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Steinberg, David S.
Seleccin del micro hbitat acutico por Norops oxylophus (Polychrotidae) en San Luis, Costa Rica
Riparian microhabitat selection by Norops oxylophus (Polychrotidae) in San Luis, Costa Rica
Microhabitat studies of anoline lizards have traditionally measured small-scale characteristics, such as perch height and perch diameter. Riparian anoles, however, may inhabit a more complex environment,
since they are in direct contact with rivers or streams; thus, a broader spectrum of variables, particularly those related to river qualities, may be necessary to garner a greater understanding of microhabitat choice in these lizards. In this study, I examine microhabitat selection by Norops oxylophus in relation to both traditional variables and three river variables in the San Luis Valley, Costa Rica. Results show that male lizards prefer higher perches (mean = 0.900 0.301 m, N = 12) than do females (mean = 0.192 0.152 m, N = 13) or juveniles (mean = 0.338 0.337 m, N = 31), while females prefer wider perches (mean = 0.138 0.067 m; N = 5) than do other lizards. Also, N. oxylophus in general seem to prefer riverside sites that are wider and deeper (meanwidth = 3.077 1.375 m, meandepth = 0.183 0.062 m, N = 56) than are unoccupied riverside sites (mean width = 1.852 0.748 m, meandepth = 0.138 0.137 m, N = 56). This study supports the hypothesis that factors related to rivers influence riparian anole microhabitat selection.
Las investigaciones del micro hbitat en las lagartijas (Polychrotidae: Anolinae) usualmente se le miden las caractersticas como la altura de percha y el dimetro de percha. Las lagartijas acuticas, sin embargo, pueden habitar en un ambiente ms complejo porque ellas estn en contacto directo con los ros y quebradas; as que, un espectro mas ancho de las variables puede ser necesario para tener una mejor comprensin de la seleccin del micro hbitats en estas lagartijas. En esta investigacin, yo examin la seleccin del micro hbitat en Norops oxylophus en relacin a variables tpicas y algunas variables del ro en el Valle de San Luis, Costa Rica.
Text in English.
Lizards--Behavior--Costa Rica--Puntarenas--San Luis
Environmental impact analysis
Lagartijas--Comportamiento--Costa Rica--Puntarenas--San Luis
Analisis del impacto ambiental
Tropical Ecology 2007
Ecologa Tropical 2007
Lagartijas (Polychrotidae: Anolinae)
t Monteverde Institute : Tropical Ecology
1 Riparian microhabitat selection by Norops oxylophus (Polychrotidae) in San Luis, Costa Rica David S. Steinberg Department of Biological Sciences, Vanderbilt University ABSTRACT Microhabitat studies of anoline lizards have traditionally measured small scale characteristics, such as perch height and perch diameter. Riparian anoles, however, may inhabit a more complex environment, since they are in direct contact with rivers or streams; thus, a broader spectrum of variables, particularly those related to river qualities, may be necessary to garner a greater understanding of microhabitat choice in these lizards. In this study, I examine microhabitat selection by Norops oxylophus in relation to both traditional variables and three river variables in the San Luis Valley, Costa Rica. Results show that male lizards prefer higher perches (mean = 0.900 0.301 m, N = 12) than do females (mean = 0.192 0.152 m, N = 13) or juveniles (mean = 0.338 0.337 m, N = 31) while females prefer wider perches (mean = 0.138 0.067 m; N = 5) than do other lizards. Also, N. oxylophus in general seem to prefer riverside sites that are wider and deeper ( mean width = 3.077 1.375 m, mean depth = 0.183 0.062 m, N = 56) than are unoccupied riverside sites (mean width = 1.852 0.7 48 m, mean depth = 0.138 0.137 m, N = 56) This study supports the hypothesis that factors related to rivers influence riparian anole microhabitat selection. RESUMEN Las investigaciones de microhÂ‡bitat en lagartijas (Polychrotidae: Anolinae) usualmente miden caracterÂ’sticas como la altura de percha y el diÂ‡metro de percha. Las largartijas aquÂ‡ticas, sin embargo, pueden habitar un ambiente mÂ‡s complejo porque ellas estÂ‡n en contacto directo con rÂ’os y quebradas; asÂ’ que, un espectro mÂ‡s ancho de variables puede ser necesario para tener una major comprensiÂ—n de la selecciÂ—n de los microhÂ‡bitates en estas lagartijas. En esta investigaciÂ—n, yo examinÂŽ la selecciÂ—n de microhÂ‡bitat