Is Bergmann's rule valid for terrestrial vertebrates?

Bergmann's rule posits that animals tend to have larger body sizes at higher latitudes with colder climates, but its applicability across various vertebrate groups has been a subject of debate. While earlier studies have proposed that the rule predominantly applies to endotherms, this hypothesis remains contested due to inconsistent empirical support across taxonomic groups and levels. In this study, we examined species- and population-level data across four major terrestrial vertebrate taxa (amphibians, reptiles, birds, and mammals) to assess the strength and consistency of Bergmann's rule. We showed that the rule applies to endotherms (birds and mammals) as evidenced by the positive correlation between body size and latitude across species, but not to ectotherms (amphibians and reptiles). In addition, we observed a stronger support for Bergmann's rule in endotherms (34% of species, n = 50) compared to ectotherms (18%, n = 93) at the order/family level, but a slightly higher support at the species level (22% vs. 18%). Contrary to expectation, however, Bergmann's rule is not stronger at the population level (21% of species) than at the species level (24% of families or orders). Bergmann's rule is likely moderated by thermoregulatory strategy, as body size was negatively correlated with temperature along latitudinal clines in endotherms but not in ectotherms. In addition, Bergmann's rule was more strongly supported in non-tropical than in tropical species and in terrestrial than in aquatic species. These findings suggest that Bergmann's rule is not a universal ecological principle but rather a context-dependent pattern influenced by ecological traits and phylogenetic history. This study underscores the need for a more nuanced understanding of Bergmann's rule across taxa and highlights the differential sensitivity of endotherms and ectotherms to environmental changes.