Aspects of the Thermal Biology of the Texas Tortoise, Gopherus berlandieri (Reptilia, Testudines, Testudinidae)

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Abstract
The thermal biology of G. berlanderi was studied on a free access grid near Laguna Vista, Cameron County, Texas [USA]. Active tortoises had temperatures ranging from 24.1-39.0.degree. C, but 90.8% of these fell between 30-35.degree. C. There was a significant positive correlation between air temperature and body temperature of active tortoises and between substrate temperature and body temperature of inactive tortoises. No correlation existed between size and body temperature. Mean body temperature of tortoises active in spring was significantly lower than mean body temperature of tortoises active in summer. Tortoises in a laboratory thermal gradient had a mean temperature below the field activity mean. Heat stress (as evidenced by frothing) occurred at 37.5.degree. C, but the critical thermal maximum was near 43.7.degree. C. Apparently, tortoises only occasionally encounter heat stress in nature, for only 3 (of 287) temperatures were recorded that approached or exceeded heat stress conditions. G. berlandieri rarely encounters damaging cold in this area.

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