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We describe a method for measuring metabolic rates (MRs) of ectotherms using successive measurements of radionuclide body burdens, which may provide an alternative means of measuring field metabolic rate (FMR). Although MRs can be measured in the laboratory, variability in food supply and quality, temperature, activity, and other factors preclude the direct application of such data to field conditions. Recently, the doubly labeled water (DLW) technique has been successfully applied to estimate FMR in a variety of animals, but this method is expensive, requires special equipment, necessitates the sampling of blood or other body fluids, and may be unsuitable for certain species. We compared the rates of elimination of seven radionuclides with estimates of MRs (CO₂ production) from DLW measurements in the desert iguana, Dipsosaurus dorsalis. We observed temperature-dependent elimination for five of the radionuclides, and found a high correlation (r² = 0.93) between the slopes of individual $^{86}Rb$ eliminat...

Eggs of painted turtles (Chrysemys picta) absorb water across that portion of the eggshell in contact with the substrate and simultaneously lose water by transpiration from that part of the eggshell exposed to air inside the nest chamber. Depending upon the rates of influx and efflux, eggs may experience increases or decreases in mass during incubation, or mass may remain essentially constant between oviposition and hatching. Water exchanges are especially sensitive to such factors as substrate water potential, relative surface exposed to the nest atmosphere, hydraulic conductance of the eggshell, and conductance of the eggshell to water vapor. Hatchlings emerging from eggs absorbing and storing large quantities of water are heavier than hatchlings emerging from eggs taking up smaller quantities of water from the substrate. Furthermore, water absorption equal to, or in excess of, water loss by transpiration assures that the original shape of the egg will be preserved, thereby guaranteeing that sufficient ...

The effectiveness of thermoregulatory postures in the black dragon (Hagenius brevistylus Selys; Odonata) was tested relative to wind speed and air temperature on live and dead dragonflies in a closed circuit wind tunnel. The postures appeared to allow the dragonflies to behaviorally select body temperatures exceeding a range of 20 C and reach temperatures as low as the ambient air or as high as the sun-warmed substrate. At least one posture appeared to be transitory and appeared to be used primarily to warm up from low body temperatures. Live dragonflies kept individually in a large flight cage showed different patterns of posturing depending on the amount of insolation. Free-ranging dragonflies were not active at low air temperatures, and postured differently in response to different substrate temperatures. We concluded that black dragons can select from a broad range of potential body temperatures as the result of behavioral posturing.

Packard and Boardman (1987, 1988) have provided timely advice on how analyses of ratios of ecophysiological data (e.g., percentages, mass-specific physiological data) can lead to incorrect conclusions. They have illustrated how attempts to increase precision of measurements or attempts to remove confounding effects of body size in ecophysiological measures by using ratios can yield "erroneous conclusions" and have recommended that "ecological physiologists discontinue using ratios to scale data and . . use the ANCOVA [analysis of covariance] instead." While analyses of ratios can be misleading, it is also possible (a) that ANCOVA can lead to erroneous conclusions and (b) that ratios have properties pertinent to biology. In this paper, we review examples in which ANCOVA can lead to incorrect conclusions, as an extension of the cautionary message of Packard and Boardman, and we present an example of ratios that have biological meaning separate from the meaning of the components of the ratios. Packard and Boardman (1988) presented data from a study in which two groups of five eggs of painted turtles (Chrysemyspicta) were incubated in different hydric environments. The hatchlings from these eggs were subsequently killed, and the carcasses (less residual yolk) were dried to a constant mass. They then asked the question, "[Did the] different conditions of incubation affect the level of hydration of tissues in hatchlings?" They performed an analysis of variance (ANOVA) on the masses of body water divided by the wet masses of the carcasses (i.e., percent hydration) and found a significant difference in the percent hydration of each experimental group (F[1,8] = 16.35, P < 0.01). However, they found that the amount of water in the

A mathematical model consisting of semiempirical equations was used to calculate boundary-layer conductances for transport of water vapor from bird eggs in free convection regimes. The model accurately predicts the boundary-layer conductance of model "eggs" fabricated from 2% agar. Predictions based on the mathematical model show that bird eggs smaller than about 10 kg have boundary-layer conductances greater than 10 times the total (boundary-layer, cuticle, shell, and membrane) conductance of the eggs. Thus, it can be inferred that boundary layers probably never constitute a significant barrier to loss of water from eggs of birds.

Eared grebes (Podiceps nigricollis) and American coots (Fulica americana) lay eggs in nests that float on water. Eggs of most grebes come into contact with liquid water during incubation, whereas those of coots do not. The rate of change in the mass of grebe eggs incubated in natural nests was not significantly different from that of coot eggs, even though the water-vapor conductance of grebe eggs is much greater. When eggs of coots and of grey Leghorn chickens (Gallus gallus) were incubated in contact with liquid water in the laboratory, they increased in mass, whereas grebe eggs, incubated under the same conditions, decreased in mass. When eggs of grebes and chickens were placed in contact with a solution of methylene blue, the contents of the chicken eggs were stained blue, whereas the contents of the grebe eggs were not stained. When chicken eggs were incubated in nests of eared grebes, they increased in mass and rotted; eared grebe eggs in the same nests declined in mass and did not rot. These data s...

Small American alligators (200-400 g) were heated and cooled from 20 to 35 to 20 C in a temperature-controlled wind tunnel. Blood flow to the limbs and tail was periodically occluded, using remotely controlled occlusion cuffs. When blood flow in all appendages was occluded, the rate of heating was significantly reduced from that when blood flow was unrestricted. The rate of cooling was not significantly affected by occlusion of blood flow in all appendages, however. Blood flow to appendages, rather than a generalized perfusion of the skin, appears to be primarily responsible for the differences in the rates of heating and cooling in alligators.

Herbivorous reptiles are a comparatively rare group of animals, likely in part because of constraints resulting from interactions between ectothermy and herbivory. Herbivorous lizards, as a group, are characterized by a unique set of morphological, anatomical, and physiological properties. Additionally, most herbivorous lizards are found in (and perhaps partially confined to) the tropics. The most northerly distributed herbivorous lizards, the chuckwalla (Sauromalus obsesus) and the desert iguana (Dipsosaurus dorsalis), occur only in very hot regions of the desert southwestern United States. Herbivorous lizards digest their food much less efficiently than nonherbivorous lizards, and digestive efficiency is affected little by body temperature. However, the rate at which food passes through the gut is affected by body temperature in chuckwallas; specifically, food passes through the gut more slowly at lower body temperatures. Throughout the activity season, free-ranging chuckwallas exploit microhabitats all...