Thermoregulatory Influences on the Ecology of Two Sympatric Varanids in Sri Lanka

Abstract

We studied the interaction between thermoregulatory behavior and ecology of two species of sympatric varanids, Varanus salvator and Varanus bengalensis, in south Sri Lanka. V. salvator was active by early morning but sought shade in the afternoon; any afternoon activity was in an aquatic environment. Although the regression of V. salvator cloacal temperatures on daytime ambient temperatures is significant (P = 0.02) and indicate thermoconformity, they maintain a relatively stable body temperature behaviorally by selecting appropriate thermal microhabitats. During the night they sought stable thermal microhabitats under dense bushes, thickets, and even in water (which is warmer than the night air temperature). This presumably enabled them to be active early the following morning. V. bengalensis commenced activity later in the day and tend to elevate body temperatures (P < 0.01) by basking. They also forage in the open thus thermoregulating while feeding. By evening they retreat into burrows and other refugia where body temperatures drop below air temperatures. Thus, both species tend to thermoregulate behaviorally by selecting appropriate thermal microhabitats and exhibit temporal partitioning of activity times. THERMOREGULATION IN LIZARDS has been extensively studied and documented (see reviews by Avery 1982 and Huey 1982); however, the varanids, despite being the largest lizards, have received comparatively little attention (Avery 1982). Although some studies have documented varanid body temperatures and, to some extent, thermal behavior (Anderson 1963, Stebbins & Barwick 1968, Pianka 1969, Pianka & Pianka 1970, McNab & Auffenberg 1976, King 1980), they have concentrated on allopatric situations or on the autecology of species. However, thermoregulatory behavior is known to be influenced by competitors and predators (DeWitt 1967, Regal & Connolly 1980), food (Swingland & Frazier 1979), time of day (Regal 1967), and climatic conditions (Licht et al. 1966). Thus, interspecific differences in thermoregulatory behavior of sympatric lizards can be related to activity times, foraging behavior, habitat and microhabitat choice, and other ecological factors (Pianka & Pianka 1970, Lee 1980). Varanus salvator is a large, semiaquatic lizard commonly found in close proximity to water. It is a good swimmer (Smith 1932), and Auffenberg (1981) states that a major factor determining their distribution is availability of aquatic habitat. The smaller Varanus bengalensis is primarily terrestrial and rarely takes to water. It is also the more active of the two species (Dryden et al., pers. comm.). Both species are fairly common and are sympatric throughout most of their range in Sri Lanka. In this study we compare the thermal biology of these two species of varanids and relate it to their activity patterns, foraging strategies, and habitat and microhabitat choice. STUDY AREA AND METHODS The study was conducted at Ambalantota on the south coast of Sri Lanka, and the study area consisted of the peninsular strip formed by the sans serif L bend in the Walawe River and the mainland bordering the river (Fig. 1). The peninsula is a flat meadow with mangroves and Pandanus bushes along most of the river bank and dense scrub thickets adjacent to dunes on the sea side. The adjacent mainland is a mosaic of small villages, paddy fields, cocoanut plantations, scrub jungle, and meadows. The study area is in the dry zone (Fig. 1) where the climate is hot and arid with little rainfall throughout the year. Observations of lizards took place during March, April, May, and August of 1986. Some observations and temperatures were of lizards outfitted with radio transmitters to facilitate recaptures for a study on metabolic rates of free-ranging varanids. This helped in obtaining behavioral and temperature data on lizards that had sought shelter and were otherwise difficult to locate. Animals were captured and cloacal and ambient air temperature (in the open) were taken immediately with a Schultheis thermometer. When animals were found on the ground, ambient temperatures were taken at ground level, and when found above ground, ambient temperatures were measured approximately 1 m above ground level. Weights were taken to the nearest 10 g, and snoutvent lengths (SVL) were recorded. I Received 16 March 1987, revision accepted 31 May 1987. 74 BIOTROPICA 21(1): 74-79 1989 This content downloaded from 157.55.39.217 on Mon, 18 Apr 2016 07:36:03 UTC All use subject to http://about.jstor.org/terms