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La funcin del tamao, la interaccin social y la depredacin de metachrosis de la lagartija Espinosa verde (Sceloporus malachiticus)
The role of size, social interaction and predation on metachrosis of the green spiny lizard (Sceloporus malachiticus)
Metachrosis, or color change, in lizards may have social or antipredator functions. The purpose of this study was to determine impact of size, social interaction and predatory pressures on metachrosis in S.
malachiticus. Nine lizards were collected and photographed to quantify their initial color. The snout-vent length and photographed color (as hue, saturation and brightness) were measured and compared (Spearman Rank Correlation, P > 0.60), showing no significant correlation. Lizards were then paired and allowed to interact for thirty minutes, once with every lizard and photographed after. Interactions were broken down into three groups (same size, with larger, with smaller), where color after interaction was compared to initial, showing significance only for increasing hue and decreasing brightness for individuals interacting with smaller lizards (Paired t-test, P < 0.015). Lizards were also handled for 15 seconds to imitate
predation. Before and after photographs were compared, showing a significant decrease in brightness (paired t-test, p <0.0001). Lizard darkening during predator imitation trials provides more evidence for thermoregulation and possibly crypsis, while changes in color during social interaction suggest selective pressures beyond thermoregulation for color change. Therefore, metachrosis here is not just a response to temperature and light, but also suggests that interactions and stress contribute to this phenomenon.
Metachrosis o cambio de color, en las lagartijas puede tener funciones sociales o de anti depredador. El propsito de este estudio fue determinar el impacto del tamao, la interaccin social y las presiones predatorias en metachrosis en S. malachiticus. Se recolectaron nueve lagartijas y fueron fotografiados para cuantificar su color inicial. Se midieron la longitud desde la nariz hasta el ano y color fotografiado (como matiz, saturacin y brillo) y se comparan (rango de correlacin de Spearman, P > 0.60) que no muestran correlacin significativa. Luego se puso a las lagartijas en pareja y se dejaron a que interactuaran durante 30 minutos, una vez con cada lagartija y se fotografiaron despus. Las interacciones fueron divididas en tres grupos (mismo tamao, con el ms grande, con el ms pequeo) donde el color despus de la interaccin fue comparado al iniciar, mostrando significancia solo para aumentar la tonalidad y disminuir el brillo para los individuos que estn interactuando con lagartijas ms pequeas (Paired t-test, P < 0.15). Las lagartijas tambin se manejaron durante 15 segundos para imitar a la depredacin. Antes y despus de que las fotografas fueron comparadas, mostrando una disminucin significativa en el brillo (paired t-test, p < 0.0001). El oscurecimiento de las lagartijas durante los ensayos de imitacin de depredadores ofrece ms evidencia para la termorregulacin y posiblemente cripsis, mientras que los cambios en el color durante la interaccin social sugieren presiones de seleccin ms all de la termorregulacin para el cambio de color. Por lo tanto, metachrosis aqu no es slo una respuesta a la temperatura y la luz, sino que tambin sugiere que las interacciones y el estrs contribuyen a este fenmeno.
Text in English.
Costa Rica--Puntarenas--Monteverde Zone
Costa Rica--Puntarenas--Zona de Monteverde
Tropical Ecology Summer 2010
Malachite Spiny Lizard (Sceloporus malachiticus)
Ecologa Tropical Verano 2010
Lagartija Malaquita Espinosa (Sceloporus malachiticus)
t Monteverde Institute : Tropical Ecology
The Role of Size, Social Interaction and Predation on Metachrosis of the Green Spiny Lizard ( Sceloporus malachiticus ) Daniel Whonsetler, Wittenberg University Department of Biology ________________________________________________________________________ Abstract Metachrosis, or color change, in lizards may have social or antipredator functions. The purpose of this study was to determine impact of size, social interaction and predatory pressures on metachrosis in S. malachiticus. Nine lizards were collecte d and photographed to quantify their initial color. The snout vent length and photographed color ( as hue, saturation and brightness) were measured and compared (Spearman Rank Correlation, P > 0.60), showing no significant correlation. Lizards were then pa ired and allowed to interact for thirty minutes, once with every lizard and photographed after. Interactions were broken down into three groups (same size, with larger, with smaller), where color after interaction was compared to initial, showing significa nce only for increasing hue and decreasing brightness for individuals interacting with smaller lizards (Paired t test, P < 0.015). Lizards were also handled for 15 seconds to imitate predation. Before and after photographs were compared, showing a signifi cant decrease in brightness (paired t test, p <0.0001). Lizard darkening during predator imitation trials provides more evidence for thermoregulation and possibly crypsis, while changes in color during social interaction suggest selective pressures beyond thermoregulation for color change. Therefore, metachrosis here is not just a response to temperature and light, but also suggests that interactions and stress contribute to this phenomenon. Introduction Metachrosis, or color change, is an ability that is