en Norops oxylophus en relaciÂ—n a variables tÂ’picas y algunas variables de rÂ’o e n el Valle de San Luis, Costa Rica. Los resultados indican que los machos prefieren perchas mÂ‡s altas (promedio = 0.900 0.301 m, N = 12) que las hembras (promedio = 0.192 0.152 m, N = 13) o los jÂ—venes (promedio = 0.338 0.337 m, N = 31), mientras las hembras (promedio = 0.138 0.067 m; N = 5) prefieren perchas mÂ‡s anchas que los otros estadios. TambiÂŽn, N. oxylophus en general parece preferir sitios ribereÂ–os que son mÂ‡s anchos y profundos (promedio anchura = 3.077 1.375 m, promedio profundidad = 0.1 83 0.062 m, N = 56) que otros sitios (promedio anchura = 1.852 0.748 m, promedio profundidad = 0.138 0.137 m, N = 56). Esta investigaciÂ—n apoya la hipÂ—tesis que los factores relacionados con los rÂ’os influyen en la selecciÂ—n de los microhÂ‡bitates en la s lagartijas aquÂ‡ticas. INTRODUCTION Anoline lizards have long served as a model system for the study of such diverse topics as convergent evolution (e.g., Leal et al. 2002), adaptive radiation (e.g., Warheit et al. 1999), behavioral ecology (e.g., Leal and Rodriquez Robles 1995), and microhabitat selection (e.g., Scott et al. 1976). The last theme, microhabitat selection, is of extreme
2 importance to anoline ecology, for it can directly affect both the fitness of an individual and the dynamics of an enti re population (Morris 2003) by influencing reproductive success (Doligez et al. 2002) and community organization (Losos et al. 2003). Recent studies on microhabitat selection of anoles have tended to focus on small scale, abiotic conditions, such as perch characteristics (e.g., Scott et al. 1976) and local temperature (e.g., Birt et al. 2001). These measurements are of obvious importance to any study of forest anoles, the recipients of much research attention, since they affect a great deal of the ecologica lly and physiologically pertinent aspects of lizard biology. However, a microhabitat investigation of riparian anoles, organisms that have typically been overlooked by the numerous scientists researching lizard ecology, must also consider variables other than those strictly associated with perches and forest structure. Semi aquatic lizards are in direct contact with rivers or streams, which likely produce significant effects on anoline microhabitat preference, possibly by serving as areas of refuge (Dappen 2003) or feeding sites (Leenders 2001). Thus, microhabitat studies of riparian anoles must broaden the scope of previous anoline research by including a consideration of river characteristics along with traditionally quantified variables. Norops oxylophus a moderately sized anole that occurs from Nicaragua through Panama (Savage 2002), is one of many semi aquatic lizards that researchers have yet to examine in detail. Vitt et al. (1995) measured body temperature, diet, and daily activity budget, and Dappe n (2003) studied escape behavior; yet few other data on this species are available. Despite this paucity of information, N. oxylophus serves as a perfect model organism to study the effects of a river on the microhabitat selection of an anole. Observations of water route escapes (Pianka and Vitt 2003; Dappen 2003) and predation of invertebrates on rocks within streams (Leenders 2001) suggest that rivers may play a large role in producing favorable or suitable perch sites in some aquatic anoles and may shape Norops distribution along rivers. In this study, I examine a population of N. oxylophus in San Luis, Costa Rica, in an effort to discern any possible influence of river variables on the microhabitat preference of this riparian anole. MATERIALS AND METHOD S Study Sites This study was conducted on segments of three rivers in the upper San Luis Valley, Puntarenas, Costa Rica, during the final period of the dry season (21 April 2007 06 May 2007). The three rivers RÂ’o Alondra, RÂ’o Bruja, and RÂ’o San Luis run through a mosaic of tropical premontane moist forest (Zone 1) and pasture (Holdridge 1967). Each is characterized by rocky banks closely bordered by trees, lianas, vines, grasses, and shrubs. The specific study sites were 800 m stretches of each riv er, which emanated from the confluence of the three waterways and ranged in elevation from 1000 1200 m. Lizard Classification Each river was surveyed for a total of 12 hours, and an ad libitum sampling method was used to maximize the number of N. oxyl ophus individual's located (Martin and Bateson
