Pigmentation varies greatly across and within species — and
color may be more than skin deep: In 2008, Anne-Lyse Ducrest,
Laurent Keller and Alexandre Roulin, three ecologists at
Switzerland’s University of Lucerne published a review article
in Trends in Ecology and Evolution [Pleiotropy in the
melanocortin system, coloration and behavioral syndromes
September 2008] on the relations between pigmentation,
sexuality, and aggression in 45 vertebrate species. They found
that darker-colored individuals had higher levels of aggression
and sexuality than lighter colored individuals across three
species of mammals (African lion, soay sheep,  and white-tailed
deer), four species of fish (mosquito fish, guppy, green
swordtail, and Arctic char), four species of reptiles (asp
viper, adder, fence lizard, and spiny lizard), one amphibian
species (spadefoot toad), and 36 species of birds. Ducrest and
her co-authors’ explanation: increased levels of melanocortin
hormones, which determine coloring, are linked with increased