Systematic approaches to assessing high-temperature limits to fertility in animals

: Critical Thermal Limits (CTLs) gauge the physiological impact of temperature on survival or critical biological function, aiding predictions of species range shifts and climatic resilience. Two recent Drosophila species studies, using similar approaches to determine temperatures that induce sterility (Thermal Fertility Limits; TFLs), reveal that TFLs are often lower than CTLs, and that TFLs better predict both current species distributions and extinction probability. Moreover, many studies show fertility is more sensitive at less extreme temperatures than survival (Thermal Sensitivity of Fertility: TSF). These results present a more pessimistic outlook on climate change consequences. However, unlike CTLs, TFL data are limited to Drosophila, and variability in TSF methods pose challenges in predicting species responses to increasing temperature. To address these data and methodological gaps, we propose three standardised approaches for assessing thermal impacts on fertility. We focus on adult obligate sexual terrestrial invertebrates, but also provide modifications for other animal groups and life history stages. We first outline a "gold-standard" protocol for determining TFLs, focusing on the effects of short-term heat shocks, simulating more frequent extreme heat events predicted by climate models. As this approach may be difficult to apply to some organisms, we then provide a standardised TSF protocol. Finally, we provide a framework to quantify fertility loss in response to extreme heat events in nature, given limitation in laboratory approaches. Applying these standardised approaches across many taxa, similar to CTLs, will allow robust tests of the impact of fertility loss on species responses to increasing temperatures.