25th Annual Meeting and Symposium of the
Desert Tortoise Council, April 21-24, 2000
Abstracts

Trace Element Systematics of Desert Tortoise Shell

Gordon B. Haxel^1,3, Roy J. Knight^1,4, Kristin H. Berry^1,5, Bruce L. Homer², and Maurice A. Chaffee^1,4
1U.S. Geological Survey, ²University of Florida, Gainesville, Florida; ³Flagstaff, Arizona; ⁴Denver, Colorado; ⁵Riverside, California

Among the suspected causes of accelerated morbidity and mortality of the desert tortoise (Gopherus agassizii), a threatened species, is elemental toxicity related to several heavy metals and metalloids. We and several colleagues are evaluating this hypothesis by investigating the biogeochemical pathways that link rock, soil, vegetation, and tortoises; comparing trace element concentrations in the shells and internal organs of healthy and diseased tortoises; and studying the chemical effects of geological environment and various human activities and products on tortoise health.

Here we report data that establish for the first time the basic trace element patterns that characterize the two major components of desert tortoise shell - bone and scute. Shell materials from six apparently healthy tortoises (accidentally traumatically injured or killed by vehicles) and from 14 diseased tortoises were analyzed by instrumental neutron activation analysis (INAA). Powdered samples were irradiated by thermal neutrons in the USGS research reactor in Denver. Five gamma ray counts were performed: < 1 hour; and 5, 7, 14, and 60 days after irradiation. Approximately 18 elements were determined with satisfactory accuracy and precision in tortoise bone, scute, or both; and six additional elements could be determined at moderate to high concentrations. As the measured abundances span more than seven orders of magnitude, from major (structural) elements such as Ca in bone (about 23 percent) to trace elements such as Sc or Sb (10 - 40 ng/g (ppb)), presentation of the data is facilitated by normalization to the average composition of upper continental crust (UCC) (Fig. 1).

Despite the contrast in bulk composition between bone (apatite-like) and scute (composed of keratin, an insoluble protein), the normalized trace element patterns of bone and scute from healthy tortoises are rather similar, differing by no more than one-half an order of magnitude for most elements (Fig. 1). The higher Ca and Na content of bone is to be expected. These basic trace element patterns evidently are determined primarily by biologic processes rather than by geologic factors, as the analyzed tortoises lived on diverse geologic substrates in widely separated areas of the Mojave and Colorado Deserts, southern California. Local geologic factors may influence details of the patterns.

The trace element spectra (Fig. 1) comprise three segments: alkali and alkali earth elements on the left side, transition metals (Sc-Ni) in the middle, and chalcophile ("sulfur-loving") metals and metalloids (Zn-Se) on the right. The transition metals plotted either are trace nutrients (Cr, Fe, Co, Ni) or are of little or no biological significance at parts-per-billion levels (Sc, La - Lu, Th, Ta). Zinc, Se, and probably As are essential trace nutrients, although all are toxic at high concentrations; Sb and Au have no known natural biological role. In both bone and scute, the transition metals, except Cr and Ni, are effectively "diluted" to ~10-2 times their upper-crustal abundances. In contrast, the chalcophile elements Zn, As, Sb, and Se, along with Au, have markedly higher normalized abundances. This relative enrichment of the chalcophile elements in both bone and scute suggests that sulfur may be important in controlling these trace element patterns.

Desert tortoises spend six to nine months of the year underground in their burrows, in close contact with soil. Scute (the outer layer of the shell) is not appreciably enriched relative to bone in lithophile trace elements such as Le, Ce, Th, or Ta nor in potentially labile elements such as As and Se, suggesting minimal elemental input to the shell through contact with soil, at least in healthy tortoises.

Our trace element data also reveal several interesting and possibly important differences between healthy and diseased tortoises. Scute from diseased tortoises is strongly enriched, relative to scute from healthy tortoises, in several trace metals, most notably Au and W (!). The significance of this anomaly is unclear. Three diseased tortoises have much higher scute As and slightly lower scute Se than the other healthy and diseased tortoises. In tortoise bone, As and Sb are particularly informative. These two trace elements evidently are incorporated into tortoise bone in approximately the same ratio as they occur in UCC: As/Sb about 7.5. Two diseased tortoises have highly elevated bone As and Sb, with somewhat low As/Sb ratio. These pronounced anomalies in the chalcophile trace elements As, Se, and Sb in several diseased tortoises suggest some perturbation in the concentration, siting, chemical state, or utilization of sulfur within the organism. Whether such a disturbance is more likely a cause of disease or a result of disease remains to be determined. We shall continue to evaluate these ideas as we acquire data for more tortoises and for additional potentially toxic chalcophile elements such as Cd, Hg, and Pb.