Variation in thermal sensitivity of performance among colour morphs of a pygmy grasshopper

Populations of pygmy grasshoppers, Tetrix subulata, display genetically cod ed discrete variation in colour pattern and there are differences among mor phs in the capacity to achieve body heating. To determine whether colour mo rphs differ in thermal physiology, I assessed reaction distance and jumping performance of individuals belonging to different morphs at two different temperatures. Individuals allowed a potential predator to approach less clo sely and jumped longer distances at high than at low temperature. My analys es also uncovered variation among morphs in average reaction distance and j umping capacity, as well as in thermal sensitivity of these two traits. Mat rix correlation analysis further revealed that pair-wise differences betwee n morphs in thermal sensitivity of jumping performance (but not reaction di stance) could be accurately predicted by differences in body temperatures p referred in a laboratory thermal gradient. These results support the view t hat morphology, behaviour and thermal physiology of ectotherms may evolve i n concert. The relationship between reaction distance and jumping performan ce varied among colour morphs at high temperature, and the common within-mo rph relationship between these two traits deviated from the corresponding a mong-morph relationship. This suggests chat the variation among morphs has partially arisen through active divergence, with selection having influence d both traits and modifications having occurred to different degrees in dif ferent morphs. My data further suggest that pale colour morphs, with a limi ted capacity to attain high body temperatures, may not necessarily be at a selective disadvantage, because their physiology may be adapted to lower bo dy temperatures.