3 1986). All observed lizards were captured by hand. Using calipers (error = 0.02 mm), snout vent length (SVL) of each lizard was measured to the nearest tenth of a millimeter in order to ensure proper classific ation. Individuals greater than 53 mm possessing a large burnt orange dewlap were classified as males, while those greater than 49 mm and lacking any semblances of a dewlap were considered females (Vitt et al. 1995). Individuals of sizes less than 49 mm or between 49 53 mm, but possessing incipient dewlaps (not fully formed) were considered juveniles (Vitt et al. 1995). The age class or sex of lizards that were sighted, but not captured, was determined only by visual inspection from a distance; however, t hese identifications are likely accurate owing to the conspicuousness of male dewlaps and the ease of estimating relative size. Each sighted individual was marked with white out in an effort to avoid measuring the same lizard twice. Microhabitat Character ization Perch height and horizontal distance of perch site from the river edge were measured to the nearest centimeter for each lizard. Perches located directly on the ground (e.g., a rock on the bank) were assigned values of 0 cm, and perch sites within or above the water itself were assigned negative distance values. Perch circumference for lizards on live or dead vegetation was measured to the nearest centimeter, and original perch substrate (live vegetation, dead vegetation, rock within the river, or r ocky bank) was also noted. All perches were marked with flagging tape in order to avoid re measurement. Small quadrants, encompassing the entire area of the river between 1 m upstream and 1 m downstream of each perch site, were created around the perch si tes of all lizards (Figure 1). Three variables width, depth, and area of exposed rocks were then measured for each quadrant. Width (distance from one bank to the other) was calculated by averaging width measurements taken at three locations along the q uadrant (1 m upstream of the perch site, at the perch site, and 1 m downstream of the perch site), and depth by averaging eight depth measurements taken at various points within the quadrant (Figure 1). Relative area of exposed rocks was determined by esti mating the amount of rock surface protruding from the water a process made simple by the creation of the quadrant and the subsequent visual segmentation of the river. Quadrants were considered not rocky (zero to one eighth area covered by protruding rock s), slightly rocky (one eighth to one fourth), moderately rocky (one fourth to three eighths), rocky (three eighths to one half), or extremely rocky (one half to five eighths). Random quadrants were created 20 m upstream of each used perch site in order t o characterize areas of the river not utilized by N. oxylophus The three river variables width, depth, and area of exposed rocks were also measured for each random quadrant.
4 1 m 1 m = River edge Legend = Location of depth measurements = Perch site Figure 1. Diagram illustrating a quadrant around a perch site (red s tar) by a river. Three width measurements were taken along the dotted lines (at 1 m intervals). Eight depth measurements were taken at locations indicated by green dots. Statistical analyses ANOVA tests were run to elucidate the effects of sex/age class on perch height, perch circumference, horizontal distance of perch to river edge, river width, and river depth. Post hoc Fisher's PLSD tests were employed after each ANOVA test. T tests were used to examine differences in river width and depth between used and unused sites along the river. Chi squared tests were performed in order to evaluate the differences in relative rock cover between sex/age classes and used and unused sites, as well as the differences in preferred perch substrate between sex/age class es. Due to the close proximity and general similarity of the three rivers, data for all rivers were combined for the aforementioned statistical analyses RESULTS Sex and age class had a significant effect on lizard perch height (ANOVA test, F = 20.610, df effect = 2, df error = 53, P < 0.001). Males perched higher than either females (Fisher's PLSD, P < 0.001) or juveniles (Fisher's PLSD, P < 0.001) (Figure 2). Females and juveniles did not perch at different heights (Fisher's PLSD, P = 0.141) (Figure 2).
