Cryptic Reproductive Costs of Heatwaves for Animal Populations

Heatwaves are among the most serious and immediate threats posed by climate change to biodiversity. While their impacts on mortality are obvious and often severe, heatwaves also have hidden sublethal effects that are only now beginning to be understood. Sublethal effects are particularly significant in reproduction, where heatwaves not only have immediate impacts on fertility and reproductive success, but also trigger transgenerational effects with potential long-term consequences for populations. Heatwaves impact fitness across all reproductive stages, from gamete formation to offspring production, development, and beyond. These impacts can be cryptic and difficult to observe, potentially extending across generations via parental and epigenetic effects that influence offspring fitness. Crucially, these changes to reproductive phenotypes can then further impact populations by altering sexual selection processes. By increasing variance in reproductive traits, heatwaves can influence both intra- and inter-sexual selection processes and affect pre- and post-copulatory episodes of selection. However, empirical evidence on how heatwaves modulate sexual selection and its evolutionary consequences remains scarce. We argue that identifying and quantifying the cryptic reproductive costs of heatwaves is critical to understanding their impacts on populations at both demographic and evolutionary timescales.