The Role of Metachrosis in Thermoregulation of Sceloporus malachiticus Joel Patrick Napier Stonedale Department of Animal Behavior, University of Texas at Austin ________________________________________________________________ ABSTRACT Metachrosis is a change in color, common to many amphibians and reptiles. The purpose of this study was to determine if rate of metachrosis correlates with weight in Sceloporus malachiticus. Twenty one lizards were collected and photographed every half hour , and the change in reflected light in the photographs was measured. There was a significant negative correlation between rate of metachrosis and weight (Simple Regression, P < 0.02), which is an indication that metachrosis aids in thermoregulation. Larger liza rds must remain darker longer in order to absorb heat more quickly, because their lower surface area to volume ratio causes them to need more heat for their large bodies that have limited ability to absorb heat due to their proportionately lower surface ar ea. This need for larger lizards to be darker may also explain why other reptiles such as Clelia clelia, Clelia ceytalia, Ctenosaura similis, and Iguana iguana become darker as they grow older and increase in size. RESUMEN Metacrosis es un cambio en color, comÃºn en muchos anfibios y reptiles. El propÃ³sito de este estudi o fue determinar si la tasa de m etacrosis tiene correlaciÃ³n con el peso en Sceloporus malachiticus. Se colectÃ³ VeintiÃº n lagartijas y se fotografia ron cada media hora, y se midiÃ³ el cambio en la luz reflejada en las fotografÃas. Hubo una correlaciÃ³ n negativa y significativa entre la tasa de metacrosis y el peso (RegresiÃ³ n simple, P < 0.02), lo cual es una indicaciÃ³n de que la metac rosis ayuda en la termorregulaci Ã³n . Las lagartijas mÃ¡s grand e deben quedarse mÃ¡ s oscuras por m Ã¡s tiempo para absorber calor mÃ¡s rÃ¡pidamente, por su Ã¡rea de superficie mÃ¡s baja de proporciÃ³n de volumen, ellos necesitan mÃ¡s calor para sus cuerpos grandes que tienen la habilidad limitada de absorber el calor . Esta nece sidad para lagartijas mÃ¡ s grandes ser mÃ¡s oscura puede explicar tambiÃ©n por que otros reptiles como Clelia clelia, Clelia ceytalia, Ctenosaura similis, y Iguana iguana llegan a ser mÃ¡ s oscuros como ellos crecen mÃ¡s viejo y mÃ¡ s grande. INTRODUCTION Sceloporus malachiticus, or the green spiny lizard, is found from southern Mexico to Panama above 600 meters. It is the southernmost representative of a genus of iguanids known in the United States as fence lizards. Commonly found basking in the sun on rooftops, fence posts, walls , and rocks, it is almost entirely heliothermic (Robinson 1983). It also goes through a color change, or metachrosis, based on temperature, being dark at low temperatures and becoming malachite green as its body temperature rises (Tucker 1993). The selecti ve pressure favoring metachrosis has been attributed to camouflage to aid in predator avoidance and prey capture, but it may also aid in thermoregulation (Carter 1999). I have noticed in my observations of S. malachiticus in the field that there is extreme color variation even among individuals sharing a common substrate at the same
time of day. It also appeared that larger lizards were generally darker in the morning, which would be consistent with the idea that metachrosis aids in thermoregulation, becau se larger lizards, which have limited heat absorption abilities due to their lower surface area to volume ratios, would need to remain darker longer to obtain their optimal body temperature. As a lizard grows, its surface area increases in two dimensions w hile the volume increases in three dimensions. Because heat can only be absorbed through the surface and the volume increases more quickly than the surface area, larger lizards cannot absorb adequate heat as fast as smaller lizards. The purpose of this exp eriment is to see if larger lizards do in fact have a lower rate of metachrosis than smaller lizards. If they do, it would be an indication that thermoregulation is responsible for the evolutionary pressure that selected for metachrosis in S. malachiticus. MATERIALS AND METHODS I collected 21 Sceloporus malachiticus from the area around Monteverde, Puntarenas, Costa Rica, consisting of elevations ranging from 1250 1400 m. All lizards were kept overnight in an aquarium, which was lined with black constructio n paper to standardize reflection and reduce light entering through the sides. Each aquarium had a 100 watt light bulb in the corner, 13 cm from the bottom of the aquarium. The lights turned off at 7:30 p.m. the day they were caught and turned back on at 7 :30 a.m. the following day. The lizards were photographed with a Sony DSC S85 at approximately 7:20 am and every half hour after the lights went on at 7:30 am. If the lizards moved more than 15 cm away from the light, they were no longer photographed, bu t otherwise were photographed until 1:30 p.m. They were photographed with standardized settings with the camera 0.5 m away from the bottom of the aquarium, a focal length of 0.5 m, an F ratio of 2.1, a shutter speed of 1/80 second, and the flash on. After they were done being photographed, all of the lizards were weighed, and their snout vent lengths were measured. Each lizard was cropped from the photograph, excluding its toes, and the luminosity was measured with Adobe Photoshop 7 software (see figure 1). There was unexpected variance among the luminosities of lizards that were photographed on different days, so all luminosities were divided by the lizard's individual mean in order to facilitate comparison among subjects from different days. The luminositi es (as proportions of their means) were plotted against time to show the rate at which each lizard changed luminosity, or the rate of metachrosis . The slopes of the logarithmic regression curves for each lizard were then plotted against weight, and a simpl e regression was performed. Two model lizards of a similar volume were constructed with clay in order to show that a lizard with a lower luminosity would heat up faster than a lizard with a high luminosity in the aquarium conditions. One was constructed fr om black clay and the other from white clay, and both were constructed with the tip of a thermometer inside of them. They were placed in the same conditions as the lizards and their temperatures were measured every 30 minutes. They were als o photographed t o determine their luminosities.
