Figure 1. X-Ray of a chameleon tongue extended and enveloping a prey item in the tongue pad. Black dots are markers used to track anatomical points during feeding movements.
Evolutionary Consequences of Dietary Specialization– My dissertation work expanded on my functional morphology background by integrating ecological studies. My primary goal has been to examine behavioral and functional adaptations in response to dietary specialization in North American horned lizards. In many organisms, ecological specialization is reflected in morphological and behavioral adaptations that are thought to confer a functional advantage in resource use. Among the 12 species of horned lizards, for example, an ant-eating lifestyle has resulted in unique but similar morphologies. However, dietary analyses revealed a lack of dietary fidelity and hinted at the possibility that genus may represent a continuum in the degree of specialization. To examine this variation in specialization, I took an integrated approach through a detailed morphological analysis, dietary analyses and measurement of performance traits (bite force and prey capture).
Figure 2. Feeding sequence of desert horned lizard (Phrynosoma platyrhinos) feeding on a harvester ant. Note how the lizard captures the prey using the tongue and swallows the ant whole.
I found that horned lizards do indeed represent a continuum of specialization in morphology, performance and diet. Similarly to ant-eating mammals, horned lizards that eat predominately ants have undergone changes in both morphology and feeding behavior. These ant-eating species exhibit a loss of jaw musculature and dentition, as well as reduced bite force capacity mitigated by an increase in prey capture rate. Yet our examination also revealed that a nested clade within horned lizards exhibits the opposing trend, with a robust morphology and performance capacity and also rapid prey capture rate. This morphologically robust group appears to have the best of both worlds: being able to eat hard prey items while at the same time still able to capture prey quickly. These findings indicate that becoming specialized may not always be an evolutionary dead end as is sometimes suggested and emphasizes the importance of examining ecomorphological studies in a comparative context. My current interests in this group revolve around the evolution of horn development and the relationship of horns to the functional and morphological variation displayed in the feeding system. Movies of lizard feeding behavior can be found here as well as on my links page.
Organismal Performance Capacity - My post-doctoral research has focused on three main projects that are based on the central tenet that organismal performance capacity can be used to examine behavioral, ecological and evolutionary relationships in communities of organisms. First, we have begun to examine the link between performance traits and fitness among four sympatric species of Caribbean Anolis, representing different ecomorphs (morphologically divergence related to differences in habitat use). Over the past three years, we have measured performance traits (such as bite force and sprint speed) and determined survivorship in a population of Anolis. Our preliminary results suggest that selection on performance and morphological traits may differ depending on the ecomorph and imply that seemingly important performance tasks may not be equally relevant for all species in a community.
Second, we are examining the role of performance traits in structuring a community of lizards by addressing not only the resources used, but also how well species perform at tasks that allow them to utilize these resources. In this study, we simultaneously examine performance, resource use and resource availability in a community of 10 species of lizards. We have found differing results for our two performance traits: bite force (linked to diet), and vertical sprint speed (linked to the ability to utilize arboreal habitat). Bite force capacity is sufficient for 70% of the species to access 95% of the available prey, and even the poorest biters can access more than 70% of the prey, suggesting that dietary constraints are unlikely in this community. In contrast, poor vertical locomotor performance by half of the species indicates an inability to access both vertical and horizontal components of the habitat. These findings reveal the importance of performance traits in community studies because they indicate how key aspects of community structure can be constrained by differential performance capabilities.
Third, in the communities of lizards that we are studying multiple species display large and conspicuous sexual signals and we have been examining the relationship between sexual signal size and performance capacity. In many species of lizards males fight over territories that allow them access to females, and during these interactions males will display sexual signals to competing individuals. It is thought that these signals provide information about an invdividuals "quality" (click here to see a movie of the ornate tree lizard displaying), such as fighting ability, and that they potentially reduce the number of fights by allowing individuals to asses competitors before engaging in potentially damaging fights. So far we have begun to investigate the relationship between performance capacity (bite force & sprint speed) and sexual signals in several populations of the ornate tree lizard (Urosaurus ornatus) which exhibit variation in the size of their sexual signals (Figure 3).
Figure 3. Variation in throat coloration (left) and belly patch size (right) in a single population of Urosaurus ornatus from Arizona.
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