New research questions the validity of Bergmann’s rule, a scientific principle established in the 19th century. According to this rule, animals from high latitudes and cooler climates tend to be larger than their close relatives living in warmer climates. However, the fossil record examined for this study revealed conflicting results.
The concept of thermal regulation
The Bergmann’s rule is an ecological principle stated in the 19th century by the German biologist Carl Bergmann. This rule postulates that within the same species or group of closely related taxa, individuals that inhabit regions with colder temperatures have tend to be taller than those of populations living in regions with warmer temperatures. This rule often applies to mammals and birds, but it has also been studied in other groups of organisms.
The theory behind Bergmann’s rule is based on the concept of thermal regulation. According to this theory, larger animals have a lower surface area-to-volume ratio, meaning they lose less heat relative to their body volume. In cold environments, this adaptation is advantageous, as it helps keep animals’ body temperatures within functional limits. In contrast, in warmer environments, a smaller body size may be advantageous to avoid overheating.
Other factors at play
Note that while Bergmann’s rule has been widely observed in many groups of organisms, it is not universal. Other factors can also influence animal body size, such as availability of food resources et predation pressure. In environments where resources are abundant and predation pressure is low, animals may indeed tend to be larger due to advantages in competition for resources and defense against predators. In contrast, in environments where resources are scarce and predation pressure is high, animals may be smaller to minimize energy requirements and escape predators more efficiently.
L’A species’ evolutionary history can also play a role in its body size. Evolutionary pressures exerted by the environment over time can indeed result in specific adaptations that do not necessarily follow Bergmann’s rule. For example, some species may have evolved to become smaller due to constraints related to resource availability or interspecific competition.
In addition to Bergmann’s rule, other ecological principles can also influence animal body size depending on climate. For example, Allen’s rule states that animals in cold regions tend to have shorter limbs than those in warm regions, reducing body heat loss. These rules may interact in complex ways with Bergmann’s rule and contribute to the variation in body size observed in nature.
A study calls into question this well-established rule
Despite exceptions, Bergmann’s rule remains an important concept in ecology and evolutionary biology. This is why it continues to be studied and discussed by scientists as part of understanding geographic variation in animal body size. Recently, researchers from the University of Alaska and the University of Reading looked into the case of dinosaurs. Were they also subject to this rule of nature?
To find out, the team analyzed hundreds of data points from the fossil record, particularly those from Alaska’s Prince Creek Formation. We know that during the time of the dinosaurs, local temperatures were already freezing. The region also experienced regular snowfall. However, despite these rigorous environmental conditions, no significant increase in body size of arctic dinosaurs was observed.
The researchers then attempted the same assessment with modern mammals and birds, descendants of prehistoric mammals and dinosaurs. The results were largely the same. There existed a slight relationship between body size of modern birds and temperaturebut it was not not the same for prehistoric birds.
More generally, researchers have found that Bergmann’s rule is actually not applicable only to a subset of homeothermic animals (those who maintain a stable body temperature), and only when we consider temperature ignoring all other climatic variables. This suggests that Bergmann’s theory is ultimately more the exception than the rule.
The results of this study highlight that Bergmann’s rule, although historically important, is not universal and is subject to significant exceptions. Variations in animal body size do not appear to be driven solely by temperature, but rather by a complex set of environmental, biological, and evolutionary factors. This puts the application of this principle into perspective and calls for a more nuanced view of how organisms adapt to their environment. These discoveries pave the way for new research on the influence of interactions between climate and species biology.
Details of the study are published in the journal Nature Communications.
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