Male-biased sexual dimorphism in head, limbs, and tail scaled to snout-vent length has been reported in many lizard species. Consequently, various hypotheses have been proposed to explain observed body-shape dimorphism. According to the majority of them, the proportions of body components are adaptively related to sexual differences in ecology as well as in reproductive behaviour. Our study shows an alternative, much more parsimonious explanation in the common lizard (Lacerta vivipara). According to our analyses, the exaggeration of a single trait, specifically trunk length in females, may explain the whole pattern of sexual differences in body shape. The only consistent adaptive hypothesis, then, is that females with a larger abdominal cavity, and consequently a longer trunk, have a reproductive advantage. Size-adjusted heads, limbs, and tails traditionally reported to be larger in males than in females, features ascribed to numerous evolutionary mechanisms, thus appear to be just an artifact of inappropriate scaling to a sexually dimorphic trait (snout-vent length). As scaling to a single trait has been routinely used in many studies carried out in animals, we warn against quick interpretations based on such analyses and recommend more cautious inspection of allometries.Résumé : On signale un dimorphisme sexuel des tailles de la tête, des membres et de la queue corrigées par rapport à la longueur museau-évent chez plusieurs espèces de lézards, dimorphisme qui favorise les mâles. Diverses hypothèses ont été avancées pour expliquer ce dimorphisme de la forme du corps. La plupart d'entre elles concluent que les proportions des segments du corps sont adaptées aux différences sexuelles d'écologie et de comportement reproducteur. Notre étude met de l'avant une explication de rechange, beaucoup plus parcimonieuse, chez le lézard vivipare (Lacerta vivipara). Selon nos analyses, l'exagération d'un seul caractère, spécifiquement la longueur du tronc chez les femelles, peut expliquer l'ensemble des différences sexuelles de la forme du corps. La seule hypothèse adaptative conséquente est alors que les femelles qui ont une plus grande cavité abdominale, et donc un tronc plus allongé, possèdent un avantage reproductif. Les tailles corrigées de la tête, des membres et de la queue, qui sont alors décrites comme plus grandes chez les mâles que chez les femelles et rattachées à divers mécanismes évolutifs, semblent alors n'être que des artéfacts causés par une correction impropre par rapport à la longueur museau-évent, un caractère à dimorphisme sexuel. Comme la correction par rapport à un seul caractère est couramment utilisée dans plusieurs études sur les animaux, nous mettons en garde contre l'interprétation hâtive des résultats et nous recommandons un examen plus attentif des allométries. [Traduit par la Rédaction] 1117Kratochvíl et al.

Abstract. The snout-vent length (SVL), a conventional measure of overall body size in lizards and snakes, is used in a wide variety of ecological, evolutionary, and taxonomical studies. Trends in SVL variation are often analysed using data from several researchers (observers), but possible confounding effects due to inter-observer differences in measurement protocols have never been appropriately examined. This study reports inter-observer biases between eleven herpetologists who measured the same specimens of the Eurasian common lizards (21 adult specimens were examined by eight observers and additional 192 specimens by two observers). Intra-observer bias over time (1.5-15 months between measuring sessions) was also estimated. In the vast majority of comparisons, mean difference between the first author and another observer varied from −1.0 to +0.8 mm, or from −1.9 to +1.6% if expressed as a percent of the specimen's SVL value. Some non-regular effects of sex and study sample on the studied bias were revealed, and their possible reasons are discussed. We are advising the researchers who intensively collect SVL and other morphometric data to consider testing intra-observer and inter-observer biases and to establish etalon samples available for re-examinations.