Nesting ecology of reptiles: Effects of maternal nesting behavior and egg incubation environments on offspring phenotypes and fitness

Abstract

Understanding the role of the environment in shaping phenotypic variation has been a goal at the heart of evolutionary biology since its inception. Through the work that led to the modern synthesis framework, we see that the environment can act as a force in natural selection, leading to the “survival of the fittest” in a given environment. However, from more recent work, we also see that the environment can shape variation through phenotypic plasticity, or the ability of a genotype to produce more than one phenotype in response to the environment. The role of plasticity in evolution is still a complicated topic, particularly with regard to how selection can act on plastic traits and how plasticity contributes to an individual’s fitness. My PhD work aims to understand the adaptive nature of phenotypic plasticity with a particular focus on maternal nest site choice and its effects on offspring development. An organism’s developmental environment is composed of several dynamic components (e.g. temperature, moisture) that can influence survival and phenotypic outcomes at this very critical early-life stage. While there is a significant amount of literature on developmental plasticity, there is a lack of studies that examine these topics under ecologically relevant conditions. Oviparous reptiles serve as excellent systems for studies on developmental plasticity because many species exhibit no parental care after the point of nesting. This allows for examining the effects of nest site selection and nest environment on development without the confounding effects of parental care throughout development. My work uses these systems to test the adaptive significance of nesting behavior by examining its effects on offspring fitness across life stages in reptile systems. The first chapter of my dissertation assessed nest-site choice and its effects on offspring in the Western painted turtle (Chrysemys picta). I used long-term data on nest-site selection and natural nest microclimates to design a cross-fostering experiment that tested the effects of canopy cover on egg hatching success. I found that female painted turtles choose nest sites that are relatively warm with open canopy, which increase egg hatching success. The results from this experiment support nesting as an adaptive behavior. For the remaining chapters of my dissertation, I focused on a different study system, the brown anole (Anolis sagrei). My second chapter sought to assess brown anole nesting behavior in the field. Though anoles are a popular system for research across several biological disciplines, very little was known about their nesting behavior beyond chance natural history encounters and lab studies. I collected the largest data set to date on anole nest-site choice in the field, and established that females actively choose nest sites with specific environmental parameters. Once this was established, I also incubated anole eggs in the field to determine which conditions improve hatching success and hatchling condition. Taken together, the results of my experiments show that females select nest sites that improve survival probability of offspring, again suggesting that their nesting behavior is adaptive. In my third chapter, I tested the effect of predation pressure on nesting behavior in the brown anole. Curly tailed lizards (Leiocephalus carinatus) overlap in their native range and are established predators for brown anoles, though they are confined to the ground and cannot prey on anoles that are perched in trees. Previous work on their relationship in the field has shown that curly tail presence leads to reduced exploration and higher perching in the brown anole. We housed a group of brown anoles in enclosures with and without curly tails and gave them a choice of a nesting container that was on vs above the ground. Females that were in enclosures with predators used above-ground nesting containers more often than females in predator-free enclosures. Finally, for my fourth chapter, I conducted a study that examined the effects of a continuous range of constant temperature incubation treatments on offspring development and survival to adulthood in the brown anole. I incubated eggs across eight treatments and released the resulting hatchlings into the field to assess survival to adulthood. We found that optimum incubation conditions for survival varied among life stages. Though these results are novel on their own, these data have more potential to be used in theoretical models estimating environmental impact on lifetime fitness. Overall, my dissertation work supports the idea that nesting is an adaptive behavior and underlines the importance of developmental conditions in shaping offspring fitness across life stages.