Symmetry, size and stress

Abstract

In their recent TREE perspective, Bjorksten et al.1xWhat does sexual trait FA tell us about stress?. Bjorksten, T.A. et al. Trends Ecol. Evol. 2000; 15: 163–166Abstract | Full Text | Full Text PDF | PubMed | Scopus (129)See all References1 suggested that fluctuating asymmetry (FA) is a poor measure of susceptibility to stress. Unfortunately, their article is based on selective citation of the literature, which biases their conclusions in a particular direction. Recently, a meta-analysis (N. Cadee, PhD thesis, Universite Pierre et Marie Curie, France, 2000) of all available studies (20 in total compared with the few cited by Bjorksten et al.) has been made, and responses of FA and size to environmental stress have been estimated. The mean effect size (Pearson correlation coefficient adjusted for sample size2xMeta-analytic Procedures for Social Research. Rosenthal, R. See all References2) for FA in the 20 studies was 0.16, whereas it was 0.22 for size for exactly the same studies. Interestingly, this conclusion was not significantly biased by the quality of the studies included, whether the studies were experimental or observational, or by the year of publication. However, one factor seriously biased the effect size for body size: in studies with mortality effect size was significantly larger (mean r = 0.33) than in studies without mortality (mean r = 0.21). There was no such bias for effect size of character asymmetry. Thus, mortality can seriously bias estimates of effect size of body size. Interestingly, the effect size for asymmetry was unrelated to the effect size for FA (r = −0.0001 for the 88 samples). This indicates that size responses to stress estimate different phenomena than FA. Thus, size and asymmetry are not alternatives – they are complementary.Bjorksten et al.1xWhat does sexual trait FA tell us about stress?. Bjorksten, T.A. et al. Trends Ecol. Evol. 2000; 15: 163–166Abstract | Full Text | Full Text PDF | PubMed | Scopus (129)See all References1 also chose not to cite a paper investigating the relative importance of size and FA as a predictor of success in sexual selection3xThe relative importance of size and asymmetry in sexual selection. Thornhill, R. and Moller, A.P. Behav. Ecol. 1998; 9: 546–551CrossrefSee all References3. This study investigated the effect size for size and asymmetry in exactly the same studies, and these were approximately of a similar magnitude independent of whether samples, studies or species were used as units of analysis. Again, this suggests that size is not a better predictor of success than FA.Finally, FA has been known for some time to be a poor estimate of developmental instability because it is a variance estimate based on a single observation: the length of a character on the left and the right side of the body4xThe heritability of fluctuating asymmetry and the genetic control of developmental stability. Whitlock, M. Proc. R. Soc. London Ser. B. 1996; 263: 849–854Crossref | PubMed | Scopus (142)See all References, 5xHow repeatable is the estimation of individual fluctuating asymmetry. Van Dongen, S. Proc. R. Soc. London Ser. B. 1998; 263: 1423–1427CrossrefSee all References, 6xPhenotypic plasticity of fluctuating asymmetry and fitness. Shykoff, J.A. and Moller, A.P. Oikos. 1999; 86: 152–158CrossrefSee all References. A better estimate can be obtained by calculating the hypothetical repeatability of developmental instability5xHow repeatable is the estimation of individual fluctuating asymmetry. Van Dongen, S. Proc. R. Soc. London Ser. B. 1998; 263: 1423–1427CrossrefSee all References, 6xPhenotypic plasticity of fluctuating asymmetry and fitness. Shykoff, J.A. and Moller, A.P. Oikos. 1999; 86: 152–158CrossrefSee all References. This measure estimates the proportion of variation in individual FA owing to between-individual variation in developmental instability. Because the upper limit to the hypothetical repeatability is 0.64 (5xHow repeatable is the estimation of individual fluctuating asymmetry. Van Dongen, S. Proc. R. Soc. London Ser. B. 1998; 263: 1423–1427CrossrefSee all References, 6xPhenotypic plasticity of fluctuating asymmetry and fitness. Shykoff, J.A. and Moller, A.P. Oikos. 1999; 86: 152–158CrossrefSee all References), effect sizes for FA have to be divided by a value that is maximally 0.64 to obtain an effect size estimate for developmental instability. Thus, the mean effect sizes for FA listed above reflect considerably larger effect sizes for developmental instability, because they should at least be multiplied by a factor of 1.57 (the reciprocal of 0.64). In conclusion, there is little evidence to suggest that size is a better measure of ability to cope with stress than developmental instability. The contrary appears to be the case.