A Comparative Study of Habitat Complexity, Neuroanatomy, and Cognitive Behavior in Anolis Lizards

Abstract

<p>Changing environmental conditions may present substantial challenges to organisms experiencing them. In animals, the fastest way to respond to these changes is often by altering behavior. This ability, called behavioral flexibility, varies among species and can be studied on several levels. First, the extent of behavioral flexibility exhibited by a species can be determined by observation of that species' behavior, either in nature or in experimental settings. Second, because the central nervous system is the substrate determining behavior, neuroanatomy can be studied as the proximate cause of behavioral flexibility. Finally, the ultimate causation can be examined by studying ecological factors that favor the evolution of behavioral flexibility. In this dissertation, I investigate behavioral flexibility across all three levels by examining the relationship between habitat structure, the size of different structures within the brain and total brain size, and behavioral flexibility in six closely-related species of Puerto Rican <italic>Anolis</italic> lizards. <italic>Anolis</italic> lizards provide an excellent taxon for this study as certain species, including those used here, are classified as belonging to different ecomorphs and are morphologically and behaviorally specialized to distinct structural habitat types.</p><p>In order to determine the presence of behavioral flexibility in <italic>Anolis</italic>, I first presented <italic>Anolis evermanni</italic> with a series of tasks requiring motor learning and a single instance of reversal learning. <italic>Anolis evermanni</italic> demonstrated high levels of behavioral flexibility in both tasks.</p><p>To address the pattern of brain evolution in the <italic>Anolis</italic> brain, I used a histological approach to measure the volume of the whole brain, telencephalon, dorsal cortex, dorsomedial cortex, medial cortex, dorsal ventricular ridge, cerebellum, and medulla in six closely-related species of Puerto Rican <italic>Anolis</italic> lizards belonging to three ecomorphs. These data were analyzed to determine the relative contribution of concerted and mosaic brain evolution to <italic>Anolis</italic> brain evolution. The cerebellum showed a trend toward mosaic evolution while the remaining brain structures matched the predictions of concerted brain evolution. </p><p>I then examined the relationship between the complexity of structural habitat occupied by each species and brain size in order to determine if complex habitats are associated with relatively large brains. I measured brain volume using histological methods and directly measured habitat complexity in all six species. Using Principal Component Analysis, I condensed the measures of habitat structure to a single variable and corrected it for the scale of each lizard species' movement, calling the resulting measurement relevant habitat complexity. I tested the relationship between relative volume of the telencephalon, dorsal cortex, dorsomedial cortex, and whole brain against both relative habitat complexity and ecomorph classification. There was no relationship between the relative volume of any brain structure examined and either relevant habitat complexity or ecomorph. However, relevant habitat complexities for each species did not completely match their ecomorph classifications. </p><p>Finally, I tested the levels of behavioral flexibility of three species of <italic>Anolis</italic>, <italic>A. evermanni</italic>, <italic>A. pulchellus</italic>, and <italic>A. cristatellus</italic>, belonging to three distinct ecomorphs, by presenting them with tasks requiring motor and reversal learning. <italic>Anolis evermanni</italic> performed well in both tasks, while <italic>A. pulchellus</italic> required more trials to learn the motor task. Only a single <italic>Anolis cristatellus</italic> was able to perform either task. <italic>Anolis evermanni</italic> displayed lower levels of neophobia than the other species, which may be related to its superior performance.</p><p>In combination, this research suggests that <italic>Anolis</italic> of different ecomorphs display different levels of behavioral flexibility. At the proximate level, this difference in behavioral flexibility cannot be explained by changes in the relative size of the total brain or brain structures associated with cognitive abilities in other taxa. At the ultimate level, the size of the brain and several constituent structures cannot be predicted by habitat complexity. However, behavioral flexibility in certain tasks may be favored by utilization of complex habitats. Flexibility in different tasks is not correlated, rendering broad comparisons to a habitat complexity problematic.</p>