Body Temperature Differences Between Green And Brown Grasshoppers Do Not Result From Thermal Physiology or Thermal Preferences

Colour polymorphisms correspond to the co-occurrence of several distinct colour morphs that vary in hue and/or brightness, independently of sex, age or any other state-dependent modifiers. Colour morphs could represent different life-history strategies, maximising their fitness locally in the trait space through trade-offs between all their biological functions. This mechanism could play a role in the maintenance of the green-brown polymorphism in Orthoptera. Grasshoppers are characterised by a widespread green-brown polymorphism and continuous variability in brightness within colour morphs. It has previously been found that brown individuals are warmer in the field than green conspecifics, but it is unclear if these differences are related to thermal physiology and/or thermal preferences. Therefore, we experimentally tested the thermal physiology and thermal preferences of three green-brown polymorphic species of acridid grasshoppers. We found no differences between green and brown grasshoppers, either in heat-up and equilibrium temperature patterns or in temperature preferences. Nor did we find support that the brightness variation is involved in the thermal physiology of these species. Instead, we show that body mass shapes the thermal physiology, with heavier individuals heating more slowly, and that males heated up faster and reached higher equilibrium temperatures than females. As females are heavier than males, the sex differences might be largely explained by size differences. Our results suggest that neither the thermal physiology nor the thermal preferences explain temperature differences in the field. However, green and brown individuals might still select different microhabitats in the field, which may indirectly lead to differences in body temperature. The persistence of the green-brown polymorphism may result from other mechanisms such as niche partitioning via microhabitat choice, mating preferences or frequency-dependent apostatic selection.