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Of mice and men

Rapid anatomical changes in rodents linked to increases in human population density, precipitation

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6:11pm, August 3, 2009
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No one ever said that rodents aren’t adaptable.

Subtle yet discernable changes in rodent skull shape and body length at sites worldwide may be linked to human population density and precipitation in those locales, new research has shown.

Previously, scientists have noted rapid anatomical changes, those occurring over just a few decades, only in populations of geographically isolated rodents, says Oliver Pergams, an evolutionary biologist at the University of Illinois in Chicago. Such changes typically took place when a species was introduced to a new area and adapted to the ecosystem there, he notes. However, he adds, “I suspected that those examples weren’t unique.” Now, online July 31 in PLoS ONE, Pergams and University of Washington in Seattle colleague Joshua Lawler link such changes to increases in human population density and precipitation.

Pergams and Lawler studied more than 1,300 museum-collected rodents captured at 22 sites worldwide — most located in North and South America — between 1892 and 2001. The team made more than 17,000 measurements of the creatures, which represented 25 species and subspecies of mice, rats and other rodents. Then, Pergams and Lawler split their measurements of length, skull size and 13 other physical traits into two roughly equal subsets for each site: data for creatures captured before 1950 and data for rodents collected after that date. Estimates of human population density were made using local census data for a year chosen to represent the pre-1950 collection and one chosen to represent post-1950 collection from each site. Climate data for the site, including the average annual precipitation, were calculated by analyzing the trend for the interval between the years the first and last rodents were collected.

The team didn’t identify any common trends in rodent anatomy among all the species, but some traits varied more often than others. In all, the researchers noted 61 anatomical changes among the rodents. While six of the 25 species didn’t show any substantial changes from the pre-1950s era to the post-1950s era, at least one of the 15 traits did change in the other species.

Not all of the anatomical changes could be easily linked to a particular possible cause, but many of the changes were strongly associated with an increase in human population density at the sites where the rodents were collected, Pergams says. The team also linked some of the anatomical changes to an increase in precipitation at the sites.

One of the species — Peromyscus leucopus noveboracensis, the northern white-footed mouse — changed in six distinct ways, the researchers found. In particular, Pergams says, the northern white-footed mice collected after 1950 at a site northwest of Chicago have longer ears, a longer snout and a broader skull than those trapped there before 1950. Other species exhibiting many long-term anatomical changes include P. maniculatus anacapae (a subspecies of deer mouse found in California) and Lophuromys flavopunctatus zena (the yellow-spotted brush-furred rat, found in central Kenya).

Some changes were rapid indeed. During an interval of 69 years, the average skull width for the northern white-footed mice collected near Chicago increased by more than 40 percent. Similarly, the average length of the ears of members of a species of pocket mouse at a site near San Diego increased 50 percent over the course of 79 years.

Although some of the rodents’ anatomical changes are statistically linked to changes in precipitation or human population density, the underlying reasons for the changes remain unclear, Pergams and Lawler report. As human population in an area increases, the quality and abundance of the rodents’ food supply probably increases, the researchers speculate. The same is likely true of an increase in precipitation, which typically boosts the growth of vegetation and seeds, Pergams says. Changes in the type, quality and abundance of food may drive anatomical changes that influence a particular species’ chewing efficiency or its sense of smell, for example.

The new study’s “strength is its strong documentation of changes” even though it is difficult to explain why those anatomical traits are changing or to determine what their consequences may be, says Erik Svensson, an evolutionary ecologist at Lund University in Sweden. Nevertheless, he notes, the changes aren’t surprising: “Urbanization is one of the most dramatic environmental changes you can find.”

Roland Kays, an evolutionary ecologist at the New York State Museum in Albany, adds: “It’s nice to have such a large sample size to show all the variation at once, rather than have it exposed one small study at a time.” With this data, he says, scientists can take a closer look at the most interesting cases to determine the mechanisms behind the changes.

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