Date of Award


Document Type


Degree Name

Master of Science (M.S.)


Biological Sciences

First Advisor

Zach Stahlschmidt

First Committee Member

Ryan Hill

Second Committee Member

Eric Thomas


Cities are rapidly expanding, and global warming is intensified in urban environments due to the urban heat island effect. Therefore, urban animals may be particularly susceptible to warming associated with ongoing climate change. Thus, I used a comparative and manipulative approach to test three related hypotheses about the determinants of heat tolerance or critical thermal maximum (CTmax) in urban ants—specifically, that (1) body size, (2) hydration status, and (3) preferred micro-environments influence CTmax. I further tested a fourth hypothesis that native species are particularly physiologically vulnerable in urban environments. I manipulated water access and determined CTmax for 11 species common to cities in California's Central Valley that exhibit nearly 300-fold variation in body mass. Inter- (but not intra-) specific variation in body size influenced CTmax where larger species had higher CTmax. The sensitivity of ants’ CTmax to water availability exhibited species-specific thresholds where short-term water limitation (8 h) reduced CTmax in some species while longer-term water limitation (32 h) was required to reduce CTmax in other species. However, CTmax was not influenced by the preferred foraging temperatures of ants. Further, I did not find support for my fourth hypothesis because native species did not exhibit reduced thermal safety margins, or exhibit CTmax values that were more sensitive to water limitation relative to non-native species. In sum, understanding the links between heat tolerance and water availability will become critically important in an increasingly warm, dry, and urbanized world that may be selecting for smaller (not larger) body size.





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