Twenty-Third Annual Meeting and Symposium of the
Desert Tortoise Council, April 3-5, 1998
Abstracts

The desert tortoise (Gopherus agassizii) is a federally listed threatened species in the Mojave and Colorado desert portions of its range in the Southwest. Diseases contributing to accelerated tortoise mortality include upper respiratory tract disease, cutaneous dyskeratosis (lesions of the scutes, the horny plates covering the shell), and degeneration of the liver and kidney. Necropsies by B. L. Homer and colleagues indicate elevated concentrations of several potentially toxic metals in the liver and kidney of some animals. We are investigating the abundances and sources of trace elements, including potentially toxic metals, in desert tortoise shell and scute by instrumental neutron activation analysis (INAA), and by lead isotopic analysis, which should enable us to distinguish natural and anthropogenic sources of lead.

Our initial, reconnaissance INAA investigations have two purposes: to learn which chemical elements can be measured in desert tortoise bone and scute, and to determine overall abundance levels. Two tortoises from the northern Colorado Desert were analyzed: a healthy subadult male (killed on a highway) and a diseased subadult female. Gamma ray counts were performed at <1 hour, and 5, 7, 14, and 60 days after irradiation. At least 33 elements, including many of the potentially toxic metals, can be measured with acceptable accuracy and precision in tortoise bone, scute, or both.

Provisional inferences from these initial analyses are: (1) The more electropositive (“lithophile”) transition elements (Sc, Ti, V, Zr, Hf, Ta, rare earth elements, Th, U) are more readily accommodated in the apatite of bone than in the keratin of scute. (2) Although desert tortoises spend much of their time underground in burrows, the data tentatively suggest minimal elemental input to the scute through contact with soil or bedrock. (3) Trace element abundances in tortoise bone are generally similar to those in human bone; tortoise scute is likewise fairly similar to human nail material. (4) Scute from the diseased tortoise shows relative depletion in most of the elements determined. Whether this apparent difference is an effect or a cause of disease, or simply variation between two individuals, remains to be determined.