SPRING IS THE TIME FOR GROUND SQUIRRELS

It’s hard to conceive of a species spending most of its life asleep, but that’s the case for the spring ground squirrels.

Ground squirrels feed on herbaceous vegetation, and many of them are active throughout the summer and then hibernate during the winter. Some ground squirrels, however, live in arid deserts or grasslands in which lush growing vegetation is present only during the spring or during a summer rainy season. These species are active only during that time, so they are active above ground for only about a third of the year.

Two species that exemplify this scenario are the Washington Ground Squirrel (Spermophilus washingtoni) and Piute Ground Squirrel (Spermophilus mollis). Both are small, short-tailed species. Piute Ground squirrels occur in a limited area of sagebrush steppe in southern Washington between the Cascades and the Columbia River. Washington Ground Squirrels occur east of that river in the Columbia Basin. Both species have declined substantially in recent years as their habitat has been lost to agriculture.

Belding's Ground Squirrel (Spermophilus beldingi) is a slightly larger and more southerly species of the high desert in eastern Oregon and northern California. It has adapted very well to agriculture, so it is still abundant, but its life history is much like that of the other two.

Washington Ground Squirrels have litters of about eight young. They are born in late winter and emerge from their burrows about a month after the adults. A visitor to a ground squirrel colony around the beginning of April is treated to the sight of playful young on all sides, while the adults go about the business of serious eating.

After a few months of activity, with an abundance of fresh salads on the daily menu, first the adults and then the juveniles retire back into their burrows and spend the summer in a state of torpor called aestivation. Aestivation then grades into the usual winter hibernation. The longest an individual of these species spends above ground is about four months. One adult Washington Ground Squirrel in captivity was dormant for 244 days.

Juveniles grow to adult weight in two months after leaving the burrow. All individuals begin to deposit fat 6-8 weeks after emerging and end up with lipids comprising about 65% of their body weight when they go into the burrow for good. They then live through the summer, fall and winter on the fat deposited during the spring.

High temperature and low humidity are stressful on any animal, and as there is no new growth on the plants they favor, the spring-active ground squirrels have no source of water during the summer; thus aestivation. Aestivation is an adaptation for survival in hot, dry climates, just as hibernation is an adaptation for survival in cold climates. These small animals use both of these strategies, surviving by the fine tuning of their seasonal activity to their climate. How will global warming affect them?

Dennis Paulson

WASHINGTON CHIPMUNKS

Mammal watching has never achieved the popularity of bird watching for at least one reason: most mammals are nocturnal. Not only are most of them small and brown, but you can’t even see them! Mammalogists survey mammals by trapping them, netting them, looking for their tracks and scat, etc. Their survey methods don’t usually include walking around with a pair of binoculars on a nice, sunny day in spring.

But there are exceptions. Some mammals are much more visible than others, primarily because they are diurnal. These include many of the large grazing and browsing hoofed mammals (ungulates), the cute pikas of western mountains, and the squirrels. The Pacific Northwest is abundantly provided with squirrels, encompassing the taxonomic range of the squirrel family: flying squirrels, tree squirrels, ground squirrels, marmots, and chipmunks.

Chipmunks come close to being birdwatchers’ mammals. They are active during the day, with an emphasis on the “active,” they are brightly marked, they are territorial, they vocalize frequently, and they come readily to bird feeders. They vary from very shy to very inquisitive, even tame where they encounter people regularly. They are still basically brown, but their conspicuous stripes make them easily recognizable as chipmunks.

These small squirrels are usually associated with rock piles and fallen logs, where they nest. They forage on the ground and in shrubs, sometimes ascending well up into the trees. Hyperactivity describes them best, as they move jerkily along with tail cocked up into the air. When disturbed, they dart into cover, appearing some distance away for another look at the disturbance.

Basically seed eaters, chipmunks will take anything that comes along, including fruits, fungi, and arthropods. They are accomplished nest robbers. taking bird eggs whenever they can find them. During the fall, they busily gather seeds in cheek pouches and cache them in their protected nests. They can then hole up for the winter and feast on these caches without leaving their protected shelter. Caches can contain tens of thousands of seeds.

Most western states have several species of chipmunks; Washington has four. The most common and widespread species in Washington is the Yellow-pine Chipmunk (Tamias amoenus), so named because it is common in the ponderosa (or yellow) pine zone east of the Cascade Mountains crest. Absent from the dense forest of the western lowlands and mid elevations in the mountains, it is again common in the subalpine-alpine zone of the Cascades and Olympics. It is easily seen by hikers in the high country and drivers through almost any of the interior national forests. It is at home in trees, ascending high into pines to forage for the seeds.

The Red-tailed Chipmunk (Tamias ruficaudus) looks about like the Yellow-pine but is slightly larger and longer tailed, and the tail is more intense reddish below. It occurs in the mountains of far northeastern Washington, in Stevens, Pend Oreille, and northern Spokane counties. Its habitat zone is the montane conifer forest and open subalpine zone above that, mostly above the elevation range of the Yellow-pine.

This species would not have been separated from the Yellow-pine but for its copulatory organ. Rodents have a bony structure in the penis called the baculum, and this structure differs among different species of chipmunks. That of the Red-tailed is distinctly longer than that of the Yellow-pine. I have not read of any structural characteristics that differentiated the females!

The largest Washington chipmunk is the Townsend’s (Tamias townsendii), restricted to the forested western lowlands and similar dense habitats up to treeline in the Olympics and Cascades. It occurs in both forested and open (e.g., clear cuts) microhabitats. It is not found in the more open ponderosa pine woodlands below the wet conifer forests on the east side. In addition to being larger and darker, not as brightly marked, it differs from all the other species in not having a clearly defined dark stripe extending from nose to eye. This and the Yellow-pine often occur together in ecotones between open alpine or ponderosa pine habitat on one side and dense conifer forest on the other. Washington Pass is one such location.

Finally, the Least Chipmunk (Tamias minimus) is restricted to sagebrush habitats in the southern part of the Columbia Basin. It is a smaller species, generally paler and grayer than the others, with the back stripes brown instead of blackish and very little reddish or brown color anywhere. It overlaps with Yellow-pine where sagebrush, grassland, and ponderosa pine come together. Least Chipmunks occur at higher densities than the other species, and one place to see this is at the Ryegrass rest stop east of Ellensburg on eastbound I-90. It is full of Least Chipmunks most of the year, gathering by the dozens at the feast of sunflower seeds put out by DOT employees.

There is also a pair of chipmunk lookalikes in Washington. The Cascade Golden-mantled Ground Squirrel (Spermophilus saturatus) is found throughout the Cascades, in both semiopen conifer forest and alpine meadows. The Golden-mantled Ground Squirrel (Spermophilus lateralis) occurs in similar habitats in the Blue Mountains and, less commonly, the mountain ranges east of the Okanogan River. Both are larger than chipmunks, with an unstriped golden-orange head. Most ground squirrels eat leaves rather than seeds, but these species are more chipmunk-like in their diet.

Dennis Paulson