Polymorphic nature of cranial fluorescence in the fox squirrel (Sciurus niger) from Texas and Oklahoma.

Abstract. -- Congenital erythropoietic porphyria (CEP) is a rare pathological condition in man and some domestic animals. Reported diagnostic features include skin lesions, shortened life span, darkened bones and fluorescent skeletal tissues. Literature sources report the condition is characteristic of fox squirrels (Sciurus niger), and that the species suffers no ill effect of the disease. Examination under an ultraviolet light of 157 skulls and associated mandibles of S. niger from Texas and Oklahoma revealed that skeletal fluorescence is a polymorphic feature that affects only about 70 percent of examined specimens. Sexes did not vary in frequency of occurrence of this phenomenon, although adult specimens were significantly more likely to fluoresce than those of younger age classes. Among other findings are the purple fluorescence of nestling and juvenile specimens, in contrast to the orange coloration of older age classes. These results suggest several potential avenues for future study relative to the biogeography and genetics of this phenomenon in S. niger.

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Congenital erythropoietic porphyria (CEP) is a rare pathological condition that has been documented in humans and some domestic species of mammals. Onset of the diagnostic skin lesions and hemolytic anemia occurs in humans shortly after birth or early in childhood, and life expectancy is limited to a few years. The disease results from a defect in heme biosynthesis and the resulting porphyrins are distributed in the urine, feces, and tissues such as spleen, blood, bones and teeth. Deposition of these porphyrins imparts both a dark color to bone and a fluorescent quality is reported for the skeletal and other affected tissues of diseased individuals when exposed to long-wave ultraviolet light (Schmid et al. 1954; 1955; Bloomer et al. 1993).

The unusual dark pink to dull reddish hue of the skull and post-cranial skeleton of specimens of the fox squirrel (Sciurus niger) is a distinctive feature that has long been noted by naturalists and has even been used by some biologists (Jones et al. 1983; Lowery 1974) as a means to distinguish the skull of this species from that of the morphologically similar skull of the gray squirrel (S. carolinensis). Turner's (1937) documentation of porphyria as the causative factor of this phenomenon in S. niger was followed up in more detail by Levin & Flyger (1973) and Flyger & Levin (1977). Laboratory studies by these workers of the urine, blood and bone marrow demonstrated the presence of CEP in their series of fox squirrels, as well as an absence of the condition in a comparable series of gray squirrels.

Flyger & Levin (1977) noted that S. niger is asymptomatic for pathological aspects of CEP, which prompted them to suggest the species as a laboratory animal model for future studies of this physiological condition, but perhaps because of the rarity of the disease in humans, there has been no answer to their call for follow-up investigations of the species in this role. However, this study has been cited as the basis for a series of statements in the literature (Caire et al. 1989; Fitzgerald et al. 1994; Flyger 1999; Koprowski 1994) proclaiming CEP and the resulting fluorescent qualities of bones and teeth as characteristic of the fox squirrel.

The impetus for the present study was provided by a recent mammalogy laboratory exercise conducted by the third author, which unexpectedly demonstrated that a large proportion of fox squirrel skulls did not fluoresce when exposed to ultraviolet light. This study details the regional polymorphic aspect of this unusual feature in S. niger, and describes the frequency of fluorescence between sexes and relative age groups.

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