5 Std. Dev. Std. Err. Mean SEX HEIGHT cm -0,2 0,0 0,2 0,4 0,6 0,8 1,0 1,2 1,4 male female juvenile Figure 2. Mean perch heights (m) of male, female, and juvenile Norops oxylophus individuals in the San Luis Valley, Costa Rica. Standard deviation and standard error are shown. Horizontal distance from perch to river edge for males (mean = 0.237 0.299 m, N = 12), females (mean = 0.223 0.366 m, N = 13), and juveniles (mean = 0.398 0.742 m, N = 31) did not differ significantly (ANOVA test, F = 0. 542 df effect = 2, df error = 53, P = 0.585). Sex and age class signi ficantly affected perch circumference of N. oxylophus (ANOVA test, F = 8.385, df effect = 2, df error = 27, P < 0.05). Females were found on perches that had larger circumferences than those used by males (Fisher's PLSD, P < 0.001) and juveniles (Fisher's P LSD, P = 0.002) (Figure 3). However, males and juveniles perched on similarly sized substrates (Fisher's PLSD, P = 0.456) (Figure 3). SEX/AGE CLASS HEIGHT (m) Ma le Female Juvenile
6 Std. Dev. Std. Err. Mean SEX CIRCUM cm 0,00 0,04 0,08 0,12 0,16 0,20 0,24 male female juvenile Figure 3. Mean perch circumferences (m) for male, female, and juvenile Norops oxylop hus in the San Luis Valley, Costa Rica. Standard deviation and error are shown. Males preferred live vegetation, females preferred rocks in the river, and juveniles preferred the rocky bank (chi squared = 23.462, df = 6, P = 0.003) (Figure 4). 0 2 4 6 8 10 12 Live Vegetation Dead Vegetation Part Rock in River Rocky Bank Substrate Type Number of Lizards male female juvenile ÂŒ Figure 4. Number of male, female, and juvenile Norops oxylophus encountered on various substrates in the San Luis Valley, Costa Rica. Males (mean width = 3.690 1.724 m; mean depth = 0.204 0.063 m), females (mean width = 3.291 0.957 m ; mean depth = 0.205 0.058 m), and juveniles (mean width = SEX/AGE CLASS CIRCUMFERENCE (m) PERCH CIRCUMFERENCE (m) Male Female Juvenile
7 2.750 1.316 m; mean depth = 0.166 0.060 m) did not select areas along the rivers with significantly different widths (ANOVA test, F = 2.331, df effect = 2, df error = 53, P = 0.107) or significan tly different depths (ANOVA test, F = 2.786, df effect = 2, df error = 53, P = 0.071). However, juveniles tended to be found in shallower river stretches than either males or females (Figure 5). Std. Dev. Std. Err. Mean SEX DEPTH m 0,08 0,12 0,16 0,20 0,24 0,28 male female juvenile Figure 5. Mean river dept h in the vicinity of male, female, and juvenile Norops oxylophus perch sites in the San Luis Valley, Costa Rica. Standard deviation and error are shown. Males, females, and juveniles did not occupy areas along the river with significantly different rel ative rock cover (chi squared = 12.857, df = 8, P = 0.117). Sites used by lizards were closer to wider river stretches than unoccupied sites (t test, t = 5.321, df = 110, P < 0.001) (Figure 6). SEX/AGE CLASS DEPTH (m) RIVER DEPTH (m) Male Female Juvenile
8 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 Used Sites Random Sites Mean Width (m) Figure 6. The mean river width (m) at sites used by Norops oxylophus individuals along rivers in San Luis, Costa Rica, along with the mean river width at unoccupied sites along the same rivers. The bars show standard deviation. River stretches at sites used by N. oxylophus individual s were also deeper than those at unoccupied sites (t test, t= 2.357, df = 110, P = 0.020) (Figure 7). 0 0.05 0.1 0.15 0.2 0.25 0.3 Used Sites Random Sites Mean Depth (m) Mean Width (m) Used Sites Unused Sites Mean Depth (m) Used Sites Unused Sites
9 Figure 7. The mean river depth (m) at sites used by Norops oxylophus individuals along rivers in San Luis, Costa Rica, alon g with the mean river depth at unoccupied sites along the same rivers. Bars show standard deviation. The relative amount of exposed rock area in rivers did not differ between used sites and unused sites (chi squared = 3.540, df = 4, P = 0.472). DISCUSS ION The documented variation in perch height, perch circumference, and perch substrate used by different sex and age classes of N. oxylophus is not surprising, for these characteristics have traditionally been measured in anoline microhabitat studies (e.g ., Scott et al. 1976; Birt et al. 2001) and have previously been proven important to lizard ecology. The higher perch height of males is likely a consequence of differential selective