Climate and mating phenology in ants

Plastic or evolutionary shifts in phenology are key ways organisms respond to their environment.  Reproductive timing incurs strong selective pressure because it directly impacts fitness.  We lack comprehensive data on reproductive timing in one of the world’s most prominent insect groups—the ants.  Ecological studies often focus on ant workers—the agents through which colonies interact with their environment—rather than on queens and males whose survival and reproduction constitute the direct fitness of ant lineages.  We characterize the mating phenology of ants across the USA by synthesizing data from museum records dating back over a century.  We characterize how ant mating seasons vary across time and space and test whether they have shifted in response to climate change.  Average mating date across the ants is delayed by about 1 day per degree of latitude and per 100 meters elevation, and has been advancing about 0.1 day per year.  Reproductive timing is nearly twice as variable at low latitudes than high, such that mating seasons can occur year-round in southern areas but occur mostly in late summer in the far north.  Consistent with warming temperatures, a fourth of native species analyzed have advanced their mating seasons by 0.9 days per year in recent decades.  One species has delayed its mating season by 1 day per year.  Our results highlight the sensitivity of ants to climate change mediated by shifts in mating season and present a novel mechanism for climate-driven changes in ant community composition.