Improving natural history research with image analysis: the relationship between skin color, sex, size, and stage in adult Red-spotted newts (Notophthalmus viridescens viridescens)

Natural history researchers are increasingly using digital cameras and computer software to measure their studyanimals. 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 andpost-breeding (emigrating to the terrestrial habitat) stage adult newts. From 232 individuals (53% male, 47% female) wedocumented a wide but subtle range of skin colors, measured as the average hue value of all pixels in digital images of eachnewt body. We found that terrestrial post-breeding individuals were browner than aquatic individuals, consistent with themorphological adaptations of emigrating individuals for the terrestrial habitat. However, we also discovered that maleswere statistically greener than females but this effect depended on their stage. We suggest this difference may representdifferent degrees of adaptation for the terrestrial habitat between the sexes. Finally, we used image analysis to obtainmeasures of body length comparable to traditional snout-vent length and a measure of total body surface area, whichprovided a better correlation with newt mass than did body length and therefore could potentially serve as an alternative tomass or body condition. We suggest that image analysis methodology offers great promise for future questions relating tosize and color in amphibians and provides researchers with an improved way to study aspects of the natural history ofamphibians.