Examining mammalian thermoregulation clines across North America using Bergmann's rule

Increased body size across latitudes confers great thermoregulatory advantages to species living in colder climates (i.e., higher latitude or elevation) compared to warmer areas (i.e., lower latitude or elevation). This relationship, known as Bergmann's rule, has been widely tested among endothermic organisms with some evidence of variations between species in a genus. Interestingly, this relationship has been far less examined across heterothermic species that use specific thermal and metabolic strategies, described as daily torpor and hibernation, to cope with environmental challenges. This study aims to evaluate the adherence to Bergmann's rule of mammalian hibernators and daily heterotherms, and to compare this to homeothermic endotherms. We hypothesize that the adherence to Bergmann's rule will be less evident in heterotherms compared to homeothermic endotherms due to more efficient thermoregulatory strategies. We found that adherence to Bergmann's rule was limited, especially in the hibernator and daily heterotherm groups in intraspecies relationships; however, we did find strong evidence supporting Bergmann's rule when looking at mean body length versus mean latitude in the hibernator thermoregulatory class and when looking at all species together regardless of thermoregulatory strategy. These findings shed light on how different thermoregulatory strategies help predict the effect of climate change on organisms' adaptive responses and provide further evidence supporting Bergmann's rule in hibernators.