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For the past 20 years, Professor
Phil Myers of the U-M Museum of Zoology and Department of Ecology and Evolutionary Biology has been studying the distribution of mammal species in Michigan.

Professor Myers says, "We've been looking at all small mammal species — what we're focusing on here is those that are changing."

Myers is seen in this photo weighing a woodland deer mouse as part of his research.


In addition to trapping and releasing
animals found at the U-M Biological Station and other locations, Professor Myers and his students study U-M Museum of Zoology specimens and field notes made by other scientists throughout the 20th century to learn where the animals have been found over the past 150 years.

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Opossums are heading north
Opossums (scientific name Didelphis virginiana) are originally native to more southern regions of North America. For many decades their range has been expanding north. In Michigan, the northward movement of opossums in the past 25 years has been dramatic.


Professor Myers has documented the opossum’s northward spread with observations of road kills he sees while driving up north on his research trips. When he finds a dead opossum by the road, he carefully notes the location with his GPS, and makes a record of it.




The red dots on the map show documented opossum collecting locations dating  from 1889 to 1979. During this time opossums were limited to the southernmost part of the Lower Peninsula, with just one record in the western Upper Peninsula. The red dots show Professor Myers' oppossum road kill documentation for 1980-2006. The sample dots line up vertically following the roads used by Dr. Myers, and are clear evidence of the oppossums northward spread—all the way to the northermost parts of the Lower Peninsual as well in the southwest Upper Peninsula.


Look out for frostbite!
Although opossums are heading north as the climate warms, Michigan's winters are still hard for them. The opossum doesn't hibernate in the winter. It will often hole up during very cold weather because it runs the risk of getting frostbite on its hairless ears, tail, and toes. The opossum in the photograph above has lost the end of its tail due to frostbite.
This is a study specimen of a juvenile opossum. Adult opossums get a bit larger than this.

The range of the white-footed mouse is expanding northward...

...while the range of the woodland deer mouse is shrinking









Specimens mounted like this are called "study skins." They provide an invaluable historical record, one that lets us not only document where species live now and in the past, but also study evolutionary change in traits, including color, size, and shape of the skull.



Collect the data, but don't hurt the mice!
These two species of mice are hard to identify, so to gather reliable information about their changing ranges, Professor Myers and his colleagues must trap them to get a closer look. They use Sherman Live Traps, which leave the animals unharmed.

To the left are two of Professor Myers’ collaborators setting Sherman Live Traps.
Below is one of the traps.

After being carefully removed from the trap, a mouse's measurements are taken to help identify it – the two species of mice are very similar. Each mouse's sex and age are also recorded
Spit samples are taken from the mice. The spit provides a protein called salivary amylase. This protein is analyzed in the lab to confirm the identification of the mice. To the left and below is spit sample equipment.


Plenty of data
The results of the past 20 years of Professor Myers’ research are based on data gathered on roughly 100,000 trap nights (one trap out for one night constitutes a single trap night). To the left, Myers and his colleague Barb Lundrigan record mouse statistics. Records from other scientists in the early and middle 20th century show that both species of mice were present then in roughly equal numbers. Today, white-footed mice far outnumber deer mice, and have completely replaced them, throughout the Lower Peninsula and in some areas of the Upper Peninsula.
Is climate change the cause of changing mice ranges?
It looks that way. Research at the U-M Biological Station in Northern Michigan has shown that the shorter the winter, the more white-footed mice. Are winters getting shorter, making conditions better for white-footed mice? To find out, Professor Myers studied records of ice break-up on Grand Traverse Bay going back more than 100 years.

Professor Myers found that in the 1970s, ice broke up in early April; now it usually happens a month sooner. This chart shows that the ice break up is happening earlier now than it has in the past 160 years. So it appears that the warming climate may be driving the northward march of the white-footed mice… at the expense of the woodland deer mice.

What's the big deal?
Does it matter that one species of mouse is invading another's territory? According to Professor Myers: "These are the most abundant mammal species in their habitat. We don't know enough about how different they are to predict the effect of replacing one with another. We do know that these mice play important roles in their environment: scattering seeds, eating insect pests, and serving as food for a variety of predators. They affect the composition and appearance of our forests. White-footed mice also carry the bacteria that cause Lyme Disease, which is now spreading through the southern and western Upper Peninsula. What's happening to the woodland deer mouse may affect us directly, and it’s a clear warning that the effects of climate change are already having an impact on natural communities in Michigan."
What's happening to mice and opossums is also happening to other mammals…



Climate change: Warmer... but why? | Upsetting the balance | Mammals on the move
Connecting climate and disease | How do sea shells record temperature change?
Taking Earth's temperature