Abstract
Phenotypic plasticity often entails coordinated changes in multiple traits. The effects of two alternative environments on multiple phenotypic traits can be analyzed by multivariable binary logistic regression (LR). Locusts are grasshopper species (family Acrididae) with a capacity to transform between two distinct integrated phenotypes or “phases” in response to changes in population density: a solitarious phase, which occurs when densities are low, and a gregarious phase, which arises as a consequence of crowding and can form very large and economically damaging swarms. The two phases differ in behavior, physiology and morphology. A large body of work on the mechanistic basis of behavioral phase transitions has relied on LR models to estimate the probability of behavioral gregariousness from multiple behavioral variables. Mart́ın-Blázquez and Bakkali (2017; [10.1111/eea.12564]10.1111/eea.12564) have recently proposed standardized LR models for estimating an overall “gregariousness level” from a combination of behavioral and, unusually, morphometric variables. Here I develop a detailed argument to demonstrate that the premise of such an overall “gregariousness level” is fundamentally flawed, since locust phase transformations entail a decoupling of behavior and morphology. LR models that combine phenotypic traits with markedly different response times to environmental change are of very limited value for analyses of phase change in locusts, and of environmentally induced phenotypic transitions in general. I furthermore show why behavioral variables should not be adjusted by measures of body size that themselves differ between the two phases. I discuss the models fitted by Mart́ın-Blázquez and Bakkali (2017) to highlight potential pitfalls in statistical methodology that must be avoided when analysing associations between complex phenotypes and alternative environments. Finally, I reject the idea that “standardized models” provide a valid shortcut to estimating phase state across different developmental stages, strains or species. The points addressed here are pertinent to any research on transitions between complex phenotypes and behavioral syndromes.
Highlights
A central question in animal behavior is the extent to which individual differences in multiple behavioral traits are integrated together into behavioral syndromes (Sih et al, 2004; Dingemanse et al, 2010; Wolf and Weissing, 2012) or with other phenotypic dimensions such as morphology and physiology to form complex integrated phenotypes (Pigliucci, 2003; Murren, 2012; Armbruster et al, 2014; Kern et al, 2016)
All analyses are documented in the Supplementary Material of the present paper, which includes a report in PDF format and the .Rmd source code that generates the report reproducibly from the raw data of Martín-Blázquez and Bakkali (2017)
Martín-Blázquez and Bakkali (2017) concluded that their S. gregaria models did not perform well in L. migratoria, they recommended their “Sg_nonmorphometric” model, which was fitted to final instars, for use in adults: “For testing adults or the same nymphs at different time points, we suggest using the “Sg_non-morphometric” model.” (p. 23)
Summary
A central question in animal behavior is the extent to which individual differences in multiple behavioral traits are integrated together into behavioral syndromes (Sih et al, 2004; Dingemanse et al, 2010; Wolf and Weissing, 2012) or with other phenotypic dimensions such as morphology and physiology to form complex integrated phenotypes (Pigliucci, 2003; Murren, 2012; Armbruster et al, 2014; Kern et al, 2016). Locusts can transform between two very distinct integrated phenotypes or phases in response to changes in population density (Pener and Simpson, 2009). A trivially obvious prerequisite for any analysis of a specific phase trait is a meaningful measure of that trait To this end, Martín-Blázquez and Bakkali (2017) have recently proposed standardized logistic regression (LR) models for estimating “the gregariousness level” of individuals of the desert locust (Schistocerca gregaria Forskål) for adoption by the research community, together with suggestions for extending their approach to other locust species including the migratory locust (Locusta migratoria L.). The present paper is not intended as a comprehensive and detailed critique of the paper by Martín-Blázquez and Bakkali (2017); instead, I discuss specific conceptual and methodological shortcomings of that work that are germane to any research on transitions between complex phenotypes
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