Embryos are largely understudied in a representative sample of journals in conservation physiology

Understanding how animals respond to environmental stressors across their life cycle is essential for predicting species' vulnerability to climate change. Here, we systematically reviewed the literature to quantify the variation in research effort on different life stages in the field of conservation physiology. Specifically, we reviewed experimental studies measuring physiological and life-history responses to climatic stressors across three representative scientific journals: Conservation Physiology, Journal of Thermal Biology and Journal of Experimental Biology. Our systematic map of 1276 studies revealed a pronounced underrepresentation of studies on embryos, representing only 8% to 9% of studies. This pattern was remarkably consistent across all axes considered (i.e. journals, taxonomic groups, physiological traits and environmental stressors). We also found that 80% of studies only investigated single life stages, and over 5% of studies did not clearly report the life stage(s) used. Despite the increasing recognition of the ecological importance and sensitivity of early life stages to environmental stressors, we found no evidence that research on embryos has gained traction over the past decade (2013-2024). We argue that these ontogenetic biases likely reflect a combination of historical precedents and enduring methodological and logistical constraints that continue to shape research agendas. To build a more holistic understanding across the life cycle, we: (i) call for a paradigm shift placing embryos at the center of experimental agendas, (ii) outline emerging methodological advances that increase the feasibility of research on early life stages, (iii) demonstrate how studies on embryos align with ethical considerations for animal research, (iv) highlight perspectives for future evidence syntheses and study reporting and (v) promote investigations of the mechanisms underlying physiological variation across ontogeny. Closing the ontogenetic gap will be key to improving our ability to predict population-level impacts of climate change and guiding more effective conservation and management interventions.