Evolution of Pelage Luminance in squirrels (Sciuridae)

Sciurids have one of the greatest diversities of color patterns and hues
among mammalian families, but whether these color patterns conform to
Gloger’s Rule has not been investigated. Gloger’s rule is the
ecogeographic trend that describes the tendency of animals’ pelage to be
darker in warmer and wetter environments. Several mechanisms have been
proposed explain this trend that relate to various environmental
variables. However, these variables frequently correlate with each other
which can cause increased Type I error rates in conventional analyses with
a single dependent variable. Here we use a new phylogenetic comparative
method that implements path analysis to produce a hypothesized series of
cause and effect relationships between predictor variables to form a path
model. This multi-regression approach allows for the simultaneous
exploration of the effects of numerous predictor variables. Our path
analysis results using 137 Sciurid species show that squirrel pelage
luminance conforms to Gloger’s rule and that there is a significant
evolutionary relationship between precipitation and pelage luminance. Also
significant were that other commonly found variables (humidity,
temperature, and vegetation cover) did not have strong relationships with
pelage luminance. Our findings demonstrate that utility of path analysis
in phylogenetic comparative studies in studying a trait influenced by
several covarying variables.