Niemeier, S., J. Müller & M.-O. Rödel

Fluctuating asymmetry – appearances are deceptive. Comparison of methods for assessing developmental instability in European Common Frogs (Rana temporaria). pp. 14-26 plus Supplementary material.

Abstract. Developmental instability provides a powerful monitoring tool to detect threats prior to population declines. Consequently, assessing the level of developmental instability by measuring fluctuating asymmetry (FA) of bilaterally symmetrical traits in association with environmental stress has become increasingly attractive. However, many studies failed in detecting a clear connection of FA to environmental stressors. Some of these may have suffered from large measurement error (ME) or the use of inappropriate methods. Here, we compared measurement accuracy and FA outcome from manual calliper measurements with those from non-destructive micro-3D-computed tomography (μCT) based skeletal measurements. Amphibians are assumed to be ideal models for measuring fluctuating asymmetry due to their sensitivity to environmental stress. For our study, we chose two bilateral, metric traits (femur and radio–ulna length) of the European Common Frog, Rana temporaria. Calliper measurements revealed meaningful FA estimates (i.e., FA exceeded ME) for radio–ulna length only. In contrast, μCT-based measurements delivered meaningful FA estimates for both traits. ME was about twice as high for calliper measurements compared to μCT-based measurements, resulting in inflated levels of FA. Using callipers, we observed higher ME for femur measurements than for radio–ulna, meaning that ME strongly depended on the respective trait. When using μCT, however, we observed comparable ME between both traits. Our study revealed that analyses of developmental instability using manual measurements should be treated with caution. For smaller vertebrates we recommend skeletal measurements with μCT as a valuable alternative due to its greater reliability, thereby allowing for multi-trait analyses with equal accuracy.

Key words. Agreement, amphibians, Bland-Altman, developmental stability, environmental stress, measurement error, Micro CT, morphology, Rana temporaria, traits.