Non-invasive biomarkers of reproduction in individuals and populations of wild vertebrates

An increasing number of vertebrate species are under threat of extinction, and conservation efforts are vital to the survival of many species. Knowledge of reproductive processes across taxa, and how different factors can impact reproductive success, is essential for supporting conservation breeding and mitigating threats to reproductive success of wild populations. Although historically much of our knowledge came from model species and more invasive techniques, non-invasive monitoring of biomarkers related to reproduction can be invaluable to monitor individuals and populations over prolonged periods, crucial to gaining fundamental knowledge on reproductive physiology across a wider range of species and locations, and for evidence-based management of reproduction. Advances over the last few decades have increased the sample types that can be used and the potential biomarkers that can be investigated, but the proportion of vertebrates to which these techniques have been applied remains relatively low. The aim of this review is to highlight some of the hormones and other biomarkers that can inform on reproductive function, the sample types and considerations for non-invasive monitoring, and where efforts are required to further our understanding of reproduction in both individuals and populations of wild vertebrates. Increased application of these techniques across species could not only improve the success of conservation efforts but also help to assess the increasing threats to reproduction, such as by endocrine disruptors in natural environments, that have the potential to impact all species on the planet.