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<p>Reliable body size estimates are central to ecological and evolutionary inference, yet the increasing use of digitized specimens raises concerns about measurement consistency and comparability. Here, we evaluate precision, agreement, and systematic differences between direct (digital caliper) and image-based snout–vent length (SVL) measurements in two hylid frogs representing contrasting size ranges, <i>Dendropsophus elegans</i> (♂ 2.9&#xa0;cm SVL; ♀ 3.1&#xa0;cm SVL) and the larger <i>Boana faber</i> (♂ 8.8&#xa0;cm SVL; ♀ 8.9&#xa0;cm SVL)<i>.</i> Using repeated measurements, we evaluated differences between approaches. Image-based measurements did not produce equivalent results across species. In <i>D. elegans</i>, image-based measurements showed a mean difference of 6.3&#xa0;mm relative to calipers, corresponding to a relative error of 24.7%. In <i>B. faber</i>, the mean difference was close to zero (0.9&#xa0;mm), but individual measurements varied widely between methods, with a relative error of 6.8%. Caliper measurements were consistently more precise and repeatable, whereas image-based measurements showed greater variability. Bland–Altman analyses and Deming regression revealed constant and proportional differences, indicating that caliper and image-based SVL measurements should not be assumed to be interchangeable. These differences likely arise from transforming three-dimensional preserved specimens into two-dimensional images, where curvature, positioning, scale placement, and landmark identification can affect linear measurements. Digital images are valuable resources, but morphometric use requires standardization, validation, and caution when integrating datasets obtained with different methods. Our results demonstrate that measurement error is structured and context-dependent, and we provide practical recommendations for the appropriate use and integration of digital morphometric data in biodiversity research.</p>

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From specimen to pixel: digital standardization and bias in frog body size estimates

  • 期刊论文

  • 出版模式：

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  • 发表日期：

    2026年7月10日

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  • Ingrid R. M. Miguel,
  • Manoela Woitovicz-Cardoso,
  • Fábio Hepp

摘要

Reliable body size estimates are central to ecological and evolutionary inference, yet the increasing use of digitized specimens raises concerns about measurement consistency and comparability. Here, we evaluate precision, agreement, and systematic differences between direct (digital caliper) and image-based snout–vent length (SVL) measurements in two hylid frogs representing contrasting size ranges, Dendropsophus elegans (♂ 2.9 cm SVL; ♀ 3.1 cm SVL) and the larger Boana faber (♂ 8.8 cm SVL; ♀ 8.9 cm SVL). Using repeated measurements, we evaluated differences between approaches. Image-based measurements did not produce equivalent results across species. In D. elegans, image-based measurements showed a mean difference of 6.3 mm relative to calipers, corresponding to a relative error of 24.7%. In B. faber, the mean difference was close to zero (0.9 mm), but individual measurements varied widely between methods, with a relative error of 6.8%. Caliper measurements were consistently more precise and repeatable, whereas image-based measurements showed greater variability. Bland–Altman analyses and Deming regression revealed constant and proportional differences, indicating that caliper and image-based SVL measurements should not be assumed to be interchangeable. These differences likely arise from transforming three-dimensional preserved specimens into two-dimensional images, where curvature, positioning, scale placement, and landmark identification can affect linear measurements. Digital images are valuable resources, but morphometric use requires standardization, validation, and caution when integrating datasets obtained with different methods. Our results demonstrate that measurement error is structured and context-dependent, and we provide practical recommendations for the appropriate use and integration of digital morphometric data in biodiversity research.