The Biogeography of the Western Gray Squirrel (Sciurus griseus)

Written by James Williams for Geography 115, Fall 2003

INTRODUCTION

I chose the western gray squirrel, also known as the California gray squirrel as my organism because it is an organism that I see each day throughout the Sacramento area and in the neighborhood where I live. Since Sacramento has two rivers running through it, it has more trees than any other city in the world next to Paris, and therefore many squirrels. In much of the State of California the native western gray squirrel is slowly becoming replaced by another more commonly seen tree squirrel, the fox squirrel. However, I chose to make Sciurus griseus my subject organism because I believe it is prettier than the fox squirrel and the species native to California before the fox squirrel was introduced to our state. I will describe the natural history of S. griseus, which will include its food, habitat, and reproduction. In my concluding remarks I would like to explain why I believe S. griseus is an important species and why it should be more appreciated in the State of California.

TAXONOMY

SPECIES DESCRIPTION/NATURAL HISTORY

Sciurus griseus is gray haired with a white underbelly, a bushy long tail, and with long hooked sharp claws. Their long bushy tail serves to help them balance while climbing trees as they jump from limb to limb and travel from tree to tree. Their long hooked claws give them the grip they need for such activity. Adults can range anywhere from 17.6 to 33.4 ounces in weight and can measure from 19.7 to 23.6 inches long including their tails (Ryan, 1995). Also, they have a keen sense of smell to find their buried food and sharp eyesight to see predators coming.

Sciurus griseus feed on many other things besides just acorns as many believe. They also eat various fungi, seeds and nuts of trees and shrubs, fleshly fruits, and green vegetation (Ryan, 1995). They will also at times eat bark from trees.

Sciurus griseus is the largest native squirrel in the State of California and is an arboreal rodent meaning it lives in trees. They make nests for themselves called drays and also use cavities in trees to raise their young (Gilman, 1988). These drays are made with sticks, twigs, and leaves and are lined with mosses, lichens and shredded bark to soften the inside of their nests (Ryan, 1995). They are non-territorial creatures because they often change nests many times during the day or use a different nest to sleep in at night than the one they use to rest in during the day (Gilman, 1988). To protect themselves better they usually build a nest in a canopy of trees that also allow them to travel well above ground from tree to tree (Ryan, 1995).

Sciurus griseus breed in California from December to July and it is believed that males become sexually mature at one year and females at 10 to 11 months old (Western Washington Fish and Wildlife Office, 2003). Usually two to four males or at times as many as six males will chase a female in a mating chase. The largest most dominant male normally wins the female. It is generally believed that a female will produce only one litter per year; however, there is evidence that sometimes females produce two litters in a year and two to five young may be born in a 44 day gestation period (Ryan, 1995).

I believe S. griseus is important to man because they help spread new tree growth as a result of the seeds they bury and leave behind from their cache that are uncollected. Also, S. griseus is prey for many other organisms such as the red-tailed hawk, great horned owl, coyote, bobcat, gray fox, dogs and cats and would affect those organism’s food chain should the S. griseus become extinct (Ryan, 1995). I believe more needs to be done in the State of California to promote the growth of the population of S. griseus since it is the states’ native tree squirrel species.

GEOGRAPHIC DISTRIBUTION

Sciurus griseus have three subspecies: Sciurus g. anthonyl in southern California; S.g. nigripes in the San Francisco Bay area; and S.g.griseus in central California, the valley areas of northern California and Oregon, along the Columbia River Gorge of Oregon and Washington, in southern Puget Sound, and on the eastern slopes of the Cascade Range in Oregon north to Okanogan County, Washington. The following map shows the distribution of these three subspecies in relation to the distribution of Oregon White Oak (Ryan, 1995).

Sciurus griseus can be found living as far north as central and western Washington, through western Oregon and most of California, as far east as western Nevada and along the Sierra Nevada Mountains, and as far south to northern Baja California. It is native to the western part of the United States. They mostly live in redwood forests and woodlands of oak and pine and fir trees. They normally prefer trees between 3,000 and 8,000 feet in elevation (MacClintock, 1970). In general, S. griseus is drawn to woodland habitats near open grasslands in river valleys and is why there are many populations here in the Sacramento area. In Washington, there are small-scattered populations that follow the range of Oregon White Oak as can be seen on the above map (Ryan, 1995).

Sciurus griseus generally lives in forests and woodlands that have species of oak. They use oaks for food, shelter and travel, and as part of their predator escape routes. Also, S. griseus seem to prefer habitats of closely populated tree areas with tree canopies that allow for arboreal travel since squirrels by and large do not like to cross open prairies to remote trees (Ryan, 1995).

Besides being found throughout wilderness areas up and down the western part of the United States, S. griseus is also a prolific organism found throughout many of our cities, parks, and school grounds. As a result, it can be said that its worst enemy is the automobile since S. griseus along with other squirrel species are one of the most popular organisms to be seen dead on many western roads as evident in the photograph on page 4. The photograph was taken on my street where I live. My personal observation is that S. griseus is a slower moving squirrel species than is the eastern fox squirrel which is also a common squirrel species in our area. Therefore unfortunately, more S. griseus are run over by automobiles than are the quicker moving eastern fox squirrels. Hence, fox squirrels are more widely seen in our area and seem to have a larger surviving population.

It is believed that squirrels are animals with a great deal of site fidelity. Once they establish themselves in an area they remain in that area for life, except when they experience a shortage of food. This being the case, it is difficult to track each species migration and movement (Gurnell, 1987).

EVOLUTION

The order of the squirrel is the rodent (Rodentia), and it is thought that it was first a forest living creature similar to the squirrel. It appeared in the fossil record approximately 50 million years ago in North America, and shortly thereafter in Eurasia. Its ancestor probably lived in North America during the Eocene epoch between 54 – 37 million years ago (Gurnell, 1987). Since then, it is difficult to follow the evolution of the squirrel, but it is speculated that it experienced little change with a few occurrences of quick changes through its evolutionary progression.

It is held that the flying squirrel is probably the earliest form of squirrel and the tree and ground squirrels most likely splintered off from it somewhere down the line during the later part of the Oligocene. While I studied the phylogenetic reconstruction of the squirrel, I noticed that there are a few differences in opinion between authors when it comes to the research of the organism’s cladistic biography. For example, my main source stated that the earliest records of the tree squirrel seems to indicate that it came from the genus called Protosciurus from fossil deposits found in North America believed from the Oligocene epoch, and from Sciurus dubius from Europe from the same period. However, I did not find the genus Protosciurus anywhere when I traced the cladistic biography from the website I used for my research to make the cladogram of my organism’s genus Sciurus. Please see the cladogram of my organism Sciurus on the following figure (Maddison, 2001).

REFERENCES

Gilman, Kathleen Nettie, 1988. The Western Gray Squirrel (Sciurus griseus). Its Summer Home Range, Activity Times, and Habitat Usage in Northern California. Pp. 1-65.

Western Washington Fish and Wildlife Office. (May 30, 2003). Endangered and Threatened Wildlife and Plants; Status Review and 12-Month Finding for a Petition To List the Washington Population of the Western Gray Squirrel. Available: http://www.epa.gov/fedrgstr/EPA-SPECIES/2003/June/Day-10/e14354.htm [10/10/03]

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