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 characterized 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.