animals. Yet, the original formulation lacks differentiation
between the two primary pigments: eu- and pheomelanin. We examined
geographic variation in eumelanin and pheomelanin to unravel how various
ecological factors influence pigment deposition, and to assess support for
the complex version of Gloger’s rule. Location: South America. Time
period: Contemporary. Major Taxa Studied: Sigmodontine rodents. Methods:
We extracted pelage color data from 231 species and quantified the
variation in eu- and pheomelanin deposition at the assemblage level. We
performed linear multiple regression to investigate the influence of
temperature, precipitation, predator diversity, and UVA-B radiance in
eumelanin (lightness) and pheomelanin (redness). Results: Our findings
support the original formulations of Gloger’s rule. Rodents in warmer and
rainier regions, which also entails greater exposure to UV radiation and a
diverse range of predators, exhibit darker-colored pelage. In addition,
redder rodents prevail in warmer environments. However, contrary to the
rule predictions, we observe a reversal for reddish patterns in relation
to precipitation, with rainier regions showcasing more intense red
rodents. Main conclusions: Our study breaks new ground by investigating
previously unexplored facets of Gloger's rule in a continental
mammalian group. We discovered compelling evidence that darker and redder
coloration align closely with temperature and rainfall gradients. Although
we found support for eumelanin-pelage predictions, expectations for
pheomelanin pigmentation were only partially met. Our results might
suggest that selective pressures act differently on dark and reddish
coloration, revealing that coloration patterns in response to climate are
more intricate than previously formulated.