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29th Annual Meeting and Symposium of the
Desert Tortoise Council, February 20-23, 2004
Abstracts
Sonoran Tortoises Foraging After Summer Rains Consume Low PEP Diets
Olav T. Oftedal1, Roy Averill-Murray2, Terry
Christopher1, Daren Riedle2 and Michael Jarcho1
1Department of Conservation Biology, National Zoological Park,
Smithsonian Institution, Washington DC
2Non-Game Branch, Arizona Game and Fish Department, Phoenix, AZ
The opportunity to drink rain water permits the desert tortoise to rid the body of excess potassium via fluid urine. In such circumstances, tortoises do not need to degrade protein to produce potassium urates, so that the Potassium Excretion Potential [PEP] index of food plants becomes less important. We hypothesized that Sonoran desert tortoises that foraged after summer rains would not need to be selective with regard to the PEP value of food plants, and could consume a low PEP diet without adverse effect.
The foraging behavior of 6 adult radiotelemetered tortoises was observed August 11-18, 2002, at Ragged Top, Ironwood Forest National Monument, Pima County, Arizona. Six tortoises were observed to take more than 18,500 bites of 2342 plants representing 31 identified plant species. The diet was comprised of 51.3% bites of grasses (annual and perennial), 25.8% bites of summer annual forbs, 19.8% perennials (including shrubs and vines), and 3.2% bites of senescent material and bone. The top 5 plants eaten were a dual-season annual grass (6-weeks three-awn, Aristida adscensionis, 33.1% of bites), a euphorb subshrub (Ditaxis lanceolata, 12.3%), a summer annual euphorb (Chamaesyce florida, 11.0%), summer annual grama grass (Bouteloua spp., 9.8%), and a summer annual spiderling (Boerhavia intermedia, 6.5%). Two to six samples were collected of each food plant species (n=29) for analysis of water, protein (TN x 6.25) and potassium, from which the PEP Index was calculated. Plants of high PEP indices (PEP = 10-17) comprised 20% of bites, but only a few species appeared to be preferred relative to abundance along foraging paths. Plants with negative PEP indices comprised a substantial portion (11.4%) of bites. The overall diet, weighted by number of bites per plant species, was calculated to contain 61% water, 15% protein, 2.5% potassium and a PEP index of 1.1. By comparison to the spring diet of juvenile tortoises in the central Mojave Desert, this diet was low in water (61 vs. 72%), high in protein (15 vs. 11%), very high in potassium (2.5 vs. 1.4%), and very low in PEP (1.1 vs. 15). These data are consistent with the hypothesis that tortoises foraging after summer rains do not need to seek out high PEP plants as they have sufficient water available for excretion of excess potassium.
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