Melanistic fox squirrels (Sciurus niger) have expanded westward and increased in frequency in the Omaha, Nebraska, and Council Bluffs, Iowa, metropolitan areas. The selective advantage of melanism is currently unknown, but thermal advantages have been hypothesized, especially in winter. No difference in metabolic response curves were measured between melanistic (black) and rufus (orange) fox squirrels. When exposed to sunny skies, both melanistic and rufus squirrels had higher surface (skin and fur) temperature as ambient temperatures increased. Melanistic squirrel surface temperatures did not differ when squirrels were exposed to sunny or cloudy skies. However, rufus individuals showed significantly lower increases in surface temperatures when under cloudy skies. During fall months, rufus individuals were about 1.5 times more active throughout the day than melanistic individuals. However, in winter, melanistic fox squirrels were approximately 30% more active in the mornings (before 13:00) compared to rufus squirrels. Pre-winter body condition was higher in melanistic (25.5 ± 1.8 g/cm) compared to rufus (20.30 ± 3.6 g/cm) fox squirrels; however, there were no significant differences between melanistic (22.8 ± 1.4 g/cm) and rufus (23.9 ± 0.8 g/cm) fox squirrel post-winter body condition. The results of this study indicate that melanistic fox squirrels may have a slight winter thermal advantage over rufus fox squirrels by maintaining higher skin temperatures.
If the inline PDF is not rendering correctly, you can download the PDF file here.
Altman P. & Shapiro D. (1974) Testing assumptions of data selection in focal animal sampling. Behaviour 67115-133.
Arenz C. (1997) Handling fox squirrels: ketamine-hydrochloride versus a simple restraint. Wild. Soc. Bull. 25107-109.
Armitage K. (2009) Fur color diversity in marmots. Ethol. Ecol. and Evol. 21183-194.
Audubon J. (1843) The Missouri River Journals of John James Audubon. University of Nebraska Press D. Patterson Ed. 512pp.
Banfield A. W. F. (1974) The mammals of Canada. University of Toronto Press Toronto Canada. 438pp.
Barnes T. L. R. M. Kubik C. N. Cadaret K. A. Beede E. M. Merrick S. Chung T. S. Schmidt J. L. Petersen and D. T. Yates. 2017. Identifying hyperthermia in heat-stressed lambs and its effects on β agonist-stimulated glucose oxidation in muscle. Proc. Amer. Soc. Ainim. Sci. 68:25-29.
Bittner T. King R. & Kerfin J. (2002) Effects of body size and melanism on the thermal biology of garter snakes (Thamnophis sirtalis). Copeia 2477-482.
Campbell K. L. & Hochachka P. W. (2000) Thermal biology and metabolism of the American shrew-mole Neurotrichus gibbsii. J. Mammal. 82578-585.
Caro T. (2005) The adaptive significance of coloration in mammals. Biosci. 55125-136.
Clusella-Trullas S. van Wyk J. H. & Spotila J. R. (2009) Thermal benefits of melanism in cordylid lizards: a theoretical and field test. Ecol. 902297-2312.
Cooper C. E. Walsberg G. E. & Withers P. C. (2003) Biophysical properties of the pelt of a diurnal marsupial the numbat (Myrmecobius fasciatus) and its role in thermoregulation. J. Exper. Biol. 2062771-2777.
Ellis H. I. (1980) Metabolism and solar radiation in dark and white herons in hot climates. Physiol. Zool. 53358-372.
Foresman A. (1995) Opposing fitness consequences of colour pattern in male and female snakes. J. Evol. Biol. 853-70.
Fratto M. & Davis A. (2011) Do black-furred animals compensate for high solar absorption with smaller hairs? A test with a polymorphic squirrel species. Current Zool. 57731-736.
Geen M. R. S. & Johnston. G. R. (2014) Coloration affects heating and cooling in three color morphs of the Australian bluetongue lizard Tiliqua scincoides. J. Thermal Biol. 4354-60.
Geeslin H. (1970) A radio-tracking study of home range movements and habitat uses of the fox squirrel (Sciurus niger) in East Texas. MSc. Thesis: Texas A & M University College Station.
