Rats Changing with Environmental Changes
Climate Change Causing Rodents to Change Their Shape and Size
Aug 21, 2009
It’s not something that people would generally notice, but it’s apparently been happening over the past century: the head shape and overall size of rodents has been changing.
The scientist who made this observation – Dr Oliver Pergams, research assistant professor of biological sciences at the University of Illinois, Chicago – said the changes are due to human population density and climate change.
His research was published in the July 31, 2009 issue of PLoS One.
100 Years Of Rats Show Changes
As part of his research, Dr Pergams examined and measured 15 anatomic traits of rodent specimens from museums around the world, including large collections held at Chicago's Field Museum and the Smithsonian in Washington. The animals were collected between 1892 and 2001.
He recorded more than 17,000 body and skull measurements from 1,300 specimens from 22 locations in Africa, the Americas and Asia, comparing the specimens from before 1950 to those collected after 1950.
Although there was a slight overall trend to bigger sizes, there were also increases and decreases in the traits he measured, with changes as great as 50 percent over 80 years.
Human Populations Put Pressure on Rats
Dr Pergams also compared specimens from islands with small human populations to animals from mainland areas with much greater human density. There were greater changes among the specimens from the mainland.
He said that previously, rapid morphological change in mammals had been infrequently documented and was considered quite rare. His studies showed instead, that it was fairly common.
“Most of the cases in our study showed changes in at least one — and as many as seven — morphological traits over some portion of the last century. Thus, at least for the last 100 years, rapid morphological change (changes in shape) in some rodents seems to be frequent, and occurs on the mainland as well as on islands.
“Over the last 100+ years, rapid morphological change in rodents has occurred quite frequently. Rapid change, contrary to previous opinion, really seems to be happening quite frequently in a number of locations around the world," he said.
"There seem to be significant correlations with 'people-caused' parameters, such as population density and anthropologically-caused climate change."
Human Activity Affects Other Species
“Humans are changing the global environment at unprecedented rates,” Dr Pergams said. “Plants and animals can react to today's enormous environmental changes in one of three ways: they can move, they can adapt, or they can go extinct.”
Species that can respond quickly to these changes through phenotypic plasticity (changing its form in response to changes in the environment), or by migration, or rapid evolution have a distinct advantage in a dynamic world dominated by rapid land-use and climatic change.
Human-induced extinction of animals and plants has received a lot of attention, he said, and the movement of plants and animals in response to environmental change also gets noticed.
“However, relatively little research has addressed the ability of species to change either as a result of phenotypic plasticity or evolution in response to rapid environmental change.”
He said most cases of morphological change are caused either by pollution, such as industrial melanism in moths or heavy metal tolerance in plants, or by introduced species evolving to meet the challenges of a new environment – for example, in the cane toad in Australia.
Population Pressure and Climate Change Affects Mammals
Rapid change in mammals has been infrequently documented, Dr Pergams said. “Although rapid change has been demonstrated in some animals, such as the selection, through hunting, for smaller bighorn sheep with smaller horns, the great majority of changes have been seen in rodents.”
In the rat specimens he measured and compared, there were generally trends in larger heads and body size, although the sizes went up and down over a period of 80 years.
Climate change has played a role in morphological changes. Dr Pergams said modelling based on his measurements suggested that “changes in several of the individual morphological traits were associated with changes in human population density, current temperature gradients, and/or trends in temperature and precipitation.”
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