29th Annual Meeting and Symposium of the
Desert Tortoise Council, February 20-23, 2004
Abstracts

A Comparison of Shell and Limb Colors in Desert Tortoise Populations in California: Size, Sex, and Regional Differences

Kristin H. Berry¹ and W. P. Alley^2
1U.S. Geological Survey, Western Ecological Research Center, 22835 Calle San Juan de Los Lagos, Moreno Valley, CA 92553
²California State University, Los Angeles

Over 1500 desert tortoises (Gopherus agassizii) from 20 different study sites in the Mojave and Colorado deserts of California were evaluated for shell and limb colors between 1992 and 2002 using the Munsell ® Soil Color Chart. Hue, Value, Chroma, and color name were assigned for each of six locations on the tortoises. All measurements were made when tortoise shells and integument were dry. Color data on dry soils were collected at 18 of the 20 study sites from microhabitats commonly used by tortoises: wash bottoms, wash/washlet edges or slopes, cover sites, and intershrub spaces in both undisturbed and anthropogenically disturbed sites. For analyses, tortoises were grouped by size-age into five classes by carapace length at the midline (MCL, mm): juvenile (< 100 mm), immature (100-179 mm), sub or small adult (180-208 mm), adult 1 (208-239 mm), and adult 2 (>240 mm). The 20 study sites were grouped initially into seven regions for statistical analysis of potential differences. Based on the findings, four regions were identified: Western Mojave, Northeastern Mojave, Eastern Mojave, and Colorado Desert.

We determined that adult 1 tortoises are statistically significantly different in color (e.g., central vertebral scutes) for each of four regions: West Mojave, East Mojave, Northeast Mojave, and Colorado. For juvenile tortoises, only the West Mojave and East Mojave regional data sets were statistically significantly different, possibly because of the small sample sizes for the Northeast Mojave and Colorado regions. Trends are for the small tortoises to be light in color in the centers of scutes, and, with increasing size and age, the central parts of scutes become increasingly darker. The large adults predominantly fall in the dark gray and black colors. Preliminary statistical analysis on seam and leg colors showed similar patterns of statistically significant differences by region for immature, subadult, and adult size classes.

Soil colors are generally lighter than the tortoises, even lighter than the juvenile tortoises. Many are in the Yellow-Red Hues, in contrast to the Yellow Hues for most of the tortoises. One observation that we are testing in more detail is that juveniles are more likely to approximate soil colors than the larger tortoises and as the tortoises increase in size and age, the differences between soils and tortoise colors become more pronounced. These and other findings suggest that population differences in shell and limb colors should be a consideration in recovery strategies for the tortoise. Determining the cause(s) and contributors to the differences in color by size and region will require additional research.

Acknowledgements: Financial support was provided primarily by the Bureau of Land Management, the National Training Center at Fort Irwin, the California Department of Fish and Game, the Desert Tortoise Preserve Committee, and also by the U.S. Geological Survey. Tracy Bailey, Kemp Anderson, Tim Shields, and others collected field data from tortoises and soils.