pressures on different sexes and age classes. Females may concentrate on foraging in order to maximize nutrient acquisition for the production of eggs, a continuous process in most anoles (Warheit et al. 1976); thus, females tend to perch closer to their food source, the riverbank. Males, on the other hand, may focus a majorit y of their energy on social interactions and the maintenance of territories that are attractive to females (Warheit et al. 1976); foraging is necessary, but of lesser importance to these individuals. Males, therefore, may select a high perch in order to ga in a better view of the surrounding area, including potential male territory usurpers, as well as to broadcast their identity and status (e.g. through dewlap extension or head bobbing) to a greater audience (Warheit et al. 1976). Juveniles may perch on roc ks or debris extremely close to the riverbank in order to avoid competition with or aggression of territorial males; these individuals may not be concerned with maintaining territories, since they have yet to reach reproductive age. Data from this study on N. oxylophus appear to validate these hypotheses. Unlike the data on perch height, results of perch circumference analyses seem to contradict published theories on perch selection by anoles. The importance of social interactions in male anole activity res ults in increased movement and conspicuousness of this sex, while the sit and wait foraging strategy of females renders them motionless for most of the day (Warheit et al. 1976). Thus, males are expected to select wide perches in order to cover their "blin d spot" and locate possible predators, while females, due to their relative immobility and cryptic coloration, are expected to be less concerned with visually avoiding predation (Warheit et al. 1976). In this study females occupy wider perches than males. This trend may be related to both observed patterns of perch height preference and the tendency of males to perch above the river. Females perch low to the ground, and thus their perch substrate is likely to consist of rocks scattered within the river or f allen tree trunks; the circumferences of rock perches were not measured, resulting in a reduction of female sample size, and fallen trunks tend to have great circumferences. On the other hand, males perch higher above the ground and at negative horizontal distances to the river edge (above the water). Thus, the perches available to males consist of vines or small branches overhanging the water; perches with greater circumferences are either farther from the water (live tree trunks or large branches) or clos er to the ground (fallen trees). Therefore, the apparent contradiction between the results of this study on N. oxylophus and past research may be
10 explained by the combined effects of male perch height selection and male preference for perch sites located a bove water a possible source of diet or escape. Compared to the traditional anoline microhabitat variables mentioned above, river variables have surprisingly little effect on the microhabitat partitioning of male, female, and juvenile N. oxylophus Despi te the absence of statistically significant results, juveniles tend to favor rockier areas more than either males or females. This preference may be related to a higher frequency of attempted water escapes made by juvenile lizards (Dappen 2003). River regi ons with more rocks may be more turbid, a characteristic that could distract predators and force them to lose sight of an escaping juvenile. Similarly, juvenile lizards often swim to sides of rocks that are furthest from a perceived source of threat (perso nal observation), thereby suggesting that rocks serve as places of refuge from predation. Thus, at least one river variable may be somewhat important to microhabitat selection of juvenile riparian anoles. Although specific sexes and age classes of N. oxyl ophus do not appear to differ in their selection of sites due to the particular river variables measured in this study, all lizards, when grouped together, seem to choose microhabitats that possess certain river qualities. Norops oxylophus prefer perch sit es near regions of rivers that are both wider and deeper than other available perch sites. The selection of wider river stretches may be a reflection of predation pressure. Predators may be willing to travel short distances