Background/Question/Methods Plastic or evolutionary shifts in phenology—the timing of life events—are key ways organisms respond to their environment. The timing of reproduction, for example, incurs strong selective pressure because of its direct impacts on fitness. But comprehensive data on reproductive timing is conspicuously lacking in one of the world’s most prominent groups of land animals—the ants. Ecological studies often focus on ant workers—the agents through which mature colonies interact with their environment—rather than on ant queens and males whose survival and reproductive success constitute the direct fitness of ant lineages. Here we characterize the mating phenology of ant communities across the USA by synthesizing data from three sources that vary in temporal and spatial scale: 1) museum records dating back over a century, 2) continuous sampling of flying insects throughout one growing season from several sites in one US state, and 3) opportunistic observations of mating ants at one site over several years. Using this dataset, we characterize how ant mating seasons vary across time and space, test whether they have shifted in response to climate change, and compare the climate sensitivity of invasive and native ants. Results/Conclusions Ant mating seasons occur later at higher latitudes. The median reproductive date for northern ants occurs five weeks later than southern ones, and average mating date across all ants increases by about 2 days per degree of latitude. Reproductive timing is also twice as variable at low latitudes compared to high latitudes. Mating seasons can occur year-round in extreme southern areas but occur mostly in late summer in the far north. Consistent with warming temperatures, nearly half the native species analyzed have shifted their mating seasons in recent decades, with mating advancing over half a day per year. No species have delayed their mating seasons. Several invasive species, in contrast, showed no consistent shifts in mating phenology, suggesting they are less sensitive to climate warming. Our results highlight a previously undocumented sensitivity of ants to climate change mediated by shifts in mating season. We also present evidence that the mating phenologies, and presumably the reproductive success, of invasive species are less sensitive to warming temperatures than native ones, presenting a novel mechanism for shifts in ant community composition under climate change. Taken together, this study is a first step toward a comprehensive theory of variation in reproductive timing in one of the world’s most conspicuous animal taxa.
