Rodent Size Linked to Human Population and Climate Change
[Writer] This is research news from U-I-C – the University of Illinois at Chicago.
Today, ecologist Oliver Pergams talks about anatomic changes over the past century in rodent samples he’s studied from around the world, and how the relatively rapid changes he found may be related to human population density and climate change.
Here’s Professor Pergams:
[Pergams] Previously, I’d done work looking at rapid morphological change – rapid change in size of — rodents in the California Channel Islands and in the vicinity of Chicago. I found that all of these rodents – in this case they were all mice – were changing in size very rapidly. This is rapidly in evolutionary terms. It wasn’t millions, or hundreds of thousands or even thousands of years. They’d change in size by 10-15 % in the last 50- 100 years. And it got me curious. I was wondering if these two instances – they were about seven populations of mice in California’s Channel Islands and around Chicago – were unique, or if rodents and small mammals were changing all over the world in response to some unknown factors.
So I applied for an NSF exploratory grant to start measuring museum specimens of rodent skulls and external measurements. I located 1,300 specimens collected around the world and housed in various museums, and I performed about 17,000 measurements on these specimens.
These were series of rodents that were collected both before and after 1950 in approximately the same location, and the resolution was usually county level – or the equivalent in foreign countries. I wanted to see if any of the 15 measurements I was taking had changed in size between the period before 1950 – and it started about 1892 for the earliest specimens – and the period after 1950, which ended in 2001 for this latest series.
I found that many of the measurements did indeed change – the morphological, rapid change in rodents — was by no means unique to the California Channel Islands and the Chicago area; that it was happening in many of the places I found. All in all, I found 61 examples of rapid morphological change in the traits I looked at, and these were on the mainland as well – many were on the mainland, not just on islands, which had been the great bulk of previous evidence.
I then wanted to see why this was happening, so I started looking at possible long-term indicators that might have an effect and influence on a rodent’s size over the short term.
I looked at human population density and I looked at climate change as factors that had changed a lot over the last hundred years and things that might have an influence. In order to work on the climate change variables, I asked Josh Lawlor from the University of Washington to work with me. He’s an ecologist and climate change specialist. Together, our analysis shows that many of these variables were indeed changing in association with either human population, or temperatures, or precipitation change.
One of the traits we found associated with human population density and climate change was something called occipital nasal length – it’s a kind of hybrid measure of nasal – nose – length and width at the same time. We found that human population density and precipitation change over the last 100 years explained a great deal of the change in size.
We speculate that this might have happened because greater human population density meant greater food resources for the rodents, and greater precipitation also increased the amount of food available.
In general, of course, our study was correlation, not causation. We don’t have a time machine to go back and say exactly what happened over the last 100 years, say exactly why these changes occurred. But it is striking that we have these high correlation, high association factors, and it also seems unlikely that given the huge impacts that humans have made over the last 100 years on the environment, that these impacts wouldn’t have an effect on the rodents.
Our study is the first example of a wide-ranging study to try to see if morphological change – rapid change – is common in any group of animals. Previously it had only been done for individual populations or individual species. And we think it quite likely that, though we did this study for rodents, that these kind of changes exist in many, many groups of organisms – especially those that have been heavily impacted by humans, which is pretty much everything.
We think that further research, further large meta-analyses into these kinds of changes will very likely bring up additional changes in different groups of organisms, and we think that the differences and the commonalities between changes of different organisms will lend a great deal of insight, both into rapid evolution and into the effects that people have on their environment.
[Writer] Oliver Pergams is a research assistant professor of biological sciences.
For more information about this research, go to www.today.uic.edu … click on “news releases.” … and look for the release dated July 31, 2009.
This has been research news from U-I-C – the University of Illinois at Chicago.
