Running head: THERMOREGULATION AND LEARNING IN PYTHONS
Thermoregulation and Learning in Coastal Carpet Pythons,
Morelia spilota mcdowelli.
Lois Switzer and Bryan Hummel
Reptiles are mostly ectothermic, meaning that the heat exchanged with the environment is more significant in regulating body temperature than metabolic heat production. To maintain an adequate body temperature, ectotherms must thermoregulate. This is accomplished by basking in the sun or lying on or under a sun-warmed object (Manning and Grigg, 1997). It has been well documented that reptiles thermoregulate in the wild, but by placing animals in a thermal gradient and monitoring their preferred body temperatures, one can demonstrate behavioral temperature regulation and learning behavior in a laboratory. To establish whether or not Coastal Carpet Pythons learn, a “shuttle box” type structure was constructed with four chambers. By timing the rate at which the snakes move from a chamber below their “set-point range” to a chamber at or above this range and comparing the trials over time, one could plot out a learning curve. The results indicated that there was not a significant correlation between the rate and trial number. This finding is congruent with similar experiments under natural settings.
Thermoregulation and Learning in Coastal Carpet Pythons,
Carpet pythons are indigenous to the eastern third of the Australian continent. There are six species (sub-species according to some) that often overlap in geographical range and sometimes results in areas of natural integration between neighboring species. The species used for this experiment was the Coastal Carpet Python, Morelia spilota mcdowelli. The Coastal Carpet Python has the largest range of all the carpet pythons, ranging from the northern tip of the Cape York Peninsula in Queensland down to the Coffs Harbour, New South Wales. It is also the largest of the carpet pythons reaching lengths of up to 14 feet (4.27 meters), but usually not more than 9 feet (2.74 meters) (Barker, 1994).
Reptiles are mostly ectothermic, meaning that the heat exchanged with the environment is more significant in regulating body temperature than metabolic heat production. Many species of python, including all species of carpet pythons, are able to raise their body temperature by intense muscular activity. These species could be classified as temporal heterotherms, and they use a rapid cycle of muscle contractions during egg incubation. Although these snakes can raise their body temperature up to 13°C, this type of heat production is not typically used to raise the body temperatures of snakes that are not incubating eggs (Randall et al., 1997). To maintain an adequate body temperature, ectotherms must thermoregulate. The term thermoregulation necessarily implies an active regulatory process involving behavioral and/or physiological adjustments to maintain a body temperature as close as possible to a certain “set-point range” (Hertz et al., 1993).
Careful thermoregulation increases the amount of time that an ectotherm spends at a physiologically favorable body temperature, and reduces the possibility of exposure to a potentially lethal body temperature (Huey et al., 1989; Diaz et al., 1996). To raise their body temperature, reptiles bask in the sun or lay on or under a sun-warmed object (Manning and Grigg, 1997). The blood circulation past these warmed surface tissues then redistributes this warmed blood to the cooler internal parts of the body to achieve a certain “core” temperature (Turner, 1987). This core temperature is kept within a certain “set-point range” by the same mechanism in reptiles as in mammalian and avian organisms. These animals have a temperature-sensitive center in the hypothalamus. To briefly summarize its functioning, a reptile has a thermophilic response to a cooling hypothalamus and a thermophobic behavior to a warming hypothalamus (Randall et al., 1997).
It has been well documented that reptiles thermoregulate in the wild, but good data is lacking for numerous species of reptiles. Raymond B. Huey stated in 1989, “A study of the behavioral aspects of retreat-site selection would be very worthwhile. In a controlled laboratory setting (or field enclosure), snakes could be offered a choice of retreats differing in thermal properties” (p. 942). By placing animals in a thermal gradient and monitoring their preferred body temperatures, one can demonstrate behavioral temperature regulation. To establish whether or not Coastal Carpet Pythons learn, a “shuttle box” type structure was constructed with four chambers. Chamber 1 was at or above preferred body temperature, while 2 and 4 were below preferred body temperature, and chamber 3 was the coolest and contained the water source. By timing the rate at which the snakes move from a chamber below their “set-point range” to a chamber at or above this range and comparing the trials over time, one could plot out a learning curve. The null hypothesis for this experiment is that the rate which snakes move to the warmer chamber will not vary significantly over the number of trials, therefore implying that snakes do not learn.
The animals used were Coastal Carpet Pythons (Morelia spilota mcdowelli) hatched in captivity in July 1998, by the authors of Pythons of the World, Australia. This particular lineage purportedly descended from individuals near Port Douglas, Queensland. They were 21 months old during the experiment and had been housed together since a few weeks of age. It has been documented that there are slight temperature variations in the nocturnal versus diurnal cycles of many reptiles. Snakes occasionally selected cooler habitats at night (Peterson, 1987; Grant, 1990). The preferred temperature range of the Diamond Python (Morelia spilota), a closely related species, is documented to be an average of 29.4°C in the daytime, and 25.5°C at night (Chiras, 2000). Also in Diamond Pythons, only one basking period was observed each day in the wild (Slip and Shine, 1988).
