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

Among desert animals tortoises appear to be uniquely constrained by their simple renal tubules and lack of salt glands, making excretion of electrolytes such as potassium especially demanding. However, many desert plants are high in potassium. Ingested potassium is excreted predominantly as urates and fluid urine, but this also entails substantial excretion of both nitrogen and water. Thus these constituents of food may be particularly important. An index, the potassium excretion potential (PEP), is proposed to account for the relative amounts of potassium, nitrogen and water in foods. A high PEP indicates a surplus of nitrogen and water relative to potassium. Analytical data reveal that: 1) some plant families (e.g., legumes and cacti) typically have high PEP, 2) shrubs usually have negative PEP (i.e. more potassium than can be excreted), 3) PEP varies greatly according to phenological stage, 4) PEP may differ from year to year, likely due to differences in plant water balance, 5) PEP may vary among plants on different soils, 6) PEP in dried, senescent plants (such as grasses) is altered by weathering. The stable isotopes of carbon and nitrogen in plants also warrant investigation. It may prove possible to document historical diet shifts (e.g., from perennial grasses such as bush muhly to annual grasses such as red brome) and to assess the importance of legumes in tortoise diets from the isotope levels in tortoise tissues, including museum specimens. This may prove helpful in assessing past and present impacts of livestock grazing on tortoise populations.