Exposure to a fungal pathogen increases the critical thermal minimum of two frog species.

The ability of an organism to tolerate seasonal temperature changes, such as extremely cold temperatures during the winter, can be influenced by their pathogens. We tested how exposure to a virulent fungal pathogen, , affected the critical thermal minimum (CT) of two frog species, (gray treefrog) and (pickerel frog). The CT is the minimum thermal performance point of an organism, which we estimated via righting response trials. For both frog species, we compared the righting response of -exposed and -unexposed individuals in either a constant (15ºC) environment or with decreasing temperatures (-1°C/2.5 min) starting from 15°C. The CT for both species was higher for -exposed frogs than unexposed frogs, and the CT of . was higher than . . We also found that -exposed frogs of both species righted themselves significantly fewer times in both decreasing and constant temperature trials. Our findings show that pathogen exposure can reduce cold tolerance and limit the thermal performance range of hosts, which may lead to increased overwintering mortality.