DOI: 10.5061/dryad.tqjq2bvv8
title: Evolution of Pelage Luminance in squirrels (Sciuridae)

description: 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.
description: Luminance Data from Museum Study
 Skins To obtain color
 measurements of the 135 squirrels used in our phylogenetic analysis, we
 photographed the dorsal profile of museum study skins from the Division of
 Mammals at the Smithsonian National Museum of Natural History. We used a
 Sony Alpha 5000 to take digital photographs of specimens against a gray
 matte background. Two light-boxes containing ESDDI 85W 5500K Day Light
 Florescent bulbs were positioned at opposing ends of the specimen stage in
 an interior room without windows. All images had their backgrounds masked
 by standard image editing software. We then loaded masked images into a
 custom python script that converted the image data to CIE LAB colorspace.
 Average luminance was then recorded for the entire dorsal region of the
 study skin for each species. Higher luminance values indicate brighter
 pelage color. Environmental
 Data We obtained
 environmental data for nine predictor variables for all 135 squirrel
 species from multiple databases. First, we acquired precipitation and
 temperature values for each species from the Pantheria database (Kate et
 al, 2009). We obtained canopy-closure data as a proxy measure for
 lightness of the environment from Hansen et al (2013). Due to memory
 constraints, the resolution of this dataset was reduced by a factor of 5
 using the “aggregate” function in the <em>raster</em> package in R (R Core Team 2013; Hijmans 2014). We also downloaded soil-carbon content and soil density raster data at a 5-km resolution from the ISRIC Soil Grids web app (Batjes et al. 2019). The remaining environmental data (water vapor, net primary productivity, fire frequency, and insolation) were obtained from the NASA MODIS satellite hosted on NASA Earth Observations (https://neo.sci.gsfc.nasa.gov). These data were available for each month for approximately the past 20 years. We downloaded these data at a 11 km by 11 km resolution and averaged them into a single raster file. With all of the downloaded environmental data (except the Pantheria data), we only used data for each species that fell within their species range. To determine this, we downloaded shapefiles of extent range maps of all 135 species from the IUCN (http://www.iucnredlist.org) and then clipped the environmental data based on these map using the <em>raster</em> and <em>rgdal </em>packages in R (Bivand et al. 2013; R Core Team 2013; Hijmans 2014). <strong>Accession Numbers</strong> GenBank accession numbers corresponding to sequences used in the study.