Meeting Abstract
Environmental temperatures strongly affect the fitness of ectotherms, but these effects depend on body size. The “temperature-size rule” describes the pattern that ectotherms living in hotter environments are typically smaller, suggesting that smaller ectotherms might have selective advantages in hotter temperatures. We measured flight performance and body temperature across a range of air temperatures in field and lab settings using ten species of stingless bees varying in body mass from 2-120mg. We also measured air, leaf, flower, and bee temperatures in the forest canopy, where stingless bees forage, using a canopy crane. Smaller species flew with body temperatures much closer to air temperature than larger species, which fly at temperatures up to 10°C in excess of air temperature. This pattern is explained by the scaling of heat gain and loss; thermal conductance scaled with mass0.5 while flight metabolic rate scaled with mass1.1. The shape of thermal performance curves from 25-42°C varied strongly by species, with flight metabolic rates increasing strongly in some species, decreasing strongly in others, and being independent of temperature in others. This result suggests species may respond differently to seasonal and climatic thermal variation. The critical thermal maximum temperature at which flight ceased was lower in smaller species, possibly because larger endothermic species routinely experience higher body temperatures. Finally, we compare body sizes of these species over four years, which have increased greatly in average daily air temperature. Body size did not decrease in most species as suggested by the temperature-size rule. Instead, we find greater variation in body size, both smaller and larger among forager stingless bees as average air temperature increased annually.
