At Mating Time, These Ants Carry Their Young Queen to a Neighbor’s Nest

https://www.nytimes.com/2021/05/13/science/ants-queens-inbreeding.html

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We humans have Tinder, Hinge, eHarmony and Grindr. For other animals, there’s a real dearth of matchmaking services, not even Bumble or Plenty of Fish.

But for future queens of one ant species, sterile worker ants seem to serve this function by physically carrying their royal sisters to neighboring nests. There, the queens-to-be can mate with unrelated male ants, according to researchers in a study published this month in Communications Biology.

“This is quite exciting,” said Jürgen Heinze, a zoologist at the University of Regensburg in Germany and a co-author of the study. “It’s the first case of this assisted mate choice and assisted outbreeding that we have in animals.”

If you looked at the ground by riverbanks on the Mediterranean, you might occasionally catch a glint of sunlight reflecting off the wing of a Cardiocondyla elegans queen ant. But the queen would most likely not be flying, or even walking. Instead, she would be riding piggyback atop a worker ant, gripped firmly by a worker’s mandibles.

“Once you looked a lot for those colonies and for the carrying behavior, when you close your eyes in the evening, you only see these little ants moving around,” said Mathilde Vidal, a doctoral candidate at Regensburg and lead author of the study.

From 2014 to 2019, the researchers mapped the location of 175 Cardiocondyla ant colonies in Southern France and recorded 453 instances of this carrying behavior.

Though these workers are tiny — only 2-3 millimeters in length — they have been observed carrying the queens for up to almost 50 feet from home before dropping off their sisters at the entrance of a foreign nest. And the workers seemed to know where to take their sisters, traveling in more or less a straight line and skipping nests that were closer. Genetic experiments showed that ants in the nests the workers chose were less genetically related.

As it is for all sexually reproducing organisms, choosing the right mating partner is an important decision for Cardiocondyla elegans. But this particular species faces a particular problem: The male ants have lost their wings and remain trapped in “mating chambers” near the nest entrance where they regularly mate with related females. (Genetic data shows that more than ⅔ of all matings in Cardiocondyla involve close relatives.)

Excessive inbreeding can be detrimental. In a 2006 study, Dr. Heinze and his colleagues found that prolonged inbreeding in another species of Cardiocondyla had led to unhealthier ant colonies: shorter life spans for the queen, higher offspring mortality, altered sex ratios.

Most ant species counteract this with outbreeding through nuptial flights — spectacular single-day events during which winged queens and males from many different colonies gather, swarm and mate in large clouds. But Cardiocondyla elegans queens require some help.

There is also evidence that at least some young queens are carried from one nest to another, potentially mating with males from multiple colonies. No young queen ever returns to her home nest, spending the winter instead in a foreign nest. In spring, she is kicked out — there can be only one egg-laying queen per nest — and presumably starts a colony of her own, starting the cycle anew.

There is only one season of mating for these young queens, but that is more than enough. A queen stores and preserves her mates’ sperm in a sac called the spermatheca for the rest of her life. In some species, just two sperm cells are needed to fertilize an egg and that is all that the queen releases (somewhat more efficient than the 40 million to 150 million sperm humans use to accomplish a similar task).

Though the traditional view of social insect society has held that the queen wields all of the power over the faceless workers beneath her, research is increasingly showing that this is not the case, said Boris Baer, an entomologist at the University of California, Riverside. And this new research provides yet another example.

“It looks like that the workers have taken that power that they have in these societies in their own hands, and they make decisions about the mating of their sisters,” said Dr. Baer, who was not involved in the study.

Still, one large mystery remains: “We have no idea how they choose a specific colony,” Dr. Baer said.

So far, the researchers have not been able to get ants they have collected to perform the carrying behavior in a controlled laboratory setting. Still, the new research underscores the diverse ways that living things in general and ants in particular reproduce in our world.

“Wherever I go and find a new species of Cardiocondyla, they have a different system of mating, they have a different colony structure, they have different ways of dispersal,” Dr. Heinze said.