into water during prey capture a ttempts; but they may be hesitant to move farther into a moving body of water, since this action would likely increase chances of drowning or other bodily harm. Thus, N. oxylophus may favor wider river patches because these patches offer a larger refuge fr om predation. The preference of N. oxylophus for sites close to deeper river stretches is more difficult to explain than their preference for wide rivers. Pianka and Vitt (2003) observed many predation events by fish on N. oxylophus individuals in a Nica raguan population. Since deeper rivers may harbor larger fish predators, riparian anoles should potentially favor shallow and subsequently safe regions. The driving forces behind the selection of deeper river regions are, therefore, unclear, but may become apparent when more information regarding N. oxylophus behavior and ecology is made available. This study supports previous research on anoline microhabitat selection that has focused on specific perch characteristics, such as height and diameter. However the demonstration of the significance of river variables on microhabitat selection also underscores the need to consider a broader spectrum of variables when attempting to understand the distribution and microhabitat preference of organisms, such as ripa rian anoles, that inhabit complex environments. Future studies should measure an even wider range of river characteristics, such as water velocity, in order to increase our comprehension of the factors that shape microhabitat choice in these lizards. ACKN OWLEGMENTS I would like to thank Tania ChavarrÂ’a for her assistance with project ideas, data analysis, manuscript revision, and abstract translation. Alan and Karen Masters provided valuable advice on suitable topics of study, and their help is greatly ap preciated. Thanks also to Geovanni Leiton for granting permission to use his property for some data collection and
11 Daniel Leiton for leading me to my study sites. I would also like to thank Emily Lowe, who offered insightful comments on an early manuscript LITERATURE CITED Birt, R. A., R. Powell, and B. D. Greene. 2001. Natural history of Anolis barkeri : A semiaquatic lizard from southern Mexico. Journal of Herpetology 35(1): 161 166. Dappen, N. 2003. Microhabitat use and escape behavior of male, femal e, and juvenile Norops oxylophus (Polychrotidae). CIEE Spring Tropical Ecology and Conservation: 165 173. Doligez, B., E. Danchin,, and J. Clobert. 2002. Public information and breeding habitat selection in a wild bird population. Science 297: 1168 1170 Holdridge, L. R. 1967. Life zone ecology. Revised edition. Tropical Science Center, San Jose, Costa Rica. Leal, M., A. K. Knox, and J. B. Losos. 2002. Lack of convergence in aquatic Anolis lizards. Evolution 56: 785 791. Leal, M. and J. A. Rodriquez R obles. 1995. Antipredator repsonses of Anolis cristatellus (Sauria: Polychrotidae). Copeia 1: 155 161. Leenders, T. 2001. A guide to amphibians and reptiles of Costa Rica. Distribuidores Zona Tropical, Miami, Florida. Losos, J. B., M. Leal, R. E. Glor, K de Queiroz, P. E. Hertz, L. RodrÂ’guez Schettino, L. A. Chamizo, T. R. Jackman, and A. Larson. 2003. Niche ability in the evolution of a Caribbean lizard community. Nature 424: 542 545. Martin, P. and P. Bateson. 1986. Measuring behavior: an introductory guide. University of Cambridge Press, New York, New York. Morris, D. W. 2003. Toward an ecological synthesis: a case for habitat selection. Oecologia 136: 1 13. Pianka, E. R. and L. J. Vitt. 2003. Lizards: windows to the evolution of diversity. Universi ty of California Press, Berkeley, California. Savage, J. M. 2002. The amphibians and reptiles of Costa Rica: a herpetofauna between two continents, between two seas. University of Chicago Press, Chicago, Illinois. Scott, N. J., D. E. Wilson, C. Jones and R. M. Andrews. 1976. The choice of perch dimensions by lizards of the genus Anolis (Reptilia, Lacertilia, Iguanidae). Journal of Herpetology 10(2): 75 84. Vitt, L. J., P. A. Zani and R. D. Durtsche. 1995. Ecology of the lizard Norops oxylophus (Polychro tidae) in lowland forest of southeastern Nicaragua. Can. Journ. Zool. 73(10): 1918 1927. Warheit, K. I., J. D. Forman, J. B. Losos and D. B. Miles. 1999. Morphological diversification and adaptive radiation: a comparison of two diverse lizard clades. Evol ution 53(4): 1226 1234.