Impact of body size on critical thermal maxima in female solitary desert bees

Abstract

Abstract Solitary, size‐variable bees are adapted to a wide range of thermal environments (e.g., through critical thermal maxima, or CTmax) and are important, understudied subjects for research on species' vulnerability to climate change. Centris pallida are solitary, ground‐nesting desert bees with females varying two‐fold in body mass. We hypothesized that body size would affect CTmax in female C. pallida, predicting that an increase in body size would result in increased thermal tolerance. We tested the effects of female body mass on CTmax using a ramping CTmax assay while controlling for age. We used flow‐through respirometry to confirm that the behavioural indicator of CTmax correlated with the metabolic indicator of lethality. Body mass correlated positively with CTmax; every 100 mg increase in body mass resulted in a 1.5°C increase in CTmax. Female age (as assessed by an index score based on wing, thorax hair and mandibular wear) did not affect CTmax. Flow‐through respirometry confirmed that loss of muscle control correlated with the metabolic ‘mortal fall’, or a decline in CO2 production. As insect body sizes typically decline with higher temperatures, our data suggest rising temperatures could magnify thermal stresses on desert bee populations.