Tomorrow is yesterday: early-life conditions shape ectotherm life histories

Sammanfattning: Life history theory seeks to explain the overwhelming diversity of resource allocation strategies in nature by exploring how evolutionary forces optimise survival and reproduction within an organism’s environment. Central to this theory is that resources are finite, and thus increased investment into one trait may reduce resources available for another trait. Understanding proximate drivers of resource allocation is therefore of great interest in evolutionary biology. In ectotherms, temperature is the most influential external factor and affects most physiological and behavioural functions. While temperature can influence traits at any life stage, changes during development are often irreversible and therefore most influential to fitness. However, predicting the direction of effects is challenging as these are often context-dependent and vary considerably among species and populations. This thesis aims to add to our understanding of proximate and long-term effects of early-life thermal conditions on traits associated with survival and reproduction. This work is the outcome of a collaboration between the University of Gothenburg and the University of Tasmania. I used two well-established ectotherm systems; the Swedish sand lizard and the Tasmanian spotted snow skink, to explore early-life effects on long-term life history traits. Both these systems are ideal models with decades of accumulated research detailing their physiology, life history and population structures. Leveraging this knowledge, I combined experimental manipulations and long-term mark-recapture data to examine the effects of early-life thermal conditions on key life history traits. I initially explored how incubation temperature affected sex determination and sex-specific telomere dynamics in neonate sand lizards. Sex is a crucial factor shaping life history strategies across all life stages. Telomeres, which protect chromosome ends, are vital components for organismal health, and their shortening has been linked to individual fitness and population structures. I showed that certain incubation temperatures can override chromosomal sex causing an overproduction of daughters. Additionally, incubation temperature influenced neonatal telomere maintenance, but sex was the strongest predictor of telomere dynamics. Female neonates had longer telomeres and better maintained them compared to males. This is likely explained by sex-specific selection on telomere length, in which it is a better predictor of fitness in females compared to males. I then used longitudinal mark-recapture data to investigate the influence of early-life thermal conditions on the developmental rates of wild spotted snow skinks, and whether maturing decisions have costs to survival and reproduction later in life. Warmer early-life conditions caused females to mature earlier, grow larger, produce more offspring by midlife and live longer. This presented a life history paradox. To solve it, I applied survival trajectory models to the same long-term dataset to determine the latent costs of developmental rates. Specifically, whether females maturing early or large suffer increased rates of age-dependent declines in survival or fecundity (senescence). While maturing early did not cause increased rates of senescence, females that exhibited faster growth and matured large suffered significantly higher mortality rates later in life compared to smaller females maturing at the same age. This thesis highlights the importance of considering variation in early-life conditions as drivers of individual heterogeneity in life history traits and resource allocation strategies. With constantly rising global temperatures, thermal effects on such traits have never been more relevant and can clearly affect the future health of ectotherm populations and influence their evolution.