27th Annual Meeting and Symposium of the
Desert Tortoise Council, March 22-24, 2002
Abstracts

Can Juvenile Tortoises Obtain High PEP Forage Throughout The Spring?

Olav Oftedal¹, Scott Hillard², Lisa Hazard³, Terry Christopher¹ and David Morafka^2
1Department of Conservation Biology, Smithsonian National Zoological Park, Washington DC; 
²Department of Organismic Biology, Ecology and Evolution, University of California, Los Angeles;
³Department of Biology, California State University, Dominguez Hills, Carson CA

A broad array of annual plants germinate in response to winter rains in the Mojave Desert, providing juvenile tortoises the opportunity to be selective in feeding during the spring. In an El Nino year (1998) we observed that tortoises were very selective in both the species and parts of plants eaten, and by this means were able to ingest a high quality diet. Diet quality was measured as the relative amounts of water, nitrogen and potassium in the diet, and was expressed as the Potassium Excretion Potential (PEP) index. Based on these results, we predict that juvenile tortoises will self-select a high PEP diet whenever they have the opportunity.

To test this prediction, and to determine if the ability to self-select a high PEP diet is dependent on the time of season, we conducted a study of foraging juveniles in 2001. Fifteen juvenile tortoises in a naturally vegetated pen at the Fort Irwin Study Site (FISS, National Training Center, California) were observed over 3 one-week periods (I: April 8-14, II: April 30-May 6, III: May 19-24). All plants that foraging tortoises encountered within 1 body width to each side (except the abundant alien grass Schismus) were recorded. All effective bites of plants were recorded according to species and part. More than 33,000 bites were observed over periods I-III, including about 700 bites of non-food items (soil, gravel, bark, scats).

The major food plants (>1% of bites in a period) were as follows: Period I, evening primrose (Camissonia claviformis, 43%), desert dandelion (Malacothrix glabrata, 40%), filaree (Erodium cicutarium, 10%) and split grass (Schismus barbatus, 1.5%); Period II, filaree (66%), evening primrose (9.9%), desert dandelion (8.4%), cryptanth (Cryptantha angustifolia, 3.6%), split grass (3.3%), woolly plantain (Plantago ovata, 2.4%) and desert pincushion (Chaenactis fremontii, 2.2%); Period III, filaree (80.5%), cryptanth (8.7%), split grass (8.1%) and big galleta (Pleuraphis rigida, 1.4%).

The primary plant part eaten in all periods was leaves (92% of bites in I, 47% in II, 74% in III), but fruit (of filaree, primrose and dandelion) became very important in period II, accounting for 37% of all bites. The change in diet from a preponderance of native annuals (I) to a preponderance of introduced filaree (III) appeared to reflect changes in phenology: both primrose and dandelion matured and senesced over the course of this study, but filaree retained some green leaves and unripe fruit until the end of III.

Although we have yet to complete all nutritional analyses for this study, we estimated the nutrient composition of the ingested diet using nutritional data on these plant species and parts from the same site in 1998. The estimated PEP index dropped from >20 in I, to about 13 in II and about 10 in III. The very high PEP index in early April (I) was attributable to the high PEP values of the two major parts eaten, evening primrose leaves (42% of bites) and desert dandelion leaves (36% of bites). By early May, these parts were becoming scarce and by late May were no longer available, and tortoises had switched to other parts that were mostly lower in PEP.

A high PEP diet provides surplus protein and water beyond the needs for potassium excretion, and thus may be very important for growth of shell, muscle and other protein-containing organs. Growth is believed to be important for juvenile survival for at least two reasons: 1. An increase in shell size and strength may deter predation by ravens, and 2. Muscle and internal organs represent nutrient reservoirs, which may need to be drawn upon during prolonged food shortage, as during drought. Our study indicates that juvenile tortoises were very successful in acquiring high PEP foods in mid-spring (April) but less so in late spring (May). Given that annual plants in early phenological stages in February and March appear to have similar or higher PEP values than in April (Oftedal, in press), we predict that the entire early to mid spring period is characterized by high PEP intakes in years of good rainfall.

This study provides an additional explanation why it may be important for juvenile tortoises to become active early in the spring. Juvenile tortoises at FISS are often observed above ground during warmer and sunny days as early as January and early February, although the extent of foraging on rosettes and early emergent parts of winter annuals has not been determined. There would appear to be selective pressures favoring early foraging on high PEP parts, so long as basking behavior allows acquisition of body temperatures favorable to digestive and renal processing of plant nutrients.

Literature Cited

Oftedal, O. T. 2002. The nutritional ecology of the desert tortoise in the Mohave and Sonoran deserts. In Van Devender, T. R. The Sonoran Desert Tortoise. Natural History, Biology and Conservation. University of Arizona Press, Tucson, AZ. In press.