Bergmann’s rule
Bergmann’s rule, which states that body size decreases among congeneric species with decreasing latitude, has primarily been studied in mammals and birds. Not all birds and mammals follow the rule, and even fewer smaller organisms, such as insects, conform to it (Blackburn et al. 1999a; Meiri & Dayan 2003). However, some evidence supports Bergmann’s rule in microorganisms, such as bacterioplankton (Meadoret al. 2009), with smaller cells at higher temperatures (Daufresne et al. 2009).
We found no studies testing a mechanism behind this pattern in microorganisms, but one explanation for Bergmann’s rule among prokaryotes may be that as temperature increases, nucleic acid content decreases, resulting in smaller cells (Huete-Stauffer et al.2016). Though studying this rule in microorganisms may provide useful insights for the evaluation of the ecological impacts of global change on microbiota, there are significant challenges in defining the size of many microorganisms, such as multicellular fungal hyphae, that must be standardized before rigorous comparisons can be drawn.