Improving Natural History Research with Image Analysis: The Relationship Between Skin Color, Sex, Size and Stage in Adult Red-Spotted Newts

—Natural history researchers are increasingly using digital cameras and computer software to measure their study animals. Adult Red-spotted Newts (Notophthalmus viridescens viridescens) are known to display a range of skin colors, from brown to green, but it has not been possible to quantify this variation until the advent of image analysis technology. We used an image analysis approach to compare skin color between sexes, across a range of sizes, and between aquatic and post-breeding (emigrating to the terrestrial habitat) stage adult newts. From 232 individuals (53% male, 47% female) we documented a wide but subtle range of skin colors, measured as the average hue value of all pixels in digital images of each newt body. We found that terrestrial post-breeding individuals were browner than aquatic individuals, consistent with the morphological adaptations of emigrating individuals for the terrestrial habitat. However, we also discovered that males were statistically greener than females but this effect depended on their stage. We suggest this difference may represent different degrees of adaptation for the terrestrial habitat between the sexes. Finally, we used image analysis to obtain measures of body length comparable to traditional snout-vent length and a measure of total body surface area, which provided a better correlation with newt mass than did body length and therefore could potentially serve as an alternative to mass or body condition. We suggest that image analysis methodology offers great promise for future questions relating to size and color in amphibians and provides researchers with an improved way to study aspects of the natural history of amphibians.