There is a hypothesis (still controversial) that directly relates the size of the animals to the climate of the region in which they live. It’s called Bergmann’s Rule (proposed by Carl Bergmann, a 19th century German biologist), which states that warm-blooded species tend to be larger in cold climates and smaller in warm climates. Birds and mammals in cold regions have been observed to be larger than individuals of the same species in warm regions. It would be an adaptive mechanism to conserve or radiate body heat depending on the climate.
Therefore, it is worth thinking: in a global warming scenario, will species evolve towards smaller and smaller specimens? This is the question asked by several scientists at the University of Florida, who have published their conclusions in Scientific Reports .
Specifically, the team focused on the North American deer mouse, Peromyscus maniculatus , one of the most common mammals in the United States. In addition, it is the best documented rodent in museum collections.
Their findings indicate that deer mice in colder climates tend to be longer and have higher body mass, which is consistent with Bergmann’s rule. As the temperature changes, the body mass of the mice decreased , which was also in line with the researchers’ hypothesis. However, as rainfall increased, the researchers expected an increase in mouse body mass due to the boost that rain can give to food resources. In contrast, for this scenario, body mass also decreased.
The study also yielded a surprising result: larger-bodied deer mouse populations are getting smaller; while those with smaller bodies are getting bigger.
In the heat of civilization
Urban areas are significantly warmer than the surrounding rural landscape, a phenomenon known as the ‘urban heat island effect’. According to Bergmann’s rule, mice should be smaller in urban areas to combat the heat. But since human food and garbage abound in cities, mice could grow by tapping into that constant supply of fuel.
The data showed that as human population density increased, deer mouse populations tended to retain the same body mass, but shortened in length. This could indicate that the urban heat island effect outweighs the benefit of endless food resources, or simply that shorter mice hide better from humans.
The team will now turn its attention to analyzing changes in body size for the rest of the mammals.
Why is it important to study this?
As we pointed out at the beginning of the article, body size is a particularly important physical characteristic in warm-blooded animals because it helps maintain adequate body temperature for biological functions such as metabolism and heat transfer.
Larger-bodied animals have less body surface area, which releases heat, relative to the volume of their bodies, so they can cope with cold better than their smaller-bodied relatives. Because body size affects thermoregulation, changes in body size could influence the resistance of animals to climate change. Even in small mammals, a minor change in body mass could have really important consequences for optimizing those energy balances.
Referencias:
Robert Guralnick, Maggie M. Hantak, Daijiang Li & Bryan S. McLean. ‘Body size trends in response to climate and urbanization in the widespread North American deer mouse, Peromyscus maniculatus’ (2020) Scientific Reports.
