its`qualification' to be treated as a trait) but also to set the upper limit for heritability (the ratio between genetic variance and phenotypic variance)( Lessels and Boag, 1987 Falconer and Mackay, 1997 Roff, 1997 Lynch and Walsh, 1998 Dohm, 2002). Since this measurement includes genetic variance, repeatability appears not only to give insight into the time-consistency of a trait (i.e. Then, τ can also be decomposed asτ=(σ 2 G+σ 2 E)/σ 2 P( Lessels and Boag, 1987 Falconer and Mackay, 1997). In turn, phenotypic variance equalsσ 2 G+σ 2 E + residual variance (σ 2 e). Between-individual variance equals genetic variance (σ 2 G) + general environmental variance(σ 2 E). A crucial concept in this definition is repeatability: the time-consistency of a trait, which is measured by the intraclass correlation coefficientτ=σ 2 A/(σ 2 A+σ 2 e)where σ 2 A is the between-individual component of variance and σ 2 e is the residual variance component when multiple measurements are performed in the same individuals(i.e. The operational definition of a trait involves an instrument and a measurement scale. In evolutionary terms, traits are organism's attributes that in some way reflect biological performance, and which ultimately have an impact on fitness( Arnold, 1983). There was no evidence of strong publication bias. additional studies will not contribute significantly to the estimator). The cumulative meta-analysis revealed that repeatability studies in MR have already reached an asymptotical effect size with no further change either in its magnitude and/or variance (i.e. repeatability of MR does not change in different species, type of metabolism, time between measurements, and number of individuals). Our results show that metabolic rate is significantly repeatable and its effect size is not statistically affected by any of the mentioned factors(i.e. We performed a meta-analysis to address these questions, as well as to explore the historical trend in repeatability studies. However, despite the more than 30 studies reporting repeatability of MR, so far there is not a definite synthesis indicating: (1) whether repeatability changes in different types of animals (2) whether some kinds of metabolism are more repeatable than others and most important, (3) whether metabolic rate is significantly repeatable. An alternative estimation of repeatability, popular among physiological ecologists, is the Pearson product–moment correlation between two consecutive measurements. The original definition of repeatability, known as the intraclass correlation coefficient, is computed from the components of variance obtained in a one-way ANOVA on several individuals from which two or more measurements are performed. Repeatability studies are gaining considerable interest among physiological ecologists, particularly in traits affected by high environmental/residual variance, such as whole-animal metabolic rate (MR).

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