On the applicability of Bergmann’s rule to ectotherms: The state of the art

Bergmann’s rule (BR) is the classic ecogeographic principle that relates the body size of endotherms with environmental temperature (or latitude). The currently available data demonstrate that the latitudinal clines in body size predicted by BR are also observed in ectotherms, being fairly common in some groups. Despite plenty of published data, it is still impossible to estimate the frequency of Bergmann’s clines in ectotherms as thoroughly as has been done for endotherms. Within large taxa of ectotherms (Arthropoda, Mollusca, and Pisces), Bergmann’s clines occur along with converse Bergmann’s and U-shaped clines. Since the classic explanation of BR based on thermoregulation principles is not applicable to most ectotherms, quite a few hypotheses have been proposed that appeal to other foundations in search of such an explanation. Part of them suggests a direct modifying influence of temperature; however, most authors look for an adaptive sense of the increase in the body size of ectotherms at high latitudes and/or low temperatures. It appears that a single universal explanation of all the cases of Bergmann’s variation in ectotherms cannot be formulated. Most likely, the observed clines arise as a result of synergetic interactions between several factors covarying with latitude (or altitude above sea level). It is not always possible to estimate the exact contribution of each of these factors.