Rats! Are Rodents Getting Bigger?

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It's the stuff of low-budget sci-fi movies: rodents around the globe are growing ever larger at astonishing rates. But B movie it's not—as UIC ecologist Oliver Pergams has demonstrated, the trend is real. In a recently published report Pergams details how rodents are showing signs of rapid, worldwide changes in size and shape. Of course, the timescale and magnitude of this size change is not alarming enough to cause movie-goers to flee the cinema (we're talking about decades and millimeters here—and in some cases size decreases were observed).

But it is significant enough to capture the attention of scientists and to merit further investigation into its causes.

Pergams' curiosity about rodent size trends piqued after studying mice on California's Channel Islands and in the Chicago area. In both locations, Pergams noted signs of rapid, recent changes in the size of his study subjects and he began to wonder if these changes were unique to the California and Chicago study sites or if they might be played out elsewhere around the globe. So he applied for (and received) a grant form the NSF and set about measuring rodents in museum collections around the world.

It must have been painstaking work. Pergams made more than 17,000 measurements of 1302 rodents. He used digital calipers to make nearly a dozen cranial measurements on each specimen, recording such dimensions as the breadth of the braincase (BB), the greatest length of the skull (GL), the breadth of the rostrum (BR), the length from the supraorbitals to the nasals (ONL), and the zygomatic breadth (ZB).

Pergams also measured four external aspects of each specimen—the total length, the tail length, the length of the hind foot and the ear length.

Pergams gathered data on museum specimens from Africa, the Americas and Asia dating between 1892 and 2001. Most of the specimens came from two large rodent families: the Cricetidae (New World rats, mice, voles, and hamsters) and the Muridae (Old World mice, rats, gerbils, and whistling rats). When he completed his measurements, he took his findings back to the lab, crunched the numbers, and looked for trends.

Pergams found an assortment of changes in each feature he had measured. In some cases, features were getting larger, in others they were getting smaller. Pergams notes though that there were slightly more trends towards larger size than to smaller size. The changes in the dimensions he measured were as great as 50 percent over 80 years. If evolution is driving these size changes in rodents, then it is doing so at a rapid pace. Of course, it is too early to positively identify the mechanism behind the changes Pergams observed, but Pergams suggests that factors such as human population density, current temperature, or trends in temperature and precipitation may be worth exploring further.


Pergams, O., & Lawler, J. (2009). Recent and Widespread Rapid Morphological Change in Rodents PLoS ONE, 4 (7) DOI: 10.1371/journal.pone.0006452
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