Natural History of Sciurus Niger
(Fox Squirrel)

Order:  Rodentia
Suborder:  Sciurognathi
Family:  Sciuridae
Tribe:  Sciurini
Subtribe:  Sciurina
Genus:  Sciurus
Subgenus:  Sciurus
 
Number of known species of Sciurus:  28
(Hoffman, et al; 1993)
 
Number of recognized subspecies of Sciurus Niger:  10
(Hall, 1981)

Geographic distribution:  Eastern and central United States, extending into the southern prairie provinces of Canada.  Introduced to California, Colorado, Idaho, New Mexico, North Dakota, Oregon, Texas, Washington (United States) and Ontario, Canada.  (Adam, 1984; Knapp and Swenson, 1986; Flyger and Gates, 1982)

 Fossil record:  Sciurus fossils found in Europe and North America dating back to the Miocene period (13-25 million years ago) are indistinguishable from today’s genus.  (Emry and Thorington, 1984) 

 Physical characteristics:  Sciurus niger is a medium-sized tree squirrel with no distinction in size/coloring between males and females.  Adult weight ranges from 507-1,361 grams (1.1 – 3 pounds; Flyger and Gates, 1982); total body length ranges from 454-698 millimeters (17.87 – 27.48 inches) with the tail 200-330 millimeters (7.87 – 13 inches;  Hall, 1981). 

Coat color in Michigan is a black-ticked tan or buff on back, legs, and tail with an orange belly; white spots on chest are not uncommon. 

In other parts of their range, coat color remains grizzled but can be less colorful, leaning towards grey along the central U.S. coast (Flyger and Gates, 1982); melanism (black) is more common in the southern United States (Kiltie, 1989, 1992) while albinism (white) is rare (Baumgartner, 1943a; Moore, 1956).  

 They have a total of 20 teeth (Flyger and Gates, 1982) and unlike Sciurus carolinensis (Eastern grey squirrel) and several others, do not have an upper P3 (premolar).  Mean rectal temperature is 105.08° F (Havera, 1979a).

 Development:  Sciurus Niger is born naked, except for vibrissae (whiskers), with birth weight ranging from 13-18 grams. Claws are already well developed.

Hair begins to appear between 7-10 days, at 3 weeks the ears open and lower incisors erupt.

Eyes open and upper incisors erupt at 5 weeks.
  
Weaning begins at 8 weeks and can complete as late as 12 weeks (Allen, 1942). 

 The greatest weight gain takes place during the first week, when infants increase body mass by 87% of birth weight and this gradually slows to approximately 8% at weaning; at the age of one year weight is 97% of maximum body mass (Nixon et al, 1991).

Females typically have their first litter at the age of 1.25 years (Harnishfeger et al, 1978), but have been reported to give birth as young as 8 months (McCloskey and Vohs, 1971) with reproductive longevity potentially > 12 years (Koprowski et al, 1988).

Ecology:  Fox squirrels molt twice a year, once in the spring from head to tail, then again in autumn from tail to head, with full tail molt occurring once during July-August (Flyer and Gates, 1982).  Lactating females molt after their babies have weaned (Baumgartner, 1943a); juveniles molt into their adult coat around 2.5 – 3 months (Moore, 1957).Contrary to most other mammals, fox squirrels are smallest in the northern portion of their range east of the Appalachians, and are again smaller in the western parts of their range.

Heart rate ranges from 150-450 beats per minute; and fox squirrels demonstrate fear tachycardia (Smith and Johnson, 1984).  Mean rectal temperature is 40.6° C/105.8° F (Havera, 1979a).  Food consumption peaks in spring or autumn with voluntary decrease in winter (Knee, 1983).  Juveniles consume 50% more food per unit of metabolic weight than adults (Short and Duke, 1971).  Body mass follows food consumption trends with a peak-to-trough difference of about 10% (Knee, 1983).

Fox squirrels are considered to be diurnal (awake during the day and sleeping through the night), with bimodal activity (two distinct activity patterns) occurring spring through fall, peaking 2 hours after sunrise and 2-4 hours before sunset.  During the winter they exhibit a unimodal pattern, with activity peaking between 10:00 a.m. and 2:00 p.m.  (Adams, 1984; Geeslin, 1970; Hicks, 1949; Hilliard, 1979)

Fox squirrels are classified as granivores (“seed eater”) and feed heavily on tree seeds during much of the year; tree buds and flowers are used primarily in winter and spring.  During early summer the fruits and seeds of a limited number of species such as mulberry, hawthorn, and maple are eaten until nuts ripen (Koprowski, 1991a; Reichard, 1976).

Fox squirrels swim by a dog-paddle, holding head, dorsum, and tail above the surface of the water (Applegate and McCord, 1974; B. Manszewski, pers. comm.).

They are considered classic scatterhoarders who disperse food caches.  Nuts are buried <2 cm below the surface of the soil or covered with leaf litter (Cahalane, 1942).  Between 33% and 99% of cached nuts are recovered (Cahalane, 1942; Stapanian and Smith, 1984) with smell being likely important in locating buried nuts.  Red and black acorns are preferred over white acorns with a demonstrated preference for those lower in tannins (Ofcarcik et al, 1973), however a diet comprised solely of red and/or black acorns was the only nut diet to lead to a decrease in body mass (Baumgras, 1944; Havera and Smith, 1979).