Bergmann's Rule: The Relationship Between Animal Body Size and Climate Temperature
- 演化之聲
- Mar 14
- 4 min read
Updated: Apr 10
Bergmann's rule is a biological pattern describing how body size in warm-blooded animals tends to vary with climate. According to this rule, individuals belonging to the same lineage of endothermic animals generally become larger as latitude or elevation increases. In other words, species inhabiting colder environments often possess larger bodies than their relatives living in warmer regions. A familiar example is the polar bear, which is substantially larger than other bears. Penguins show a similar trend: species living closer to polar regions tend to have greater body size. Even humans exhibit comparable geographic patterns in body size.
To understand how Bergmann's rule may have operated during evolutionary history, researchers examined two major groups of Mesozoic animals: dinosaurs and early mammaliaforms. These lineages represent the ancestors of today's two major endothermic groups, birds and mammals. The goal was to determine whether body size variation across different latitudes during the Mesozoic followed the pattern predicted by Bergmann's rule.
The study assembled a dataset including 339 dinosaur species and 62 Mesozoic mammaliaforms. Some of these fossils came from extremely high-latitude regions, allowing the researchers to examine body size patterns across a wide climatic range. To evaluate the environmental context in which these animals lived, the team reconstructed palaeogeographic conditions and generated climate models representing different intervals of the Mesozoic.
For comparison with modern ecosystems, the researchers also analysed data from 5,496 living bird species and 2,305 living mammal species. These datasets allowed them to determine whether patterns predicted by Bergmann's rule could be detected among modern descendants of those ancient lineages.
The results were striking. Among Mesozoic dinosaurs, body size showed no correlation with latitude or with estimated environmental temperature. The same pattern appeared in Mesozoic mammaliaforms: their body size was not associated with either latitude or climate temperature.
When the researchers examined modern animals, the outcome was slightly different. In living birds and mammals, body size still showed no relationship with latitude itself. However, a weak relationship with temperature did appear. This indicates that modern endothermic animals may indeed be influenced by Bergmann's rule, although the effect is modest. Among birds in particular, the pattern was clearer: when environmental temperature decreases by 1°C, average body mass tends to increase by approximately 0.8%.
Why does this difference exist between Mesozoic animals and modern species? The researchers proposed three possible explanations.
The first factor involves environmental conditions. Bergmann's rule is generally thought to arise from thermal constraints. Analyses of modern endothermic animals show a detectable association between body size and temperature. During the Mesozoic, however, global climates were considerably warmer and more stable than those of today. Because temperature gradients were weaker, climate may have exerted less influence on body size.
The second factor concerns geographic distribution. Although dinosaurs and mammaliaforms were widespread across the globe, many of their subgroups did not inhabit high-latitude environments. If relatively few lineages occupied colder regions, the influence of temperature on body size would naturally be limited.
The third factor relates to physiology. Mesozoic endothermic animals, particularly early mammaliaforms, were physiologically more primitive than modern species. Their metabolic systems may have been less efficient than those of today's birds and mammals. If metabolic regulation was weaker, temperature may have had a reduced effect on body size evolution.
Looking across the evolutionary history of birds, the greatest shifts in body size occurred much later, during the early Miocene around 23 million years ago. At that time global climate underwent substantial cooling, and birds experienced extensive evolutionary radiation. For example, gulls diversified rapidly during this period, increasing both body size and species diversity while expanding to a global distribution.
In contrast, the bird-line dinosaurs examined in the Mesozoic portion of the study show no evidence that their body size was shaped by temperature. This suggests that the Bergmann effect likely became important only during the more dramatic climatic fluctuations of the Cenozoic. This interpretation is also consistent with modern observations: many bird populations today appear to be decreasing in body size as global temperatures rise.
Understanding ecological rules over long evolutionary timescales provides valuable insight into how ecosystems function. Such rules help scientists understand how species interact with one another and how biodiversity is maintained. These insights are particularly important in the context of modern climate change.
Research has shown that body size can influence extinction risk in many species. Therefore, understanding the mechanisms behind patterns like Bergmann's rule may help guide future conservation strategies as global climates continue to shift.
In addition, this study reveals an important aspect of early endotherm evolution. The global success and dispersal of Mesozoic dinosaurs and mammaliaforms do not appear to have depended on changes in body mass. Instead, the emergence of warm-blooded physiology itself may have allowed these animals to inhabit a wide range of climates across the planet.
Author: Bai Leng
Reference:
Wilson, L. N., Gardner, J. D., Wilson, J. P., Farnsworth, A., Perry, Z. R., Druckenmiller, P. S., Erickson, G. M., Organ, C. L. (2024). Global latitudinal gradients and the evolution of body size in dinosaurs and mammals. Nat Commun
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