Museum specimens, such as these deer mice in the National Museum of Natural History's division of mammals, provide invaluable records of how animals have changed over time / CREDIT: Bryan McLean

Mice shrinking in size? Not just climate or urban impact but more to it, says study

According to the controversial Bergmann’s Rule, species tend to be larger in cold climates and smaller in warm ones, which may shrink mice for an instance over a period of time, while humans facing the same prospect is not ruled out.

A new study tested this and published a paper in Scientific Reports, after analyzing 70 years of records of the North American deer mouse, Peromyscus maniculatus, arguably the most common and best-documented mammal in the U.S.

Unexpectedly, researchers found deer mice are generally decreasing in mass over time, but this trend may not be linked to changes in climate, say the scientists but they are surprised to find larger-bodied deer mouse populations getting smaller and smaller-bodied populations are getting larger.

No climate impact

“The most exciting aspect of this study was one that still remains mysterious – deer mice appear to be getting smaller over time, but it doesn’t seem to directly relate to climatic drivers or urbanization,” said co-author Robert Guralnick, curator of bioinformatics at the Florida Museum of Natural History. “Is this generally true for mammals?”

Body size is an important physical characteristic in warm-blooded animals because it helps maintain the right body temperature and for metabolism and heat transfer. “Even in a small mammal like this, a minor change in body mass could have really important consequences for optimizing those energy balances,” said study co-author Bryan McLean, assistant professor at the University of North Carolina Greensboro and a former postdoctoral researcher at the Florida Museum.

Larger-bodied animals have less body surface – which releases heat – relative to the volume of their bodies, so they may cope with the cold better than their smaller-bodied kin, says the thermodynamic foundation of Bergmann’s Rule. Because body size affects thermoregulation, changes in body size could influence animals’ resilience to climate change.

Sources of study

To examine changes in the deer mouse’s body size in relation to space, time, climate and human population density, Guralnick and his collaborators compiled body length and mass measurements taken by thousands of researchers across the U.S. over seven decades.

Their findings show that deer mice in colder climates tend to be longer and have bigger body mass, consistent with Bergmann’s Rule. As temperature changed over a period, deer mice body mass decreased, which also aligned with the researchers’ hypothesis. As precipitation increased, however, researchers expected an increase in mouse body but they found body mass also decreased.

According to Bergmann’s Rule, mice should be smaller in urban areas to beat the heat but due to huge food and garbage available in cities, mice could grow larger. The data showed that in urban areas, deer mice populations tended to retain the same body mass, but grow shorter in length.

When the team decoupled mouse mass from all of these factors, they still noted a general decrease in mass, hinting that climate and urbanization influence body size in a more complicated way than previously thought.

“Preliminarily, this is very intriguing, but we still don’t know what drives this decrease in mass,” Guralnick said. The team will now turn its attention to analyzing body size across all mammals, he said.


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