After the general introduction presented in Chapter 1, Chapter 2 tackles the problem of disjointed and conflicting data, and explores to what extent do loggerhead turtle populations and life stages differ in morphology. In order to take into account the possible geographic and life stage variability, I study two neighboring populations and all postembryonic life stages by comparing the ratio of carapace length, width and height of sea turtles. I conduct a detailed analysis of empirical models (growth curves, conversion formulae). One of the aims is to answer a somewhat technical question whether or not can the growth of loggerhead turtles be considered isomorphic. Considerable deviations from isomorphy would require additional steps when defining through out the life the acquisition (or use) of energy in relation to the surface area-volume ratio. The focus of Chapter 3 is on developing a full life cycle model of loggerhead turtles. Due to substantial variability present in data related to loggerhead turtles living in different sea basins, I decided to focus on a geographically defined population rather than the whole species. In this chapter the North Atlantic population of loggerhead turtles is analyzed as it has one of the largest nesting aggregations of loggerhead turtles. After estimating the parameter values using the covariation method [126] of the package DEBtool implemented in Matlab, I compare model predictions to observations, and discuss the implications of the results. In Chapter 4 another population of loggerhead turtles, the Mediterranean population, is the main focus, together with the comparison between individuals belonging to the Mediterranean, and individuals belonging to the North Atlantic population. Individuals belonging to the two populations are first compared based solely on their morphology (length, weight, and the ratio of the two) at two life events: hatching and nesting. The average egg size reported for each population is taken into account, as it has been generally reported to account for most of the variation in hatchling sizes. As the next step, I develop a DEB model for the individuals of the Mediterranean population, analyze the model predictions, and discuss the implications of the results. Then I compare the model parameters between the populations, and suggest a physiological (maturity based) explanation for the adults having such markedly different sizes at nesting. In addition, posthatchling growth is analyzed in more detail, expanding the results of the previous chapter which suggested faster growth of posthatchlings than predicted by the model. Lastly, I reproduce a pattern of biphasic growth by modifying the food availability during the first part of the life cycle. Chapter 5 showcases the applications of the DEB model to study the effects of temperature and food availability, and the effects of plastic ingestion on the energy budget and life cycle of the loggerhead turtle. I simulate a realistic range of temperatures and food densities to explore their effect on the energy budget, i.e. observable quantities such as size and reproduction output. I present a mechanism for the effects of plastic ingestion on 6 General introduction the energy budget, applicable to any species for which the DEB parameters are known. I simulate a range of observed amounts of ingested debris, and study their effects on the processes of growth, maturation, and reproduction while assuming the plastic has (a) the same, and (b) several times longer gut residence time compared to that of food. Finally, in Chapter 6 I discuss my results in a broader context, and present an outlook on future studies, applications, and possible expansions of the developed model.