Ants are mating a day earlier every decade

Somebody sat down with a hundred years of pinned ants and worked out that they are, on average, mating earlier than they used to.

That is the short version of Helms (2022), published in Insectes Sociaux. Across a long run of North American museum specimens, nuptial flight dates have been creeping forward as the climate has warmed, at something like a day per decade in the species that show the shift, which was roughly a quarter of the native species examined. Slow, steady, and in the direction you would guess.

It is a quietly clever piece of work, mostly because of where the data came from.

You cannot run a weather-controlled experiment on a century of ant flights. Nobody was taking systematic notes in 1920. But there is a record, and it has been sitting in museum drawers the whole time.

Every properly curated specimen carries a little label: what it is, where it was caught, and when. A winged queen or male on that pin was almost certainly collected on or very close to the day it flew, because that is the only time most ants are out with wings on. Gather up enough of those labels, spread across decades and states, and you have built, by accident, a log of when flights happened long before anyone thought to ask the question.

That is the part I like. The dataset was assembled by generations of collectors who had no idea they were measuring the climate.

The timing shift did not come out of nowhere. Flight dates were already known to move with the landscape. Boomsma and Leusink noted decades earlier that a species flies later where it is colder, and the museum record carries the same fingerprint at continental scale: ants fly later the higher up and the further north you go, by about a day for every degree of latitude and for every hundred metres of elevation.

So warming the whole continent by a bit nudges the calendar the same way that moving a colony downhill or south would. The trend is the same lever, pulled slowly, by the climate rather than by geography.

Here is the honest limit, because it matters.

A drift of a day per decade is a real signal about the last hundred years. It tells you close to nothing about which afternoon a flight will happen in your garden this July. The year-to-year swings from a hot summer versus a cool one, and the differences between species and between neighbouring towns, are all far bigger than a slow centennial creep. The trend is a fact about history, not a countdown.

Dealate scores the odds of a flight near you from this week's actual weather, species by species. It does not claim to forecast the climate, and it does not claim to have called any particular year right. The Helms result is a lovely bit of long-lens science. It is not a crystal ball, and neither are we.

If you want the near-term picture instead, what is a nuptial flight covers the event itself, and ants don't read the barometer covers the weather that really moves a flight.

Helms, J.A. (2022). Nuptial flights and the evolution of mating phenology in ants. Insectes Sociaux 69: 227–237. DOI 10.1007/s00040-022-00888-y.