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

Juvenile Desert Tortoises Enhance Nutritional Status by Foraging Choice: Data from the 1998 El Niño at the Fort Irwin Study Site

Olav T. Oftedal¹, Scott Hillard² and David Morafka^2
1National Zoological Park, Smithsonian Institution, Washington D.C. 20008 and;
²California State University, Dominguez Hills, Carson, CA 90747

Potassium has a profound impact on the water and nitrogen (N) economies of tortoises because absorbed potassium (K) must be excreted either in fluid urine or as potassium urates. This provides a strong incentive for tortoises to select foods low in K and/or high in water and N (as protein). In captivity, well-hydrated tortoises avoid dietary K in food choice trails but do not favor dietary N. It is not known if tortoises exhibit similar choices in the wild. A two-week observational study was initiated in April 1998 to monitor foraging behavior of juvenile tortoises (carapace length 68-98 mm, body mass 73-211 g; n = 15) in a predator-resistant enclosure at the U.S. Army National Training Center, Fort Irwin, CA. Following fall and winter precipitation of 14.7 cm, 27 species of annual plants were observed in the enclosure. Samples of these species were collected adjacent to the enclosure. Of the plant species selected by juveniles, eaten parts and uneaten parts were collected separately. Samples were frozen, dried to constant weight at 50oC to determine water content, analyzed for N in a CHN elemental analyzer and assayed for K by atomic absorption spectroscopy following acid digestion.

Individual tortoises were followed for a total of 33.2 hrs during which time more than 5,000 bites were observed and recorded by plant species and part. Tortoises took bites from 10% of the individual plants encountered (defined as those passed within one body width on either side, excluding Schismus barbatus which was too abundant to count). The primary species ingested were Camissonia claviformis (46.2% of bites), Plantago ovata (20.4%), Erodium cicutarium (18.7%), Malacothrix glabrata (5.0%), Cryptantha angustifolia (4.8%), Schismus barbatus (2.2%), and Chaenactis fremontii (1.2%); other species collectively accounted for 1.5% of bites. Leaves were the primary part ingested of all species except Schismus. Reproductive structures that included immature fruit and/or flowers were the primary eaten part of Schismus, and a major eaten part of Plantago, Erodium and Cryptantha.

The plants encountered contained on average (weighted for abundance and mass) about 66% water, 7.6 % crude protein (CP = N x 6.25; dry matter [DM] basis, and 1.5% K (DM basis). The average potassium excretion potential (PEP; an index of the relative amounts of water and protein compared to potassium) was rather low (about 5 g/kg DM). However, the diet as consumed was very different, containing a PEP of 15 even though the highest whole-plant PEP was only 13 (Camissonia). This was attributable to the propensity of tortoises to select high PEP plant parts. For example, Camissonia leaves, which accounted for 42% of all bites, had a PEP of 20. The ingested diet contained on average 72% water, 11% CP and 1.4% K; tortoises did not need to avoid K to achieve high PEP. We conclude that juvenile tortoises are able to enhance their nutritional status by dietary selection, at least in a wet year when a wide variety of annual plants germinate and grow, including high PEP species.