RESULTS Clay Models The black clay model lizard had a lower luminosity (96 luminosity units) than the white clay model (218 luminosity units) and also heated up faster. Both were at 18.5 Â°C when the lights came on, and after 30 minutes 13 cm away from the 100 watt bulb, the black model had a temperature of 35 Â°C whereas the white model had a temperature of 30 Â°C. The black model therefore heated up at a faster rate (0.55 Â°C / minute) than the white model (0.38 Â°C / minut e). Rate of Metachrosis There was a significant negative correlation between lizard weight and rate of metachrosis, as measured by the percent change in luminosity divided by the natural logarithm of time (simple regression, P < 0.02, see figure 2). Most o f the lizards changed luminosity quickly at first but then asymptoted after about 2 hours, with the larger lizards changing more quickly (see figures 3 and 4). Comparisons could not be made among starting luminosities because of the strong variance among p hotographs taken on different days. Time Spent Near Light There was a significant positive correlation between lizard weight and time spent within 15 cm of the light bulb (simple regression, P = 0.003, see figure 5). The smaller lizards moved out of the light in as little time as one hour, while many of the larger ones stayed throughout the entire period in which they were photographed (5 hours and 30 minutes). The lizards also tended to move further away from the light as the day continu ed, presumably as a form of thermoregulation, but only the smaller ones crossed the 15 cm threshold. DISCUSSION Smaller Sceloporous malachiticus do have a significantly faster rate of metachrosis. This supports my hypothesis and indicates that S. malachiticus may use metachrosis for temperature regulation. It would be advantageous for a large lizard with a low surface area to volume ratio to stay darker longer in order to increase the rate of heat absorption through its limited surface. Although th ese results indicate that metachrosis aids S. malachiticus in thermoregulation and explain why larger lizards appear darker, they are also the result of the ability of smaller lizards to change temperature faster, as metachrosis is a function of temperature in this species (Tucker 1993). However, the results of the clay model lizards are evidence of the thermoregulatory advantage of lizards with higher volumes to have lower luminosities. The darker colors displayed at lower tempe ratures reflect less light and therefore increase the rate of heat absorption when it is most needed (Carter 1999). This accelerate d rate of heat gain could give S . malachiticus an added advantage against its smaller arthropod prey that would otherwise abs orb heat much faster due to its higher surface area to volume ratio (Robinson 1983). A higher body temperature in the early hours of the day would allow
S . malachiticus to move fast enough to catch prey that might usually evade the lizard as well as help t o avoid endothermic avian predators whose metabolic rates are not as restricted by cooler morning temperatures. As the lizard reaches its optimal temperature, brighter colors would be advantageous in reflecting more light in order to prevent overheating. M any diurnal reptiles, such as Clelia cleia , Clelia ceytalia, Ctenosaura similis, and Iguana iguana, become darker as they grow larger, and although their change in color has not been attributed to thermoregulation, it can be used as an explanation (Scott 1 983, Savage 2002, Robinson 1983, Fitch 1983). For Ctenosaura similis color change has been attributed to camouflage, justified by the fact that the young hatch in the wet season, when most of the forest is green (Fitch 1983). However, this only explains wh y the young are green, does not explain why they become darker as they grow older, and offers no explanation at all for the many Ctenosaura similis that live in evergreen forests such as Corcovado National Park (Fitch 1983). Another possibility would be that it is advantageous for the lizard to become darker in order to absorb heat more quickly. Adult C. similis spend most of the daytime basking in the sun in a posture that exposes the most surface area and are known to retrea t into their burrows when it becomes cloudy (Fitch 1983). This time and effort put into absorbing heat implies that it would be advantageous to be able to absorb heat faster. As the lizard increases in size, there is more need to