Energetic consequences of field body temperatures in the green iguana

INTRODUCTION

Diurnal lizards are well known to precisely regulate their body temperature ([T.sub.b]) during their daily activity period, despite fluctuating ambient temperatures (Cowles and Bogert 1944). Presumably, thermoregulation not only serves to avoid lethal or damaging temperature extremes, but also to maximize the time spent at optimal temperatures (Cowles and Bogert 1944, Dawson 1975). Although the optimal temperatures for many functions fall in the range of selected body temperatures in the field (Dawson 1975, Huey 1982, Stevenson et al. 1985), there are indications that others do not. For example, some reptiles select higher temperatures after feeding (Cowles and Bogert 1944, Schall 1977, Huey 1982), or spend more time basking after feeding (Hammond et al. 1988), suggesting that digestion may require higher temperatures than do other functions.

Superimposed on physiological optima of some functions, physiological and ecological constraints can have a substantial impact on the costs and benefits of maintaining a particular body temperature. Metabolic rate, for example, is related to body temperature (Moberly 1968, Bennett and Dawson 1976), and lizards may sometimes select low temperatures to conserve energy (Regal 1966, Christian et al. 1984).

Many ecologically relevant processes may play a role in balancing the costs and benefits at selected field body temperatures. In thermal studies, properties such as stamina, maximal sustainable speed, or maximum oxygen consumption have received much attention (John-Alder and Bennett 1981, Van Berkum et al. 1986, Huey et al. 1989b), whereas fewer studies have compared other vital processes with selected body temperatures (Huey 1982, Stevenson et al. 1985, Huey et al. 1989a). For example, food digestion may be of crucial importance, especially in herbivorous reptiles, because digestion of plant material is often relatively time consuming. Some studies have indicated that dry matter digestibility is positively related to body temperature (Harlow et al. 1976, Kaufmann and Pough 1982, Troyer 1987). However, other studies that used food intake rates more typical of field conditions have shown a significant influence of body temperature on the transit rate of food through the digestive tract rather than on digestibility (Parmenter 1981, Zimmerman and Tracy 1989, van Marken Lichtenbelt 1992). Although an inverse relation between body temperature and gut passage time has been demonstrated in these studies, the metabolic costs and benefits associated with optimal temperatures for food digestion remain to be investigated and compared to the costs at actual field body temperatures.

Besides the energetic costs and benefits linked to body temperature, it is relevant to know whether or not animals are able to select optimal body temperatures in their habitat. Thermal constraints can arise because the body temperature of an ectotherm is a complex function of its biology and its biophysical environment (Gates 1980). These constraints may be measured by comparing the pattern of thermal microclimate availability with the use of microclimate by lizards (Christian et al. 1983). Operative environmental temperature ([T.sub.e]) has been developed as a thermal index of microclimate (Bakken and Gates 1975, Bakken 1976, 1992, Bakken et al. 1985). [T.sub.e] may be defined as the temperature of an inanimate object of zero heat capacity with the same size, shape, and radiative properties as the animal exposed to the same microclimate. For an ectothermic reptile, with no pelage, [T.sub.e] is the environmental temperature as experienced by the animal, and is an index of the thermal potential driving heat flow. [T.sub.e] can be measured directly for many species of reptiles using taxidermic mounts (Bakken et al. 1985).

We performed a study on green iguanas (Iguana iguana) on the tropical island Curacao. We collected data on [T.sub.e] using taxidermic mounts, and on [T.sub.b] by estimation and telemetry. The following questions were addressed: (1) What are the potentially available field body temperatures for green iguanas? (2) Which are the actually achieved field body temperatures? (3) What relationships might exist between field body temperatures and the metabolic costs and benefits for food processing of those body temperatures?

METHODS

Climate and study site

The study was conducted from August 1987 until April 1988 at Santa Barbara on Curacao, Netherlands Antilles. Curacao has a semiarid climate with strong spatial and seasonal variation in rainfall. Mean annual rainfall is 570 mm, to which the rains in October-January contribute 64%, with considerable year-to-year variation. Monthly mean air temperature is 27.5 [degrees] C, with a minimum of 25.3 [degrees] C in January and a maximum of 30.9 [degrees] C in September. Strong trade winds blow from the east nearly all year round (mean wind speed 7.1 m/s). Seasonality of the rainfall has a strong influence on plant production and, thus, on food for the iguanas, often resulting in a period of low food availability from February to June (van Marken Lichtenbelt 1993). The reproductive cycle is geared to this seasonality (van Marken Lichtenbelt and Albers 1993). The mating season takes place from March to April. Eggs are laid in April-May, and hatching takes place in the second half of July and August.