restricted among reptiles. In lizards, its functions are found in only the most primitive families: geckonids, iguanids, and xantusiids ( Smith, 1995 ). Despite, however, the uniqueness of this phenomenon, there have been limited experiments done on too few lizard species. What is known, is that color change accounts for such evolutionary adaptations in lizards as crypsis, aiding in predator avoidance and prey capture, social cues for mating and dominance displays, and thermoregulation (Hager, 2002). Evid ence this has been shown in the p igments found on the throat of many iguanid lizards that function in species and sex recognition and influence dominance relationships (Cooper and Greenberg, 1992). Changes in throat color are said to be a balance between s exual communication and predation, which limits the intensity of color (Hager, 2002). Many lizards are also known to have different juvenile and adult color patterns (Smith, 1995) There are seven other factors that control color change in lizards: its se x, the color of its environment, the season, the temperature, light, health, and how excited it is ( Smith, 1995 ). The Green Spiny Lizard (Family: Iguanidae; Sceloporus malachiticus ) is the southernmost representative of the sub family Phrynosomatidae This species ranges from Mexico to Panama at altitudes above 600 m (Robinson, 1983). S. malachiticus can be seen readily on sunny days, along rooftops, trees, rocks, fence posts and building sides. Both male and female adults can be easily recognized by their bright turquoise body and blue throat (Figure 1); however, this species will display color change, or metachrosis, at lower body temperatures (Savage, 2002), becoming a dark brown. In S. malachiticus metachrosis has been shown to aid in thermoregulat ion (Carter, 1999), and also have faster rates in smaller individuals, due to their higher surface area to volume ratio (Stonedale, 2003).
All previous experiments dealing with metachrosis in S. malachiticus have focused solely on one selective pressure: t he effect of temperature on color. Before starting this experiment, I observed two individuals of S. malachiticus basking on a building side by side. Both individuals were of similar size, in the same heat source, and on the same substrate, however, one w as of light tan and black coloration, while the other was bright green. This difference was likely not due to thermoregulation, but instead some other factor. This observation made me question the wide range of possibilities that may account for the vast c olor ranges in S. malachiticus. Although temperature has been investigated and shown to impact color, other factors, like size, social interaction and predation have not been explore d in this spiny lizard. Materials and Methods Lab set up Nine lizards wer e caught using a butterfly net at elevations from 1200 1500 m in vent length (SVL) was measured (Table 1) after being caught. Lizards were then placed in individual aquariums filled with leaf li tter. Aquarium sides were covered with black trash bags to standardize light and ensure that lizards were not visible to one another. One 60 watt incandescent light was shared between two aquariums, and left on between 730 1600 hours. Table 1: SVL of S malachiticus those exhibiting adult coloration are marked with an asterisk (*). Figure 1: Coloration of a S. malachiticus adult male. Males have a b lack outlining of their ventral blue patches, while females only have blue patches. Experiment 1: Social interactions After a 48 hour acclimation period lizards were photographed from 25 cm directly above them, using a Panasonic DMC FZ5 (standard settings with the flash on), three times throughout the day with a 1.5 hour span separating photographing times. These picture s were used as a standard or starting color for later comparison. In days following, Lizards were paired and allowed to interact for thirty minutes, once with every lizard and photographed after. Starting time for interactions corresponded thirty minutes prior to all standard photo times, and all lizards were paired with each other once. Photographs were uploaded onto Adobe Photoshop CS, and cropped to a frame of 25x25 d to Size (mm) 91* 88* 86 74 70 65 58 57 56*
sample the average color, where hue, saturation and brightness were measured. I am not sure a student on the next program who wanted to follow up this study would know how to do this. Be more specific. Interactions were placed into three categorie s same size, lizard with smaller lizard, and lizard with larger lizard. Individuals were considered of same size if they were within 15% of the SVL, while lizards that were larger or smaller had to have a SVL that differed by more than 15%. All interact ions that were not same sized lizards were larger lizards interacting with smaller lizards, however, in order to analyze the perspective of both lizards these interactions were separated into two groups: the perspective of smaller the lizard, and the persp ective of the larger lizard. Experiment 2: Predator Imitation To elicit possible color change in response to a predator attack, lizards were photographed, then chased inside their aquarium with a human hand for 10 seconds. Lizards were then removed and photographed on an adjacent table. This procedure was conducted three times on different days, so that each individual experienced predator imitation three times. Hue, Saturation and Brightness Color is a complex measurement, and is composed of three e lements: hue, saturation and brightness (Figure 2). Hue is what we