Ecophysiological responses of sea turtles to global change

Abstract

The global effects of climate change are ubiquitous. Whether and how species respond to these changes will determine their populations’ persistence. As long-lived marine ectotherms with temperature-dependent sex determination (TSD), sea turtles are highly vulnerable to global temperature changes. In this thesis, I explore responses to various environmental pressures on this taxon, and some of their underlying proximate mechanisms. In Chapter One, together with colleagues, I show that 35% of variation in the pivotal temperature (the incubation temperature producing 50:50 male/female offspring) among sea turtle populations can be explained by regional climate, suggesting local adaptation of this complex trait. Building various adaptive models projecting population demographics, I find neither speculated heritability nor plastic matching of the pivotal temperature to global warming would likely be sufficient to prevent the feminisation of most populations worldwide, if climate warming exceeds 2 °C. Chapter Two unveils a previously unknown mechanism in sea turtles, where maternally derived sex steroid hormones likely influence offspring sex ratios independently of temperature. This could be a possible mechanism to facilitate plasticity in the TSD response. Global change may also disrupt biotic interactions such as host-parasite dynamics. Chapter Three shows that the prevalence of leech parasites in the only significant rookery of loggerhead turtles in the eastern Atlantic has increased over the last decade. This increase has resulted in a possible size-specific trade-off, where the smallest infected turtles invest less in reproduction following a bet-hedging strategy, and the largest infected turtles terminally invest. Chapter Four provides some evidence that this immunity-reproduction trade-off is at least in part mediated by the sex steroid hormone oestradiol. Altogether, my work demonstrates how many elements of species’ ecology and evolution are impacted by climate change. Understanding their response will contribute to more effective conservation measures.