Completed Sections of the Writing Assignment
Thermal biology of two populations of the lizards Phrynosoma cornutum and Cnemidophorus sexlineatus from northeastern New Mexico.
In spite of a large body of evidence that demonstrates the influence of internal temperature on the physiology and behavior of ectotherms, an integration of such areas of investigation with whole-animal ecology remains elusive (Huey and Stevenson 1979). For example, although a number of studies have documented the fact that many ectotherms are capable of regulating their body temperatures (Tb) through behavioral means, until relatively recently few biologists have attempted to understand this behavior in other than physiological terms. Pioneering studies in the regulation of body temperature led Bogert (1949) to conclude that behavioral thermoregulation is an evolutionary stage in the process of freeing the organism from the limitations of its thermal environment. Huey and Slatkin (1976), on the other hand, formalized the view that thermoregulatory behavior should reflect a compromise between the benefits and associated costs or risks of this activity. They then present a cost-benefit model with three parameters: (1) the benefits at various Tb, (2) the frequency distribution of environmental temperatures in a habitat at a given time, and (3) the cost to achieve a particular Tb.
Magnuson et al. (1979) make a case for viewing temperature as an ecological resource, analogous to more traditional resources such as food. They argue that animals compete for and partition thermal resources, that the success of this competition contributes directly to fitness, and that the thermal niche of an organism can be quantified as well as more traditional axes of the multidimensional niche.
In the rolling hills and arid grasslands of northeastern New Mexico the Texas horned lizard (Phrynosoma cornutum) and six-lined racerunner (Cnemidophorus sexlineatus) are sympatric. Although both lizards occupy similar habitats, they differ dramatically in size, shape and foraging patterns. Horned lizards are dull-colored, relatively large but flat-bodied "sit and wait" predators with short, stocky tails that depend on cryptic coloration and spines for defense against predators (Hammerson 1982).
Racerunners, on the other hand, are brightly colored, slender lizards with, long fragile tails (up to 70% their total body length) that actively forage for prey and rely on speed and agility to escape predators (Conant 1975, Hammerson 1982). These differences suggest that six-lined racerunners and Texas horned lizards might differ in their thermal ecology as well. Specifically, I hypothesized that these two species would differ in: (1) their preferred body temperatures, (2) their thermoregulatory abilities, and (3) the timing of their daily activities.
Materials and Methods
Study Site - This study was conducted on the Kiowa National Grassland near Clayton, New Mexico in the summer of 2000. The study site consists of rolling short-grass prairie (Agropyron, Buchloe, Bouteloua, etc.) interrupted by sand blowouts and rocky outcrops. Although currant bushes (Ribes) and chollas (Opuntia) have taken hold in isolated pockets of soil, they provide little shade. The area is dissected by deep arroyos and shallow washes that enter the Cimarron River drainage via the North Canadian River. Average elevation within the study area is approximately 1,800 meters.
Field Methods - Cloacal body temperatures (Tb) of noosed lizards were taken during the summer of 2000 with a quick-recording thermocouple thermometer (Cole Parmer) within 10 seconds of capture in the manner of Gillis (1991). Attempts were made to capture all individuals seen as quickly as possible, but in those instances when lizards responded by moving away from microhabitats in which they were initially sighted, or when pursuit occupied an inordinate amount of time, temperature data were not recorded. Shaded air temperatures (Ta) were taken approximately 1 cm above the substrate as close as possible to the point at which each lizard was initially sighted. Time of capture (MDT), sex, snout-vent length (SVL) to the nearest mm and body mass (weighed with a Pesola spring scale to the nearest 0.1 g) of each captured lizard were also recorded.
Statistical Procedures - A t-test assuming unequal variances for comparing two means was used to determine if the preferred body temperatures of P. cornutum and C. sexlineatus differed significantly, with an alpha of 0.05 or less considered statistically significant. (Zar 1974). In order to evaluate the thermoregulatory performance of each species, linear regressions of body temperature (Tb) on air temperature (Ta) were compared in the manner of Huey and Slatkin (1976). In this model, a slope of 0 indicates perfect thermoregulation while a slope of 1, indicates complete thermal passivity. Thus, slopes not significantly different from zero were interpreted as evidence of thermoregulation while those not significantly different from 1 were interpreted as evidence of thermal passivity. The t-test for the significance of the regression coefficient (b) was used to compare the slopes of the obtained regressions against slopes of 0 and 1, with an alpha of 0.05 or less being considered statistically significant. (Zar 1974). Activity times for P. cornutum and C. sexlineatus were compared qualitatively by constructing a bar graph of the number of individuals of each species captured as a function of time of day.
Bogert, C.M. 1949. Thermoregulation in reptiles, a factor in evolution. Evolution 3:195-211.
Conant, R. 1975. A field guide to reptiles and amphibians of eastern and central North America.
Houghton Mifflin Company, Boston.
Gillis, R. 1991. Thermal biology of two populations of red-chinned lizards (Sceloporus undulatus
erythrocheilus) living in different habitats in south-central Colorado. Journal of Herpetology 25:18-23.
Hammerson, G.A. 1982. Amphibians and reptiles in Colorado. Colorado Division of Wildlife, Denver.
Huey, R.B. and M. Slatkin. 1976. Cost and benefits of lizard thermoregulation. Quarterly Review of
Huey, R.B. and R.D. Stevenson. 1979. Integrating thermal physiology and ecology of ectotherms: a
discussion of approaches. American Zoologist 19:357-366.
Magnuson, J.J., L.B. Crowder and P.A. Medvick. 1979. Temperature as an ecological resource.
American Zoologist 19:331-343.
Zar, J. 1974. Biostatistical analysis. Prentice-Hall, Inc. Englewood Cliffs, N.J.
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