consider color (red, blue, green), but is defined as the dominant wavelength of light, where hue is measured ( Clark and Wiebe, 200 0) Saturation is the degree of dominance of hue in a colo r, while brightness is how light or dark a color is ( Clark and Wiebe, 200 0) a). b). Figure 2: a). D iagram showing the differences in the three elements of color hue, saturation, and brightness. Here a single hue is selected (blue) and its saturation is decreased, so that the hue becomes less and less intense within each block. The central block of the same blue hue with less saturation is made darker and lighter by decreasing and increasing its brightness. b). hue increasing (left to right) from 80 to 100, each segment represents a 5 change, saturation 50%. Results Snout Vent Length and Color lation was performed. The tests revealed that there was no
relation between SVL and body color. The plot of these relationships can be seen in Figure 3. a). b). c). Figure 3: The relationship of color and snout vent length in S. malachiticus a). n = 9, R 2 = 0.0153, rho = 0.067, P =0.86, b). n = 9, R 2 = 0.004, rho = 0.067, P =0.86, c ). n =9, R 2 = 0.104, rho = 0.202, P = 0.60 Points represent single individuals. All regressions showed color and SVL are unrelated and not statistically significant. Experiment 1: Social interactions A paired t test was used to compare standard lizard color with color after interactions. Interactions were separated into three groups same size, lizard with smaller, and lizard with larger to observe trends within those interactions The test showed there was a statistically significant decrease in h ue (df = 22, t = 2.71, P = 0.012, mean difference = 4.39 ) and increase in brightness (df = 22, t = 3.97, P = 0.0007, mean difference = 5.17%) for lizards interacting with smaller individuals. There was also a trend for lizards interacting with individuals of the same size to increase their hue (df = 23, t = 1.79, P = 0.08; mean difference = 3 .5 ). The remaining data showed that changes in saturation for all interactions, changes in brightness for same sized lizards, and interactions where individuals were paired with larger lizards were all statistically nonsignificant.
a). b). c). Figure 4: Mean value of all color properties of S. malachiticus before and after social interaction trials: a). hue in degrees, b). saturation in percent, c). brightness in percent Groups that were statistically significant are marked with asterisks (*), while those that showed a trend were marked with (t). Error bars represent standard deviation. Experiment 2: Predator Imitation A paired t test was performed to compare color chan ge before and after predator imitation trials. The data showed that predation caused a statistically significant decrease in brightness (df =26, t = 11.9, P = <0.0001, mean difference = 15.9) and a trend of increasing hue (df = 26, t = 1.84, P = 0.078, m ean difference = 3.4 ). Change in saturation after predator imitation was not found to be statistically significant (df =26, t = 0.54, P = 0.29).
a). b). Figure 5 : Mean value of all color properties of S. malachiticus before and after predator imitation: a). hue in degrees, b). saturation and brightness in percent. Asterisks (*) represent statistically significant data, while (t) represents a trend. Error bars sho w standard deviation. Alternate Observations Several times while searching for the lizards I was deceived by moss and lichens of stunningly similar color. These lichens and moss were observed on every habitat where lizards were caught. Throughout the enti rety of the experiment, lizards appeared to have little to no change in color. Most remained the same color as when they were captured, with only slight variations in brightness and saturation, but not hue. The three that most consistently stayed the same color were the three largest lizards. Predator imitation was the only experiment that showed a visible color change. Smaller lizards appeared dark brown in the morning, and would lighten up slightly when the lights were turned on and as the day progressed One individual, the only known adult male of its small size (56 mm), would quickly change to its distinctive green and blue color when I turned the lights on in the morning. I think this is important. You seem to be saying that temperature and daylight or some aspect of time is the most important determinants of color. Do you really feel this way? Lizards also showed little to no acknowledgement of each other when paired in social interactions. This could be because when I was observing, their focus tu rned to me and not each other, thus observation of uninterrupted interaction was difficult. Lizards would always run away, often into the glass wall, in order to evade my hand from picking them up. Discussion Results of this study reveal that individuals of S. malachiticus decrease their hue and increase their brightness slightly while in the presence of smaller lizards. This may be a form of inter or intrasexual communication, where the lizards are changing color as a sexual behavior or display of domina nce (Cooper and Greenberg, 1992). However, this is likely not the case. The change in color was so minute that it could not be observed with the human eye. Figure 2 demonstrates this small variation. Lizards also did not interact whatsoever when placed t ogether. This may be due to the unnatural environment they were living in. They were not able to court or really establish any territorial 0 10 20 30 40 50 60 70 80 Before After Hue (t) Degrees 0 10 20 30 40 50 60 70 Before After Before After Saturation Brightness* Percent
