Locust populations that have experienced low density conditions for several generations comprise individuals in the solitarious phase, which is characterized by cryptic coloration and a cryptic behavioral strategy that includes sparse locomotor activity with a crepuscular diel pattern and an aversion to conspecifics. This capacity for “phase change” underpins the formation and break-up of swarms ( Simpson and Sword, 2008 Cullen et al., 2017). Locusts can transform between two very distinct integrated phenotypes or phases in response to changes in population density ( Pener and Simpson, 2009). Phase change in locusts is a paradigmatic example of phenotypic plasticity and integration. The points addressed here are pertinent to any research on transitions between complex phenotypes and behavioral syndromes.Ī 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).
Finally, I reject the idea that “standardized models” provide a valid shortcut to estimating phase state across different developmental stages, strains or species. 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. I furthermore show why behavioral variables should not be adjusted by measures of body size that themselves differ between the two phases. 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. 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. Mart́ın-Blázquez and Bakkali (2017 doi: 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. 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. The two phases differ in behavior, physiology and morphology. 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 effects of two alternative environments on multiple phenotypic traits can be analyzed by multivariable binary logistic regression (LR). Phenotypic plasticity often entails coordinated changes in multiple traits. Department of Neuroscience, Psychology and Behaviour, University of Leicester, Leicester, United Kingdom.
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