Sri Lanka

This involved designing research projects on reptiles in the Dumbara Mountains (Knuckles Massif) for students from Rajarata University as part of Project Hoona and statistical analysis for a range of ecological data, carried out remotely. The results were published in a monograph by ARROS (Amphibian and Reptile Research Organisation of Sri Lanka) (pdf 52pdf 53).

Students from Rajarata University, John Drake (Edinburgh University – second from left) and myself photographed while crossing the Dumbara Mountains. One side of the mountain range was experiencing the wet season, the other the dry season. The photograph was taken at the peak of the mountain range - the contact zone between weather systems. We are experiencing the high wind effect this phenomenon produces, as air is rapidly moving between zones. The lower photo further illustrates this and also shows two of the students collecting data on Calotes versicolor on the Hettipola side, which is in the dry season. Beyond the mountain peak is the wet season.


Calotes versicolor thermal ecology

Calotes versicolor (above) is a common species from a variety of habitats and presented an opportunity to examine the effects of different altitudes on thermoregulatory behaviour. Although Sri Lanka is situated in the tropics, altitude produces a cooling effect in much the same way as latitude. Reptiles must adjust behaviour accordingly due to the influence of climate on body temperatures. If basking is increased, the time available for other key activities - e.g. feeding, mating and territory defence is reduced. An alternative solution is to make no adjustment to basking intensity and accept lower body temperatures - or operate a compromise between the two. Our research on C. versicolor investigated this problem. The study localities are shown in the photographs below. The top photograph shows the more sparsely vegetated and cooler high altitude site at Pitawala. The lower photograph Hettipola, where in addition to higher temperatures the habitat was more complex, for instance, vegetation was more varied, there were aquatic areas and perhaps importantly an increased number of potential predators, all of which can influence thermoregulatory behaviour.


Monitor Lizards

The water monitor, Varanus salvator is the world’s second largest species of lizard (by body mass) with reports of specimens reaching at least 3 metres in total length. As in most monitors V. salvator is carnivorous, although they consume large amounts of carrion. The males in particular are territorial (pdf 57).

Basking is a critical activity for ectotherms, particularly large individuals or species, and is illustrated here in a large V. salvator basking near the shore of Kandy Lake. Large reptiles due to their low surface to volume geometry, experience slower heating rates than small reptiles. However, large size is an advantage for this lizard when it enters cooler water as as it slows cooling rates. Large size also reduces the number of predators and hence adult V. salvator can bask in the open almost without interruption. Smaller individuals, however, must be vigilant for predators – birds of prey for example.

Water monitor Varanus salvator swimming across a river near Hettipola, during the dry season. This large lizard swam to the opposite bank and began foraging in vegetation for about 10 - 15 minutes, then returned to the river. The behaviour was repeated further upstream. This is a good example of why certain reptiles may not be able to regulate to optimum physiological temperatures, particularly species that frequently enter cooler environments. The need to forage must take priority over precise body temperature regulation.

The lower photograph shows the same lizard a few minutes later in mid-water in typical swimming mode with the limbs held against the sides of the body and the tail used for propulsion.

Thermoregulatory theory considers the body temperatures of reptiles that are active in the field as the `activity temperature range` and those selected in a laboratory heat gradient as the `selected body temperture range` The differences are illustrated in this graph, which shows box plots of body temperatures of V. salvator measured in the field (left side of graph) and laboratory (right side). The rectangular boxes represent the interquartile ranges, vertical lines the general ranges, horizontal lines the medians and circles the means. Sample sizes are indicated above each data set. Field data were derived from individual animals, whilst heat gradient data are from a single V. salvator tracked during a single daily period (pdf 6). The body temperatures from free-living V. salvator were measured at Kandy Lake and are derived from estimates from graphs in the paper by Nimal Rathnayake and his colleagues [2003; Hamadryad vol. 27, pp179 – 184]. Heat gradient data represent body temperatures when they began following a sinusoidal pattern (heat gradient high). The category heat gradient all includes periods of heating and cooling. As can be seen body temperatures in free-living lizards were lower, particularly in those that were swimming (mean = 26.0˚C). Basking or resting lizards had the highest body temperatures (mean  = 29.0˚C) and in good agreement with those that were foraging/active (mean = 28.9˚C). The lizard in the heat gradient selected active body temperatures of up to 37.6˚C (general mean = 34.9˚C). The differences are assumed due to the laboratory heat gradient representing a cost free thermal environment; food and heat are easily accessible and the lizard does not need to forage in what may be suboptimal thermal environments. Body temperatures in free-living V. salvator therefore represent a compromise between physiological optimum temperatures and those imposed by ecological conditions.

