Swaddle, John P.

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W&M ScholarWorks Arts & Sciences Articles Arts and Sciences 2011 Assessing the developmental stress hypothesis in the context of a reaction norm John P. Swaddle William & Mary, jpswad@wm.edu Follow this and additional works at: https://scholarworks.wm.edu/aspubs Recommended Citation Swaddle, J. P. (2011). Assessing the developmental stress hypothesis in the context of a reaction norm. Behavioral Ecology, 22(1), 13-14. This Article is brought to you for free and open access by the Arts and Sciences at W&M ScholarWorks. It has been accepted for inclusion in Arts & Sciences Articles by an authorized administrator of W&M ScholarWorks. For more information, please contact scholarworks@wm.edu. Assessing the developmental stress hypothesisin the context of a reaction normJohn P. SwaddleInstitute for Integrative Bird Behavior Studies, Biology Department,College of William and Mary, LandrumDrive, Williamsburg, VA, USASpencer and MacDougall-Shackleton (S&M hereafter) compre-hensively reviewed published evidence to suggest that early lifedevelopmental conditions can play important roles in later lifeselection events. My intention here is to provide a constructivecritique of how this ‘‘developmental stress hypothesis’’ is framedand studied, which may help guide further research.Commentaries on Invited Review 13Downloaded from https://academic.oup.com/beheco/article-abstract/22/1/13/231015/ by guest on 07 October 2019First, there is a lack of clarity as to the developmental ori-gins of the phenotypic variation under consideration. In themajority of their review, S&M refer to differences in song phe-notype among individuals (i.e., different birds in separate ex-perimental treatment groups) that develop in differentenvironmental conditions (e.g., ‘‘stressed’’ and ‘‘control’’groups). The resulting phenotypic variation, therefore, origi-nates from genotypic (G) and environmental (E) differences,and potentially any G 3 E interaction. S&M also consider fluc-tuating asymmetry (FA) of morphological traits to reveal equiv-alent developmental processes due to a breakdown indevelopmental stability (DS). However, the phenotypic varia-tion produced by DS originates within genotypes and withinenvironments; hence, FA has fundamentally different develop-mental origins than among-individual and among-treatmentcomparisons of bird song.It is useful to visualize these traits and sources of variation inthe context of a reactionnorm.Each functiondepicts theproduc-tion of a phenotype by a particular genotype across anenvironmental gradient (Figure 1). Studies that examine differ-ent birds in separate environmental treatment groups comparethe phenotypes of one set of genotypes in one environment withthe phenotypes of another set of genotypes in another environ-ment (shaded areas of Figure 1). The recorded differences resultfrom a complex of genetic and environmental developmentalfactors. DS is the production of a particular phenotype by a par-ticular genotype in a particular environmental condition(Zakharov 1981) and, therefore, developmental instability (esti-mated by FA) can be thought of as phenotypic noise arounda single reaction norm; indicated as error bars in Figure 1.With these developmental differences highlighted, it isrelevant to ask what type of variation should we measurewhen assessing the developmental stress hypothesis. As the hy-pothesis posits that early developmental conditions may havesignificant later life effects for an individual, it is the among-en-vironment and within-genotype variation that appears most im-portant; it seems we need to reduce among-genotype effects intests of the developmental stress hypothesis. Therefore, I pro-pose the need for repeated-measures methods.Fundamentally, we need to assess the production of a phe-notype by the same genotype in different environments. Asearly development happens just once, this means the develop-mental stress hypothesis becomes rather intractable in many sit-uations, especially in terms of studying bird song. However, inorganisms that regrow structures (such as birds’ feathers), itis feasible to assess the development of a phenotype in one en-vironmental condition and then in another. In such studies,it may be beneficial to have a strong hypothesis about whatthe developmentally intendedphenotype is, hence giving somedirectionality to the effects of stressors; therefore, I would rec-ommend assessing FA, as we know that larger asymmetries areaccidents of development (Møller and Swaddle 1998). Forthese collective reasons, studying the later life selection conse-quences of FA differences within individuals but acrossenvironments would be a fruitful way forward in assessing theapplicability of the developmental stress hypothesis.The reaction norm depiction of DS is also helpful in assess-ingwhetherDS isheritableand ‘‘visible’’ to selection.Fundamen-tally, the DS error barsmust vary inmagnitude among genotypesforDS torespondtoselectionpressures (Figure1). It isalsohelpfulto consider whether the size of the DS error bars for a signalingtrait are large enough that receivers could reliably detect this var-iation (Swaddle 2003). As signal detection is seldom error-proof,the likelihood that DS can be an effective cue/signal will increasein proportion to the ratio of the phenotypic variation due to DS(size of the error bars) to the phenotypic variation that cannot bediscerned due to detection error.I am not aware that all the elements (measuring within-in-dividual but among-environment phenotypic variation, show-ing that the variation is heritable and visible to selection and,of course, showing that there is later life selection) have beenput together for any system; hence, there is much to be done inassessing the developmental stress hypothesis.Address correspondence to J.P. Swaddle. E-mail: jpswad@wm.edu.Received 2 February 2010; revised 2 February 2010; accepted 3May 2010.doi: 10.1093/beheco/arq089REFERENCESMøller AP, Swaddle JP. 1998. Asymmetry, developmental stability, andevolution. Oxford: Oxford University Press.Swaddle JP. 2003. Fluctuating asymmetry, animal behavior, and evolu-tion. Adv Study Behav. 32:169–205.Zakharov VM. 1981. Fluctuating asymmetry as an index of develop-mental homeostasis. Genetika. 13:241–256.Figure 1Hypothetical bundle of reaction norms, where each line representsthe expression of a phenotype by a genotype across an environmentalgradient. Only 4 lines are shown for simplicity; with 2 genotypes(solid lines) in a hypothetical ‘‘control’’ group environment A(indicated by shaded area on left) and another 2 genotypes (dottedlines) in a hypothetical ‘‘stressed’’ group environment B (indicatedby shaded area on right). This example is intended to demonstratethe phenotypes produced by a typical experimental design reviewedby S&M and illustrate how the among-treatment differences inphenotype can be influenced by unhelpful among-genotypedifferences. The error bars depict developmental instability. In thisexample, developmental instability increases from environment A toB, as indicated by the generally increasing magnitude of error barsfrom A to B.14 Behavioral EcologyDownloaded from https://academic.oup.com/beheco/article-abstract/22/1/13/231015/ by guest on 07 October 2019

Assessing the developmental stress hypothesis in the context of a reaction norm

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Assessing the developmental stress hypothesis in the context of a reaction norm

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