absorb heat more efficient ly and less pressure from predators, so the advantage of being green fades as the advantage of being dark rises. Clelia clelia and Clelia ceytalia are mostly red as juveniles and become black as they grow older. They are considered to be coral mimics when they are younger, but after achieving the length of 1 m they turn black because they are too large to be perceived as coral snakes (Scott 1983). However, while this explains a change from coral c olors, if this does not explain why they turn black. Presumab ly, as in Ctenosaura similis, the need for more efficient heat absorption may increase as the pressure from predators that causes the juvenile coloration decreases. To better understand the role of thermoregulation in S. malachiticus, the distance the liza rd moves from the light bulb needs to be taken into account, as several lizards moved further from the light source, presumably as a form of thermoregulation. It would help to isolate the effects of metachrosis to either measure the distance the lizard mov es from the light over time or to restrict the lizard's movement so that it cannot move away from the light. To better understand the role of metachrosis in thermoregulation of reptiles in general, research needs to be done in order to determine if there i s a relationship between reptile species that become darker as they increase in size and species that are diurnal, as it would be more advantageous for large nocturnal reptile to be lighter in color in order to reduce the rate at which they radiate the hea t that was absorbed during the day.
ACKNOWLEDGEMENTS I would like to thank Andrew Rodstrum, Stephen G utchalk, Amish Patel, Rodrigo A raya, Alyson Mack, Ma tt Stevens, and Jose Luis Argue das for help with catching lizards. Thanks especially to Jose Luis Argue das for his hospitality, allowing me to tromp all over his farm in search of the elusive Sceloporous malachiticus, and allowing me to extract large amounts of them from his property. Also , thanks to Alan Masters for his help with experimental design, statistical analysis, and lizard catching tips and to Karen Masters for helping me track down Alan. Thanks to Adres Vaughn for his help with research, and finally, thanks to EstaciÃ³n BiolÃ³ gica de Monteverde for lab space and supplies. LITERATURE CITED Carter, Matt 1999. The Effects of Light and Temperature on Metachrosis in S. malachiticus. Whitman College: CIEE Tropical Ecology Summer Semester. Fitch, H.S. 1983. Costa Rican Natural History, ed. Daniel Janzen. Chicago: University of Chicago Press, pp. 394 396. Robinson, D.C. 1983. Costa Rican Natural History, ed. Daniel Janzen. Chicago: University of Chicago Press, pp. 421 422. Savage, M.J. 2002. The Anphibians and Reptiles of Costa Rica. Ch icago: University of Chicago Press, pp 437 439. Scott, N.J. 1983. Costa Rican Natural History, ed. Daniel Janzen. Chicago: University of Chicago Press, pg. 392. Tucker, E. 1993. The Effects of Light and Temperature on Metachrosis in the Spiny Lizard. In CIEE Summer 1993. Council on International Educational Exchange, New York, New York.
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Napier Stonedale, Joel Patrick
The Role of Metachrosis in Thermoregulation of Sceloporus malachiticus
Digitized by MVI
Metachrosis is a change in color, common to many amphibians and reptiles. The purpose of this study was to
determine if rate of metachrosis correlates with weight in Sceloporus malachiticus. Twenty-one lizards were
collected and photographed every half hour, and the change in reflected light in the photographs was
measured. There was a significant negative correlation between rate of metachrosis and weight (Simple
Regression, P < 0.02), which is an indication that metachrosis aids in thermoregulation. Larger lizards must
remain darker longer in order to absorb heat more quickly, because their lower surface area to volume ratio
causes them to need more heat for their large bodies that have limited ability to absorb heat due to their
proportionately lower surface area. This need for larger lizards to be darker may also explain why other
reptiles such as Clelia clelia, Clelia ceytalia, Ctenosaura similis, and Iguana iguana become darker as they
grow older and increase in size.
Text in English.
Costa Rica--Puntarenas--Monteverde Zone
Costa Rica--Puntarenas--Zona de Monteverde
Tropical Ecology Fall 2003
Ecologa Tropical Otoo 2003
t Monteverde Institute : Tropical Ecology