Spatially-explicit Thermal Modeling
Spatially-explicit modeling across scales is critical to understanding ecological processes. Surprisingly, thermal resources are rarely considered in spatially-explicit ecological models. The thermal environment has largely been ignored because existing techniques for thermal mapping do not adequately represent the spatiotemporal complexity of the thermal environments of animal habitats. The objective of this research is to develop procedures to generate detailed, dynamic, thermal maps of small animals. I am developing models using artificial neural networks that process thermographic images, concurrent microclimate and operative temperature data, vegetation, and topography to produce thermal maps of animal habitats. Using GIS, thermal maps of specific organisms can be projected over large landscapes with fine scale resolution. These thermal maps, along with other landscape characteristics, will set the stage for examining the links between physiological ecology and behavior of individuals to landscape distributional patterns.
Thermoregulation and Movement
As Edwin Abbott eloquently illustrated in his classic novel, Flatland, the dimensionality of space can be a difficult but enlightening concept. For ecologists, it means that many of our theories may need to be modified to accommodate the behavioral responses of individuals to a landscape composed of spatially-heterogeneous patchworks of resources and risks. With the increasing ability to document ecological phenomena more accurately in space (through remote sensing and GIS), our ability to frame and model ecological problems in a spatial context will only improve. Preliminary simulations using the model organism, Sceloporus virtualis, are revealing that differences in the spatial structure of thermal habitats have considerable effects on the outcome of movement patterns and effectiveness of thermoregulation. The results suggest that using currently popular thermoregulatory indices to test models of the cost of thermoregulation might be inappropriate.
Landscape Movements of Sand Dunes Lizards
To apply the thermal mapping techniques being developed in my lab to a natural system, I am examining the biophysical and geographical features of landscape that influence the distribution of sand dune lizards, Sceloporus arenicolus. Sand dune lizards are a threatened species, endemic to a system of sand dunes in SW New Mexico and nearby areas in Texas. They are habitat specialists that occur only in sand dune blowouts found within a matrix of shinnery oak habitat. These blowouts are distributed patchily across the landscape in a highly fragmented manner, and not all dunes support lizards. Initially, we are investigating three questions: 1) are there thermal features of the landscape that distinguish between dunes that support lizards and those that do not?, 2) what is the connectivity between between patches with respect to the thermal environment?, and 3) can the movement patterns of lizards between habitat patches be predicted by incorporating thermoregulatory decisions into dispersal models that simulate random walks?
Spatial Aspects of Nesting Behavior in Eastern Fence Lizards
Although we know a great deal about the influence of the thermal environment on the development and energetics of embryonic fence lizards (Sceloporus undulatus), very little is known about the actual environments of eggs or the selection of nesting sites by adult females in nature. Lab studies suggest that embryonic development is benefitted by relatively high temperatures because the duration of development is minimized and survival to hatching is maximized. Until recently, nesting locations were unknown for this species, so it was unclear how effective females are at choosing nesting environments that are beneficial to successful hatching of their young. Using radiotelemetry and modeling of thermal conditions of the soil at nest depths (using the neural network techniques), we have been able to determine the extent to which female nesting behavior potentially benefits their offspring. By following gravid females equipped with miniature transmitters, we were able to follow female lizards to their nest sites. Several features of nest site selection were quite notable: 1) females chose the the warmest microsites available within the habitat, 2) females nested at night when temperatures in the environment were cool, likely because females could not tolerate the surface temperatures where they chose to nest during the daytime, 3) females often moved several hundred meters from their normal home ranges (when non-gravid), impressive for a small lizard, and 4) females chose permanent openings in the forest canopy that provided high soil temperatures versus potential nesting sites near their home ranges that might provide appropriate nest temperatures for part of embryo development, but might fail as the forest canopy greens up over the course of the summer.
Life History Evolution in Ectotherms
As opposed to most species of animals, squamate reptiles (snakes and lizards) exhibit a pattern of growth contrary to Bergmann's rule, i.e., larger body size in cooler climates. Thermal constraints on activity have been proposed as one proximate mechanism to explain variation in suites of life history traits in snakes and lizards. For example, the longer that a lizard can maintain activity (by keeping body temperatures elevated through behavioral thermoregulation), the more time that it has to forage, but also the greater chance that it will encounter a predator and be eaten. Thus, thermal conditions can produce a trade off between growth and survival when variation in the environment favors increased activity. Using both experimental and theoretical approaches, I have shown that apparently adaptive variation in body size in nature can simply be the product of phenotypic and behavioral plasticity. Future work in this area will focus on the responses of individuals to field manipulations of food and biophysical resources to further integrate proximate mechanisms into evolutionary theory.
Proximate and Evolutionary Aspects of Physiological Performance
Population dynamics of animals depend on many factors that influence survival and reproductive success of individuals including aspects of their behavior, physiology, and environmental circumstances. Behavior and physiology often vary among individuals, resulting in multiple strategies within a population to cope with changing environmental conditions. Beyond the separate effects of behavior and physiology, the physiological capacities of individuals can affect their behavioral options, and their behavioral choices (e.g., with respect to microhabitat selection) can affect their physiological capacities. Hence, understanding the ecology of individuals will require that we comprehend not only behavior, physiology, and environment, but also how they interact. In collaboration with Jack Hayes (U. Nevada-Reno), I have examined how physiological capacity in thermogenesis influences rates of activity under extreme cold conditions in deer mice both in the lab and in nature. Findings to date show that animals either simply don't have the capacity to tolerate the cold and avoid it, that animals with higher thermogenic capacity are more active under cold conditions, or that animals willing to undergo cold exposure may be able to upregulate thermogenesis and tolerate cold. Future efforts will model how this combination of strategies might be maintained in a natural population that experiences environmental variation in annual temperatures.
Evolution of Endothermy
Many theories have been proposed to explain the evolution of endothermy in birds and mammals. Of these theories, the aerobic capacity model has gained the most support. Rather than a response to selection for thermoregulatory capabilities, the aerobic capacity model suggests that endothermy evolved as a response to selection for sustained locomotor activity (or increased maximal aerobic capacity) and that elevated resting metabolic rate evolved as a correlated response. If true, then a correlation between maximal and resting metabolic rates represents a design constraint for birds and mammals. Working with Jack Hayes, I have helped initiate an artificial selection experiment that is attempting to break the genetic correlation between maximal and resting metabolic rates. Future work in my lab will examine correlated responses of different aspects of physiology and ife history for mice from the selected lines.