Precision of Thermoregulation and Its Relation to Environmental Factors in the Desert Iguana, Dipsosaurus dorsalis

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Abstract
Desert iguanas exercise control over their body temperature by appropriate adjustment of their position in gradients of temperature. This control was analyzed through the use of techniques for continuous recording of body, soil, and air temperature. The preferred level of body temperature, defined as the median of the cumulative frequency distribution of body temperature, is 38.5[degree]C for desert iguanas in a thermal gradient in which substrate temperatures ranged from 30 to 50[degree] C. The precision of thermoregulation under these conditions is sufficient to restrict the central 50, 68, and 95 percentages of body temperatures to ranges of 37.0 to 39.5, 36.1 to 40.1, and 33.2 to 41.8[degree]C, respectively. The negative skewness of the distribution, evident in these figures, is significant and may reflect a physiological characteristic of the system for thermoregulation employed by lizards. Under natural conditions of the desert in Riverside County, California, temperatures quickly reach levels too high to permit regulation of body temperatures at the preferred level on or above the surface of the ground. Desert iguanas do not immediately retreat to burrows under these conditions, but stay above ground until they become heated to 43-44[degree]C. On the hottest summer days, this tolerance of temperatures above the preferred range may increase the time suitable for activity above ground from 0.5 to about 3 hr. This behavior obviously prolongs the period in which feeding and other activities can be conducted, and, as such, may be one of the major factors permitting occupation of hot deserts by this lizard. Body temperatures during territorial fighting behavior and in the presence of predators may rise to 46-47[degree]C.