Gillooly J. F. Brown J. H. West G. B. Savage V. M. & Charnov E. L. (2001) Effects of size and temperature on metabolic rate. Science 2932248-2251.
Grojean R. E. Sousa J. A. & Henry M. C. (1980) Utilization of solar radiation by polar animals: an optical model for pelts. Appl. Optics 19339-346.
Gustafson E. J. & VanDruff L. W. (1990) Behavior of black and grey morphs of Sciurus carolinensis in an urban environment. Amer. Mid. Nat. 123186-192.
Hamilton W. & Heppner F. (1967) Radiant solar energy and the function of black homeotherm pigmentation: an hypothesis. Science 155196-197.
Heppner F. (1970) The metabolic significance of differential absorption of radiant energy by black and white birds. Condor 7350-59.
Hubbard J. K. Uy J. A. Hauber M. E. Hoekstra H. E. & Safran R. J. (2010) Vertebrate pigmentation: from underlying genes to adaptive function. Trend. Gen. 26231-239.
Huggins G. & Gee K. (1995) Efficiency and selectivity of cage trap sets for gray and fox squirrels. Wild. Soc. Bull. 23204-207.
Ibanez A. Pellitteri D. Sacchi R. Lopez P. & Martin J. (2016) Melanin-based coloration covaries with hiding and exploratory behavior in male Spanish terrapins. Ethol. 12230-36.
Innes S. & Lavigne D. M. (1979) Comparative energetics of coat colour polymorphs in the eastern grey squirrel Sciurus carolinensis. Can. J. Zool. 57585-592.
Kawanishi K. Sunquist M. E. Eizirik E. Lynam A. J. Ngoprasert D. Wan Shahruddin W. N. Rayan D. M. Sharma D. S. K. & Steinmetz R. (2010) Near fixation of melanism in leopards of the Malay peninsula. J. Zool. 282201–206.
Keeler C. Ridgway S. Lipscomb L. & Fromm E. (1968) The genetics of adrenal size and tameness in colorphase foxes. J. Hered. 5982-84.
Keeler C. Mellinger T. Fromm E. & Wade L. (1970) Melanin adrenalin and the legacy of fear. J. Hered. 6181-88.
Kiltie R. (1989) Wildfire and the evolution of dorsal melanism in fox squirrels Sciurus niger. J. Mammal 70726-739.
Kiltie R. (1992a) Comparisons among fox squirrels from the Mississippi River Delta. J. Mammal. 73906-913.
Kiltie R. (1992b) Tests of hypotheses on predation as a factor maintaining polymorphic melanism in coastal plain fox squirrels (Sciurus niger L.). Biological J. Linn. Soc. 4517-37.
Kingsley E. P. Manceau M. Wiley C. D. & Hoekstra H. E. (2009) Melanism in Peromyscus is caused by independent mutations in Agouti. PLoS One 4:e6435.
Koprowski J. (1994) Mammalian species Sciurus niger. Mammal Species Account No. 479 9pp.
Linnen C. R. Kingsley E.P. Jensen J. D. & Hoekstra H. E. (2009) On the origin and spread of an adaptive allele in deer mice. Science 3251095-1098.
Lueninghoener E. (1973) An investigation of the melanistic phase of the western fox squirrel (Sciurus niger rufiventer) in eastern Nebraska and western Iowa. MSc. Thesis University of Nebraska at Omaha.
Luiselli L. (1992) Reproductive success in melanistic adders: a new hypothesis and some considerations on Andrѐn and Nilson’s (1981) suggestions. Oikos 6460-604.
Lurz P. W. W. & Lloyd A. J. (2000) Body weights in grey and red squirrels: do seasonal weight increases in conifer woodland? J. Zool. 252531-547.
McNab B. K. (2002) The Physiological Ecology of Vertebrates. Cornell University Press. 576pp.
McNab B. K. (2012) Extreme Measures. Chicago: The University of Chicago Press. 312pp.
McRobie H. Thomas A. & Kelly J. (2009) The genetic basis of melanism in the gray squirrel (Sciurus carolinensis). J. Hered. 100709-714.