dominance, their prey was fed to them and they were surrounded by glass. This synthetic environment seemed to leav e the lizards with the sole focus of escaping. In fact the little color change that was significant may likely have been a stress response from handling them. If repeated, this study should have a larger sample size and be performed in the al environment, where photographs are taken onsite, instead of laboratory setting. Results of the predator imitation study show that S. malachiticus becomes darker when under the threat of a predator. Since lizards were being chased, this observed color c hange was likely to increase their absorption of energy, enabling them to move faster and evade the predator. This selective pressure prove s useful in survival and has been observed in all experiments with S. malachiticus by being quick, elusive, and very difficult to catch. It was not able to be determined, however, if lizards darken in the presence of predators or only during predation, when they are running away. Future studies may benefit from exploring this unknown. Hager 2002, also shows that lizards have separate colors for crypsis and communication. Cryptic colors need to match the brightness of their surrounds, while communicative colors need to stand out (Hager, 2002) Since all lizards were on a dark brown leaf litter substrate, it is possible that they darkened their coloration so that their bright colors would not contrast as much against the aquarium background. Endler (1983) has also shown that some fish populations, in the presence of known predators, l imit their expression of exaggerated color. If this is the case, it could be that S. malachiticus uses color change as a form of crypsis, and not only decreases its brightness, but increases it e. Bright green adults also darkened but only slightly changed their hue. This preference for green could be another form of camouflage, where the lizards are mimicking moss. This function would prove useful in tropical forest edges where moss and sunlig ht are abundant. It is interesting to note that lizards had opposite reactions to predation and social interaction. During predation, lizards showed a trend to increase hue (becoming more green) and significantly decrease their brightness, but when interac ting with smaller individuals, lizards increased their brightness and decreased their hue. These diverse reactions may show that lizards have more than one use for changing their color. It also shows that since color change was not the same, the lizards w ere not reacting to the same variable in both experiments. This rules out that all color change was due to the stressful unnatural environment, and also shows that lizards interacting were likely to either be stressed, or actually interacting socially, sin ce their reactions to predation were completely opposite. Still, throughout the entirety of the experiment the most notable cause of color change was light and temperature. Lizards in the morning would change from a dark brown back to the color that was o bserved throughout the experiment. Since most lizards remained nearly the same color during the day, and that there was no correlation between SVL and color, I believe that my initial observation was a flawed assumption that lizards of similar size will be the same color. The wide var i ation in color and pattern suggests that it is possible that body color could be passed genetically. However, it is known that color changes with age in lizards ( Smith, 1995 ), so it is also possible that the lizards were not t he same age.
There are numerous possible factors that influence color change in lizards (Sex, age, excitation, environment), but not all have been examined to their full extent. Future studies would benefit from looking beyond thermoregulation, and int o the unknown to discover what else determines this change. Acknowledgements I would like to thank Jose Carlos Caldron for his lizard wizardry and nearly constant help catching the emerald swift. Alan, for wisdom, patience and guidance through the proje ct. Marvin, for taking off work to show me the prime lizard real estate within his farm. Jessica Gouveia, Tiffany Reeves, JA, and Alex Kaye for their help hunting at some point or another. And lastly, Victoria La Rocca for her skills in photography and da ily conscious of animal cruelty. Literature Cited Carter, Matt. 1999. The Effects of Light and Temperature on Metachrosis in S. malachiticus Whitman College: CIEE Tropical Ecology Summer Semester. Clark, Aron, and Eric Wiebe 2000. Color Principles Hue, Saturation, and Value. NC University College of Education. Web. Cooper, W. E., Jr., and N. Greenberg. 1992. Reptilian coloration and behavior. Biology of the Reptilia, Vol. 18, Physiology E University of Chicago Press. pp 298 422. Endler, J. A. 1992. Signals, signal conditions, and the direction of evolution. American Naturalist 139: pp. 125 153. Hager, S. B. 2002. The Southwestern Naturalist 47: pp. 299 307. Robinson, D.C. 1983. Costa Rican Natural History Ed. Daniel Janzen. Chicago: University of Chicago Press. pp. 421 422. Savage, M. J. 2002. The Amphibians and Reptiles of Costa Rica Chicago: University of Chicago Press. pp. 437 439. Smith, Hobart. 1995. Handbook of Lizards Cornell University Pres s. pp. 40 44. Stonedale, Joel Patrick Napier. 2003. The Role of Metachrosis in Thermoregulation of Sceloporus malachiticus. University of Texas at Austin: CIEE Tropical Ecology and Conservation Fall Semester.