Comparative quantification of reaction norms across closely related species in a clade is rare, but such a study can reveal valuable insights into understanding how reaction norms evolve along phylogeny. The grasshopper genus Schistocerca Stål (Orthoptera: Acrididae: Cyrtacanthacridinae) is an ideal group to study the evolution of density-dependent phenotypic plasticity because it includes both swarming locusts and non-swarming sedentary grasshoppers, which show varying degrees of plastic reaction norms in many traits. The swarming locusts exhibit locust phase polyphenism in which cryptically colored and solitary individuals can transform into conspicuously colored and highly gregarious individuals in response to increases in population density. The sedentary grasshoppers do not swarm in nature, and thus it has been assumed that they have little or no expression of plastic reaction norms in many traits, except for color, which has been shown to be a phylogenetically conserved trait. In this study, we have quantified density-dependent reaction norms in behavior, color, body size, and morphometric ratio in the nymphs of four sedentary species within Schistocerca by conducting explicit rearing experiments to induce potential phenotypic changes in response to isolation and crowding. In contrast to our previous assumption, we find that all four species show a certain level of density-dependent plastic reaction norms, which implies that these sedentary species have hidden reaction norms that can only be induced experimentally, some components of which must be phylogenetically conserved. Furthermore, we demonstrate that rearing density differentially affects the expression of reaction norms in different species, suggesting that different reaction norms must have followed independent evolutionary trajectories.
Keywords: Grasshoppers; Locust phase polyphenism; Phenotypic plasticity.
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