Bengal monitor, Varanus bengalensis. This smaller species of monitor lizard (usually about 1 metre but can be much larger) was common around the Hettipola study site. It is largely a terrestrial species although when V. salvator is not present it is more aquatic. We observed foraging and feeding behaviour (pdf 55), which agreed with observations in other areas [Auffenberg, W. (1994), The Bengal Monitor, University of Florida Press]. Free-living Bengal monitors have body temperatures in better agreement with those found in laboratory heat gradients (compared to V. salvator), particularly in areas with high ambient air temperatures and when they only infrequently or never enter water.

The top photographs show an adult at the entrance to its burrow and below the same animal foraging for food. V. bengalensis is a wide-ranging forager and may travel up to 800 meters per week in some areas although rather less in subadults (286 metres). Juveniles are the most sedentary and may remain in the vicinity of a few trees. During cooler weather activity is lower in all groups.


Other Reptiles

Two species of chelonian were found, one terrestrial and one aquatic species. The aquatic species Melanochelys trijuga thermalis, seen here basking at Kandy Lake alongside a water monitor (top) is common in Sri Lanka and found in all types of water bodies including ditches, paddy fields and larger aquatic areas. It is an omnivore feeding on invertebrates including freshwater prawns as well as carrion. The only terrestrial form in Sri Lanka is the herbivorous star tortoise Geochelone elegans (bottom). The specimen in the photograph was found at Hettipola in a scrub/wooded area during the dry season. The star tortoise has been discussed in mathematics paper in relation to its shell design [Varkonyi, P.L., Domokos, G. (2006). Mono-monostatic bodies, the answer to Arnold's question. The Mathematical Intelligencer vol. 28, 34 - 38]. The authors showed that a theoretical object with one unstable and one stable point, when destabilised would always return to the stable position. The star tortoise closely approaches this theoretical shape, with a stable point (the plastron) and unstable point (the apex of the carapace). Presumably this has evolved to facilitate self-righting in the event of being over-turned – do tortoises know about mathematics?

Lyriocephalus scutatus (hump-snout lizard). Endemic to Sri Lanka this is a shade-loving lizard found in forests. The more colourful male is on the left. The mean body temperature from three individuals at a mean altitude of 694 metres was 27.5˚C (pdf 53). It is regarded as endangered primarily due to forest clearance, which may change both light quality and temperature.

Herpetology, Viper

Another endemic Sri Lankan species is the shield snake Pseudotyphlops philippines (top photograph) a subterranean form. They are primitive snakes feeding on invertebrates with some species retaining pelvic girdles. Second from top shows a pit-viper (Hypnale hypnale) located at the top of Lakegala in cool cloud forest conditions (17 – 19˚C) with little sunlight penetration. Presumably there is limited possibility for thermoregulation since conditions where it was found are apparently relatively constant all year. It has a serious bite, the venom causes coagulopathy and renal failure. The bottom two photographs  show a green vine snake Ahaetulla nasuta seen crossing a road in the Dumbara range. This is a rear fanged (opisthoglyphous) snake with moderate venom. It is active diurnally and hunts for lizards and frogs using binocular vision. Females give birth to fully formed young and are believed capable of delayed fertilisation – the female stores the sperm and fertilises the oocyte when conditions are suitable.


  • (2005). First studies of the thermal ecology of Ceratophora tennentii: (Sauria: Agamidae) inhabiting the cloud forests of Knuckles Massif, Sri Lanka. In The Diversity of the Dumbara Mountains: The Knuckles Massif, Sri Lanka: with special reference to its herpetofauna. Lyriocephalus 6 (Vols 1 & 2 ) 65 -71. pdf 52
  • (2005). Altitudinal differences in thermoregulatory behaviour in Calotes versicolor in the Knuckles region, Sri Lanka. In The Diversity of the Dumbara Mountains: The Knuckles Massif, Sri Lanka: with special reference to its herpetofauna. Lyriocephalus 6 (Vols 1 & 2 ) 83 - 93. pdf 53
  • (2006). Natural History Note; Varanus bengalensis (Bengal monitor): unusual behaviour and feeding. Herpetological Bulletin 95, 31 – 32. pdf 55
  • (2007). Natural History Note; Varanus salvator (water monitor): intra-specific aggression. Herpetological Bulletin 101, 35 - 37. pdf 57



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