Adaptive Variation in Sex and Male Size Morph Critical Thermal Maxima in Centris Pallida Desert Bees

Desert organisms must adapt to high temperatures to survive and reproduce. Individuals using different microclimates for fitness-relevant behaviors may be faced with different thermal selective pressures, even within a population of a single species. One mechanism of adaptation to extreme temperatures is increased physiological tolerance, which can be experimentally determined by testing an organism’s critical thermal maxima (CT max ). Understanding the adaptive role of CT max for individuals in extreme environments is complicated by the fact that variation in body size may impact CT max . In size-polymorphic species, larger individuals generally having higher CT max (likely due to thermal inertia). However, body size does not always correlate with experienced thermal conditions, which may be more important for driving intraspecific CT max differences. Centris pallida bees provide a unique model system to examine whether variation in experienced thermal conditions (operative temperature) vs. body size are matched by differences in the maximal thermal tolerance of individuals within the same population. C. pallida male bees found in the Sonoran Desert employ alternative reproductive tactics, with the small-morph and large-morph males using distinct microclimates for their mating strategies. Small-morph males use a cooler microclimate at least a meter above the ground, while large-morph males frequently engage in mating behaviors in the more thermally extreme microclimate on or near the desert soil. In this study, we tested whether body size or operative temperature was a more important driver of CT max in C. pallida bees. We measured the operative temperature of dead bees of each male morph in the microclimates relevant for their mating strategies, as well as air and ground temperatures. We tested the CT max of male, and female, C. pallida across a range of body sizes. We found that small-morph C. pallida had higher operative temperatures than large-morph C. pallida during the hottest parts of the day. Consistent with the effect of operative temperature on CT max we found that morph, but not body size, predicted male CT max differences, with a higher mean CT max for small-morph compared to large-morph males. For females, which all use the same range of microclimates, body size was positively correlated with CT max . Altogether, our results supported the hypothesis that experienced thermal conditions are a more important driver of CT max than body size.