The Abert Squirrel and Its Dependence on Ponderosa Pine

The Abert squirrel (Sciurus aberti aberti) occurs throughout the ponderosa pine belt in central Arizona where it appears to have become ecologically dependent on ponderosa pine. Studies of the squirrels and their relationship to ponderosa pine were conducted at the Fort Valley Experimental Forest and adjacent areas on the Coconino National Forest near Flagstaff, Arizona, between October 1953 and June 1955. Adult squirrels molt twice each year, and their summer and winter pelages are somewhat different in color shade and pattern. In a sample of 313 adult squirrels, males weighed an average of 589 g; females, 60 g. Body measurements are similar between sexes, except that females apparently have longer tails. The squirrels obtain most of their food from ponderosa pine. The inner bark of terminal twigs is the principal food during winter, and ovulated cones are preferred in summer. Staminate flowers and apical buds of pine are also important foods. Other foods include fleshy fungi, carrion, and bones. The home range of squirrels was determined in summer by trapping and in winter by mapping areas of activity in snow. On the average, squirrels ranged over about 18 acres during summer and five acres in winter. Squirrels are strictly diurnal and in 1954 were active and fed each day even in inclement weather. Abert squirrels build nests in the branches of ponderosa pine. These nests, composed of pine twigs, have an outside diameter of about one and one—half feet and an inside diameter of six inches. Squirrels do not prefer any particular site in the tree for their nest. They may occupy several nests during a year. In 1954 squirrels bred only in May at Fort Valley. The gestation period was estimated at about 40 days, and young squirrels were born between June 10 and July 12. Litter size varied from two to five and averaged 3.4. Of 729 squirrels collected during different years, 57% were male and 43% were female. Young squirrels had thick ear tufts in September, but on adults tufts were absent or very thin at that time. This difference is the best available criterion of age. The obvious causes of mortality did not appear to be responsible for the considerable losses that evidently occurred during the winter of 1953—54; other factors must have reduced squirrel densities. The number of feeding sites evident after fresh snows is a reliable indicator of squirrel density. Counts of the total number of twigs clipped by squirrels during winter appear to be related to population densities and differences between years should indicate trends in populations. In central Arizona squirrels are now apparently less numerous than at the turn of the century. This is probably due to the deterioration of squirrel habitat caused by logging and lack of adequate pine regeneration. Short—term fluctuations in population are believed to be caused by variations in quality and quantity of food supply which in turn may be related to the reproductive cycle of ponderosa pine. Intensive forest management should result in larger and more stable populations of the Abert squirrel.