(Erioneuron pilosum and Hilaria rigida), calcium 

 (Stipa hymenoides and Hilaria rigida), and sulfur 

 (Stipa hymenoides) (table 6). Low values of ADF 

 were found in Stipa hymenoides and Hilaria rigida. 



Harper and Pendleton (1993) recently presented data 

 indicating that cryptobiotic crusts enhance the nutri- 

 ent and mineral status of plants in desert habitats. 

 They suggested that the associated animals, includ- 

 ing desert tortoises, may benefit fi*om the cryptobiotic 

 crust nutrient input. We did not examine this aspect 

 but did note that cryptogams are present at our study 

 sites (McArthur and Sanderson 1992a). 



Bostick (1990) suggested that desert tortoises evolved 

 as dung eaters, presenting anecdotal evidence that cow 

 excrement was a useful food for tortoises. Our data 

 included only current-year cow excrement (n = 11 fi-om 

 2 years, three seasons, and two Beaver Dam Slope 

 sites). Cow excrement ranked 21st, last, for several 

 important parameters when compared to the 20 spe- 

 cies in appendix B. These were for moisture content 

 (14.00 percent, cow excrement dries out quickly), po- 

 tassium (0.28 percent), TNC (5.15 percent), and fat 

 (3.87 percent). Cow excrement ranked first in ADF 

 (51.48 percent), a negative food quality characteristic. 

 Cow excrement ranked 12th for nitrogen (1.53 per- 

 cent - 9.56 percent protein), ahead of perennial grasses 

 and cacti, but behind most other species. Cow excre- 

 ment generally retained or concentrated high levels 

 of minerals: phosphorus (first, 0.23 percent), zinc (first, 

 30 ppm), iron (first, 3,400 ppm), manganese (first, 

 150 ppm), copper (first, 10 ppm), calcium (third, 3.37 

 percent), sodium (third, 0,037 percent), magnesium 

 (fourth, 0.59 percent), and sulfur (fifth, 0.45 percent). 

 On the whole, our data do not demonstrate that cow ex- 

 crement is a quality food source. We question Bostick's 

 (1990) thesis that desert tortoises evolved as dung 

 eaters. It is, however, beyond question that tortoises 

 ingest a diverse array of items, including insects, soil, 

 bones, feathers, and excrement (Esque and others 1991; 

 Hansen and others 1976). 



The data presented here can be compared with other 

 mineral and nutrient data for plants in semiarid envi- 

 ronments and can be extrapolated to determine the 

 adequacy of diets of desert tortoises and other herbi- 

 vores to the extent requirements for those animals 

 are known (Cook and Harris 1950; Dietz and others 

 1962; Nagy and Medica 1986; National Academy of 

 Sciences 1975, 1977, 1984). 



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