Animals that attract mates using visual signals are casting their message into a world full of hostile entities - predators, prey, competitors - all of which are eager to intercept their message. Creating signals that will capture the attention of the intended target without attracting unwanted attention is a constant challenge for wild animals. The objective of this study is to uncover how variation in signal color influences how competing selective forces act on the individual using lizards in the neotropical genus Anolis. Specifically, the ornament color of males in the wild will be manipulated to see how this affects individual survival and reproductive success. Another component of this study will be to determine how an individual's outcome can change depending on population-level variation in the strength of competition, which will be altered by manipulating the sex ratio in six experimental populations.
This study will help elucidate how several competing forces can act in conflict or in concert to shape animal signals, a key question in the study of ecological and evolutionary biology. The diversity of animal signals, especially visual signals, is thought to be an important force in creating species diversity; the results of this research may therefore also contribute to our understanding of how natural processes create and maintain species diversity.
The diversity of animal species - both in terms of sheer numbers, and in terms of the variety of shapes, sizes and colors - is the result of evolution, driven primarily by natural selection. Understanding the origins of diversity therefore requires an understanding of the mechanisms of selection. Natural selection is sometimes discussed as if it is a single force, but in fact it is a collection of many forces acting in concert or conflict. The evolution of a population will be determined by the interaction between multiple types of selection. The goal of this project was to measure the effects of selection on a visual ornament, the dewlap, in Anolis lizards. The dewlap is a patch of brightly colored skin on the throat of these lizards that can be stretched out and displayed during social interactions, such as courtship or fights over territories (figure 1). Among the hundreds of species of Anolis found in the Americas and throughout the Caribbean, no two have identical dewlaps (figure 2). The forces that have generated this impressive diversity are poorly understood, but two major categories of selection are thought to be especially important: sexual selection and selection for survival. Sexual selection is selection related to the ability to attract and defend a mate. Usually, sexual selection favors large and brightly colored signals, either because they are attractive to members of the opposite sex (intersexual selection) or because they are intimidating to members of the opposite sex (intrasexual selection). Selection for survival may act in the opposite direction, favoring smaller, duller signals, because they are less likely to attract the attention of predators. We set out to measure the force of sexual and survival selection on the appearance of the dewlap in male A. sagrei, an abundant invasive species in Florida (and worldwide). We accomplished this by tracking the survival and reproductive success of individual lizards in six experimental populations in Florida. Survival was measured by marking each individual in the population with a unique tag in April, and resampling the population in August to determine which animals were still present. Sexual selection was measured by capturing juvenile lizards in each population in August and using a large number of random genetic markers to determine the mother and father of each. This allowed a total number of offspring to be assigned to each adult in the population. The contribution of dewlap color to an individual lizard's survival and reproductive success was determined by experimentally manipulating the dewlap at the start of the experiment. Each male in the population was given one of three treatments to permanently alter the color of his dewlap. In group 1, the edge of the dewlap was obscured using red tattoo pigments (figure 3a). In group 2, the edge of the dewlap was altered using yellow tattoo pigments (figure 3b). In the third group, the central portion of the dewlap was treated with red pigments (figure 3c). A comparison of the survival of these three groups showed that manipulation of dewlap color led to drastically different outcomes in survival. Males from group 1 and 2 had higher survival than males from group 3 (figure 4). The lowest survival was found in the treatment group that did not affect the color of the dewlap edge, suggesting that the color of the margin is a major factor in determining survival. One possible explanation for this pattern is that our edge treatments may have reduced the ultraviolet color of the dewlap, making these animal less noticeable to birds and other predators. The analysis of sexual selection in these populations is currently underway and will be completed by December of this year. In a similar parentage study which we have just completed (manuscript in prep), we found that natural variation in dewlap color affects an individual's ability to gain mates, but that this relationship depends on the reproductive strategy pursued by a male. The final analysis of the data generated by these projects will provide an important step forward in our understanding of the causal link between dewlap coloration and reproductive success. In summary, this project has already generated important insights into the evolution of a visual ornament in a morphologically diverse and species rich group of neotropical lizards. We found that ornament coloration has a significant impact on an individual's survival. Our ongoing analysis of sexual selection will provide an important counterpoint to this data. If our prediction that sexual selection acts in opposition to survival selection is supported, we will be able to make concrete predictions about how evolution will proceed when sexual selection or predation is increased or decreased. Regardless of what our results show, the data generated by this project will begin to explain how changes in selection lead populations to diverge and ultimately, to form new species.
