Developmental instability provides a powerful monitoring tool to detect threats before population declines. Consequently there have been an increasing number of studies assessing the level of developmental instability by measuring fluctuating asymmetry (FA) of bilaterally symmetrical traits in association with environmental stress. However, many studies failed in detecting a clear connection of FA to environmental stressors. Some of these may have suffered from large measurement error or the use of inappropriate methods. Notably, few studies have compared the accuracy of different approaches to estimate FA. Here we compare methods’ accuracy and FA outcome between a manual and one computerized approach. Amphibians are assumed to be ideal models for measuring fluctuating asymmetry due to their sensitivity to environmental stress. For our study we choose two bilateral metric traits (femur and radio-ulna length) of the European Common Frog, Rana temporaria. We compared level of measurement error (ME) and FA from manual caliper measurements with those from non-destructive micro-3D-computed tomography (µCT) based skeletal measurements.
Results/Conclusions
Caliper measurements revealed meaningfully FA estimates (i.e. FA exceeded ME) for radio-ulna length only. In contrast, µCT based measurements delivered meaningfully FA estimates for both traits. Repeatability of FA results was highest for radio-ulnae measured by caliper, but ME was about twice as high for caliper measurements compared to µCT based measurements, resulting in inflated levels of FA for caliper measurements. Using calipers, we observed higher ME for femur measurements than for radio-ulna, meaning that ME strongly depended on which trait was concerned; whereas using µCT, we observed comparable ME between both traits. Our study revealed that analyses of developmental instability as an indicator of environmental stress and population health using manual measurements should be interpreted with caution. For smaller vertebrates we recommend skeletal measurements with µCT as valuable alternative owing to its greater reliability, thereby allowing multi-trait analyses with equal accuracy.