What can we learn about past environments from evidence left
by Woodrats?
Written by Ricky Greer and Jan Snyder, Ed.D.
An Internet-based activity
Grades 9-12
Key Words: • Woodrat
• Middens
• Environmental Change
• Fossil Record
National Standards:
Life Science: Content Standard C:
•Interdependence of Organisms
•Living organisms have capacity to produce populations of infinite
size, but environments and resources are
finite.
•The Behavior of Organisms
•Organisms have behavioral responses to internal changes and to
external stimuli. Responses to external
stimuli can result from interactions with the organism’s own species
and others, as well as environmental changes; these responses either
can be innate or learned. The broad patterns of behavior exhibited by
animals have evolved to ensure reproductive success. Animals often
live in unpredictable environments, and so
their behavior must be flexible enough to deal with uncertainty and
change. Plants also respond to stimuli.
Arizona State Standards:
Strand 4: Life Science
Concept 3: Interdependence of Organisms
PO1. Identify the relationships among organisms within populations,
communities, ecosystems, and biomes.
PO 2. Describe how organisms are influenced by a particular
combination of biotic (living) and abiotic (nonliving) factors in an environment
Concept 4: Biological Evolution
PO 1. Identify the following components of natural selection which
can lead to speciation:
• potential for a species to increase its numbers
• genetic variability and inheritance of offspring due to
mutation and recombination of genes
• finite supply of resources required for life
• selection by the environment of those offspring better able to
survive and produce offspring
PO 4 Predict how a change in an environmental factor (e.g., rainfall,
habitat loss, non-native species) can
Affect the number and diversity of species in an ecosystem
Overview
Note: This lesson requires the use of student-available computers
that are connected to the Internet.
Through the use of resources on the Internet, students view typical woodrats and
their nests in a normal habitat. Students are also provided opportunity to see woodrat
(pack rat) middens and see examples of some that are thousands of years old. Evidence of
changes in environment over the time span of these middens is analyzed such that they
able to determine that areas that currently make up desert biomes were once less dry and
more characteristic of forested regions.
Introduction
Woodrats are commonly called Pack Rats, or Trade Rats, because they collect various
objects and bits of material to deposit in, or use in the construction of, their nests. They
are especially fond of small, bright, shiny objects which they will readily confiscate.
A popular superstition is that Woodrats are fair business-rats, who, when they expropriate
something, always leave a replacement of equal value. In fact, while carrying one trophy,
the rat may see another that is more attractive, and put down the first to pick up the
second—it can carry only one item at a time.
Range
Widely distributed throughout all the North American desert regions, north to Canada.
What do you know about the type of environment in which the pack rat middens are
found according to the following link? Look at the dots on the map and think about the
southwest and the environments you may have observed while living, or visiting, the
region.
Click here to view the distribution of Woodrats.
Description
Wood rats are pale buff, gray or reddish brown, usually with white undersides and feet .
They have relatively large ears and, normally, hairy tails. They range in length from 8 to
20 inches, including their 3- to 9-inch tail.
Behavior
Woodrats live in nests built of plant material like branches, twigs, sticks and other debris.
The huge, beaver-dam-shaped structures may be up to 4 feet across. They are usually
constructed in a tree or on the ground at the base of a tree or rocky ledge. Woodrats can
become quite a nuisance, getting into everything from attics to car engines, stealing their
treasures, damaging electrical wiring and wreaking general, noisy havoc.
In the deserts, nests are often constructed in or around cactus. The nest provides both
shelter from extremes of desert temperatures and protection from predators by using
cactus pads and cactus spines in the construction. Such construction methods helps keep
the nest much cooler than the surrounding desert floor in summer, while helping retain
the animals' body heat in winter.
Extra facts to know about Woodrats
1. Gila Monsters, as well as badgers, rip into woodrat nests without fear. A Gila can make
his living for the whole year by gobbling a couple of litters of pack rats and they do so
quite frequently.
2. Rattlesnakes, especially Western Diamondbacks (Crotalus atrox) are often found
commensally (living together) for part of the year in pack rat middens. It seems that the
snakes go deep into the middens and are sealed in by the rats during the cold part of the
year. The snakes do not touch the rats while hibernating and, apparently, the rats leave
the snakes alone. When the warm weather returns, the snakes are able to bust out and
resume activity. It is at this time that the rattler again becomes a predator to the rat.
3. The structures that Neotoma (Genus name for packrats) build are called "Middens."
They are very valuable as a resource as they are frequently built upon by generations of
rats. So, you can get a pretty deep and complicated structure that has been utilized by rats
for many years. The middens often give you glimpse into the past and can show how the
community surrounding the midden has evolved.
What is a midden?
Click on the link below and explore the site to learn more about packrat middens.
Click here to learn more about woodrat/packrat middens
Next, click the following three links and identify what the middens contain. Use the links
to answer the question below. The answer should be word processed and printed out
during this class period.
Link 1
What is the size of the midden? Remember it is constructed by a small rodent. Look at
the orange notebook and use it as a scale.
What type of texture does it have?
What is it similar to?
Click here to get an idea of the potential size of Woodrat middens
Link 2
What sort of things do you see in the midden? Realize it is found at the Grand Canyon
and the packrat carries many things back to its nest from the surrounding environment.
What can these objects tell you about the community?
Has the community changed throughout the years? Think about the Grand Canyon now.
Click here to find out what sort of things you might find in Woodrat middens
Now look at microscopic images of something found in Woodrat middens from the
Grand Canyon
Link 3
This midden is found near Yuma in southern Arizona. What objects can you identify
within it?
What do they say about the surrounding community?
Has the community changed? Think about the environment of Yuma in southwest
Arizona. Is it hot, cold, dry, wet, etc?
Click here for evidence of a former climate in what is now the desert.
Discussion Question: Use your knowledge about present day desert communities, the
information you viewed on this website, and the various links, to hypothesize how
present day desert communities looked 20,000 years ago. Include evidence from the
middens to discuss the evolution of the desert communities during the last 20,000 years.
Much of the information from this sight was taken from the website Desert Woodrats. It
can be found at Click here for more information of Desert Woodrats
Reference:
Jackson, T., Betancourt, J., Lyford, M., Gray, S., and Rylander, K. (2005). A
40,000- year woodrat-midden record of vegetational and biogeographical dynamics in
north-eastern Utah, USA. Available on the Web:
http://wwwpaztcn.wr.usgs.gov/julio_pdf/Jackson_2_ea.pdf