McRobie H. R. King L. M. Fanutti C. Coussons P. J. Moncrief N. D. & Thomas A. P. (2014) Melanocortin 1 receptor (MC1R) gene sequence variation and melanism in the gray (Sciurus carolinensis) fox (Sciurus niger) and red (Sciurus vulgaris) squirrel. J. Hered. 105423-428.
Nachman M. W. H. E. Hoekstra & S. L. D’Agostino. (2003) The genetic basis of adaptive melanism in pocket mice. Proceedings of the National Academy of Sciences of the United States of America 1005268–5273.
Protas M. E. & Patel N. H. (2008) Evolution of coloration patterns. Ann. Rev. Cell Develop. Biol. 24:425-446.
Robertson G. I. (1973) Distribution of color morphs of Sciurus carolinensis in eastern North America. MSc. Thesis University of Western Ontario. 78pp.
Rosenblum E. (2005) The role of phenotypic plasticity in color variation in Tularosa Basin lizards. Copeia 2005586-596.
SAS Institute Inc. (2002) Statistical analysis system user’s guide: statistics version 8. SAS Institute Inc. Clay North Carolina USA.
Schneider A. David V. A. Johnson W. E. O’Brien S. J. Barsh G. S. Menotti-Raymond M. & Eizirik E. (2012) How the leopard hides its spots: ASIP mutations and melanism in wild cats. PloS one 7:e50386.
Schulte-Hostedde A Zinner B. Millar J. S. & Hickling G. J. (2005) Restitution of mass-size residuals: validating body condition indices. Ecol. 86155-163.
Sherbrooke W Castrucci A. & Hadley M. (1994) Temperature effects on in vitro skin darkening in the Mountain Spiny Lizard Sceloporus jarrovii: a thermoregulatory adaptation? Physiol. Zool. 67659-672.
Short H. & Duke W. (1971) Seasonal food consumption and body weights of captive tree squirrels. J. Wild. Manage. 35435-439.
Sikes R. Gannon W. & American Care and Use Committee of the American Society of Mammalogists. (2011) Guidelines of the American society of mammalogists for the use of wild mammals in research. J. Mammal. 92235-253.
Steele M. A. & Koprowski J. L. (2001) North American Tree Squirrels. Smithsonian Institution Press. 201pp.
Tanaka K. (2005) Thermal aspects of melanistic and striped morphs of the snake Elaphe quadrivirgata. Zool. Sci. 221173-1179.
Walsberg G. E. Campbell G. S. & King J. R. (1978) Animal coat color and radiative heat gain: a re-evaluation. J. Comp. Physiol. 126211-22.
Walsberg G. E. (1988) The significance of fur structure for solar heat gain in the rock squirrel Spermophilus variegatus. J. Exp. Biol. 138243-257.
Walsberg G. (1983) Coat color and solar heat gain in animals. BioSci. 3388-91.
Walsberg G. E. (1990) Convergence of solar heat gain in two squirrel species with contrasting coat colors. Physiol. Zool. 631025-1042.
Walsberg G. E. (1992) Quantifying radiative heat gain in animals. Am. Zool. 32217-224.
Walsberg G. E. & Schmidt C. A. (1989) Seasonal adjustment of solar heat gain in a desert mammal by altering coat properties independently of surface coloration. J. Exper. Biol. 142387-400.
Walsberg G. & Wolf B. (1995) Effects of solar radiation and wind speed on metabolic heat production by two mammals with contrasting coat colours. J. Exper. Biol. 1981499-1507.
Wauters L. & Dhondt A. (1989) Body weight longevity and reproductive success in red squirrels (Sciurus vulgaris). J. Anim. Ecol. 58637-651.
Wilson J. (2013) Western expansion of melanistic fox squirrels (Sciurus niger) in Omaha Nebraska. Am. Mid. Nat. 170393-401.
Wooden K. M. & Walsberg G. E. (2000) Effect of wind and solar radiation on metabolic heat production in a small desert rodent Spermophilus tereticaudus. J. Exper. Biol. 203879-888.