Here are the key citations from the scientific literature that we used to develop this exercise:
Ducharme, M. B., J. Larochelle, and D. Richard. 1989. Thermogenic capacity in gray and black morphs of the gray squirrel, Sciurus carolinensis. Physiological Zoology 62:1273–1292.
Gustafson, E. J., and L. W. VanDruff. 1990. Black and gray morphs of Sciurus carolinensis in an urban environment. American Midland Naturalist 123: 186-192.
McRobie, H., A. Thomas, and K. J. Alison. 2009. The genetic basis of melanism in the gray squirrel (Sciurus carolinensis) Journal of Heredity 100: 709-714.
Thomsa, T.N. 1987. An investigation of the factors influencing the frequency and distribution of melanistic gray squirrels (Sciurus carolinensis). M.S. Thesis, SUNY-ESF, Syracuse.