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Oftedal, O. T., Hillard, S., & Morafka, D. J. (2002). Selective spring foraging by juvenile desert tortoise (gopherus agassizii) in the mojave desert: evidence of an adaptive nutritional strategy. Chelonian Conservation and Biology, 4(2), 341–352.
Added by: Admin (14 Aug 2008 20:35:30 UTC) |
| Resource type: Journal Article BibTeX citation key: Oftedal2002 View all bibliographic details  |
Categories: General Keywords: Ernährung = nutrition, Gopherus, Gopherus agassizii, Habitat = habitat, Nordamerika = North America, Schildkröten = turtles + tortoises, Testudinidae Creators: Hillard, Morafka, Oftedal Collection: Chelonian Conservation and Biology |
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| Abstract |
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Gopherus agassizii Testudinidae Desert tortoises (Gopherus agassizii) are known to be selective while foraging, but the nutritional consequences have not been examined. Due to the burden of potassium excretion, which entails loss of water and/or nitrogen, we predicted that desert tortoises would select plants that were high in water, protein, and an index of potassium excretion potential (PEP index), but low in potassium. The foraging behavior of 15 juvenile tortoises was studied in relation to the nutrient composition of annual plants in a naturally vegetated enclosure at the Ft. Irwin National Training Center, San Bernardino Co., California. The study was conducted during an El Nino year, when large numbers of annual species germinated. The numbers of plants eaten, the numbers of bites taken, and the number of plants bypassed while foraging were recorded. The numbers of bites per foraging session differed not only among plant species but also among plant parts. On average, the plants tortoises ate were higher in water, protein, and PEP, but not lower in potassium, than the plants they bypassed while foraging. Part of this difference was due to the low consumption of split grass (Schismus spp.) which accounted for about 86% of the biomass along the foraging routes. If split grass was omitted from the comparison, the plants eaten by tortoises were higher in protein and PEP (but not in water) and lower in potassium than the plants bypassed. Tortoises were selective in the parts of plants eaten; leaves accounting for more than 70% of all bites. In four primary food species, the parts eaten were higher in water, protein, and PEP, and lower in potassium, than the uneaten parts of these species. As a consequence of this selectivity the ingested diet of tortoises (weighted by numbers of bites per part) had an average PEP index of 15, which was very different from the mean value of 1.4 (weighted by proportion of plant biomass) for all species encountered. We conclude that in a year of abundant plant germination juvenile tortoises are able to self-select a diet of high nutritional quality, but this depends on access to species with high PEP parts, such as brown-eyed primrose (Camissonia claviformis) and desert dandelion (Malacothrix glabrata). If habitat is to be managed for the recovery of threatened tortoise populations, it is important to consider the impact of livestock grazing and other uses not only on the overall biomass of food resources, but on the nutritional quality of those resources.
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