A shuttle box apparatus was constructed from four Sterilite© 15-quart clear plastic storage containers (plastic shoeboxes). A 16-cm by 16-cm square was cut out of each lid offset to one side. This square was covered with metal window screen, and siliconed into place. Then, two 3-inch holes were melted through the other third of each lid (with an empty can of beans heated on the stove). A 2-foot section of 4-inch flexible vinyl drier tubing was used to attach each lid to two others. The final product was X-shaped, and the snakes could make one big circle through each of the four chambers. Chamber I was heated by a 100-watt ceramic heat emitter (non-light-emitting heat lamp) from Zoo Med Laboratories, Inc. suspended in a clamping aluminum basking dome approximately 2 feet from the cage floor. The floors of the chambers were covered with Hyponex © Cypress Mulch, which was consistent with what had been in the snakes’ enclosure prior to the experiment (refer to picture 1). To monitor the snakes’ temperatures during the experiment, a hand-held, non-contact infrared thermometer was used. The Raytek © gun, model number RAYST2UX is accurate to one tenth of a degree Celsius.
The method was changed throughout the project because the snakes were not really moving towards the heat consistently. The first step was to familiarize the snakes with their new enclosure. They were placed in the newly constructed “shuttle-box” apparatus for three days without heat. In the evening of the third day, the heat was turned on. The snakes went to the heat as expected. After the second trial however the female went to the heat, but the male did not. The female was coiled up in a vinyl tube near the heat, and it was decided that running two snakes at the same time could alter the results; i.e. the female blocked the male from getting to the heat. The female was removed, and the male was run alone for three days. During this time, he did not consistently go towards the heat stimulus, and the female was reintroduced to chamber I under the heat lamp as a second stimulus. The male did not seem to respond to this either, so the method was altered yet again. It was decided that because so much body temperature data was being gathered, and so little movement toward the heat source was being observed, the snakes should not be disturbed and their desired temperatures would be recorded every two hours to evaluate the extent to which these animals exploited their thermal environment (Christian and Weavers, 1996). In addition, the temperature of the heated cage, chamber 1, was also recorded. By comparing the body temperatures with the environmental temperatures, this is a common approach to the study of thermal relationships of reptiles (Gregory, 1984).
Out of the 178 hours of data collection, no quantifiable relationship could be established on the learning behaviors of Coastal Carpet Pythons. The fact that they so rapidly sought out the heat source during the first two trials, but then seemed indifferent, is likely due to external factors. Figures I and II are graphs of the snakes body temperatures versus time. For the male, the average body temperature was 24.56°C with a standard deviation of 1.98°C (see Figure I), while the average body temperature for the female was 25.86°C with a standard deviation of 2.91°C (see Figure I).
Because the snakes did not actively seek the heat stimulus (after the first two trials), no quantifiable data could be established on the learning behaviors of Coastal Carpet Pythons. The fact that they so rapidly sought out the heat source during the first two trials (1 hour and 1.25 hours for trial one), but then seemed indifferent, led the experimenters to believe that there may be external factors influencing their behavior. The fact that they had eaten 1 week prior to the experiment was thought to be sufficient time to completely digest any remaining food, but that becomes questionable when the data is reviewed. During the three days the snakes were adjusting to the new environment without heat, they may have had some food left in their digestive tracts. If this were the case, they would have liked to be at a warmer temperature more conducive for digestion, and therefore, their movements to the heated chamber would be rapid as soon as the heat is sensed by their thermoreceptors present in the supralabials and infralabials. These thermoreceptors are sensitive to temperature differences of as little as 0.002°C in rattlesnakes (Crotalus viridis) (Randall et al., 1997), and most python species are likely to be as sensitive. This is assumed because while pit vipers have two facial pits, Coastal Carpet Pythons have 20 or 22 (Barker, 1994). No research on the sensitivity of python thermoreceptors could be found. It would then follow that the drive to achieve the “set-point” range may diminish when the heat is no longer needed for digestion and this is why the heat stimulus remained unused. Because these snakes are ambush hunters, perhaps it was simply not energetically economical for the snakes to move to the heat source (Bowker, 1984).
This theory also leaves out the fact that in their home cage the snakes were often found around 30°C. If this is their preferred body temperature in their “home” enclosure, why should it seemingly be lower in the experimental apparatus when a heat source is provided and occasionally used? The data, or lack there of, leads one to accept the null hypothesis for the time being.
There are other variables that can be controlled in future experiments, which might yield more consistent results. These include always testing the animals during a certain time interval after feeding, having better controls over the chamber temperatures and/or figuring out a way to prevent the snakes from finding their preferred temperature in the vinyl tubing connecting the cages together. These pythons have strong arboreal tendencies and this may be the reason that they spent so much time in the tubes off the ground (Barker, 1999). The two temperatures related improvements might be accomplished by having strict temperature control over not only the ”hot “chamber, but the cooler ones as well.
Aside from the disappointment of finding out that the experiment did not go as expected, there was some useful information gained. It is obvious by looking at the graphs that the snakes were changing their temperatures by changing chambers. This implies what is already known; snakes are capable of active behavioral temperature regulation. The other important piece of information gained through this experiment is the average body temperatures these snakes attained during the experiment. Because they were free to move from chamber to chamber in search of their preferred temperature, their average body temperatures should be a good representation of their preferred temperature. Furthermore, the female’s average body temperature was higher than the male’s and had a greater standard deviation. This could simply represent a higher preferred temperature for the female or show that she may be slower to digest her food or had eaten a larger food item.
Although the experiment did not show learning behavior as predicted, it is still congruent with Peterson’s findings in 1987 body temperature variations in free-ranging Garter Snakes. He found that “in several situations, the snakes did not select available temperatures that were within (or closer to) their preferred temperature range” (p. 168).
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