Mice are getting smaller.
Some populations of one of America’s most common mammals, the North American deer mouse, have been shrinking over the last 70 years. Researchers at the Florida Museum of Natural History analyzed seven decades worth of records on the rodent and found that they have been gradually decreasing in mass — and it’s unclear why.
“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,” says study co-lead author Robert Guralnick in a press release.
Authors of the study, published Monday in the journal Scientific Reports, set out to test Bergmann’s Rule, a principle which contends that species in colder climates tend to be bigger and species in warmer climates tend to be smaller. This is because having more volume than heat-releasing body surface tends to make it easier for larger-bodied animals to retain heat. Conversely, having more body surface than volume helps smaller populations more quickly dissipate heat and keep cool, the rule goes.
The scientists were curious to explore whether global warming is causing animals to shrink, and compiled thousands of historic and current mouse measurements to see if deer mice are indeed growing smaller. They were surprised to find that they are.
“Preliminarily, this is very intriguing, but we still don’t know what drives this decrease in mass,” says Guralnick. Adapting to better hide from humans and responding to the effect of urban heat on food resources are both considered possibilities, although more research is necessary.
Confusingly, researchers also found that it was only the smaller-bodied deer mice populations which were shrinking; larger-bodied deer mice populations, they found, have been getting larger. This may be reflective of “an increasingly fragmented landscape,” they write.
“Even in a small mammal like this, a minor change in body mass could have really important consequences for optimizing [energy],” says co-author Bryan McLean.