Treated like a queen: Some ants carry her to mate in other nests, new study says

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There is a royal matchmaking service for a type of queen in southern France. Sisters carry their queen and drop her at her new home. But that home is a neighbouring nest — and this match is only for the would-be queens of Cardiocondyla ants.

German researchers have been studying Cardiocondyla ants to figure out why they carry their queen to other ant nests. Their study suggests the ants are trying to add genetic diversity and avoid inbreeding. 

"To survive, they need to have a bit of diversity," Mathilde Vidal, the study's lead author and doctoral candidate at the University of Regensburg, told As It Happens host Carol Off. "The only way for the future queen to mate with males is to find a male…. They first mate in their nest with their siblings — their brothers — and then they need to have some genetic diversity, so the carrying is to increase the outbreeding."

Vidal spoke about her findings, which were published this month in the Communications Biology journal. Here's part of that conversation.

Mathilde, what does it actually look like to see these queens being carried to another nest?

It actually looks like nothing if you don't look closely.

The more you look, the more you can see ... everywhere around you [are] those ants. They carry the queen on their backs. And the queen has some wings that can reflect a bit on the sun. So sometimes by just sitting on the ground and looking at the horizon … you can see something shiny. And if it's fast and going forward, that's going to be an ant being carried by another.

What is the purpose of carrying the queen away from her nest?

To survive, they need to have a bit of diversity.

In some other ant species, maybe you heard [about] nuptials of flying. When you have all the ants [with wings] in the air, flying at each other. The males fly, the females fly, they mate like this and then they form their own colony. 

In this species, they do not fly. So the only way for the future queen to mate with males is to find a male…. They first mate in their nest with their siblings — their brothers — and then they need to have some genetic diversity, so the carrying is to increase the outbreeding.

Is it her brothers who are carrying her to the other nest?

No, the sisters actually. 

The brothers are the ones for mating only. And the sisters are the workers of the colony. 

How far do these ants carry their sister queen in order to find some place where she's not going to mate with her brothers?

In this case, they can go sometimes to 14 metres.

I mean if they're close by, how do they know for sure they're not mating with their cousins and brothers?

That's a good question.... We are not even sure of the answer yet. 

Even if they go close by and it's still too related, too close to each other, it's not going to be a problem because they might just carry it again, and again, and eventually they will end up [in] some wilderness. They will be less related.

These ants could carry the queen and go right past nests that they say, "No, can't go there. We're related. Those are our cousins. Let's keep going."

That might be. We made some experiments, but so far we don't have any proof of that. So we do not know if they choose randomly ... or if they choose it because of this relatedness. We are trying to make some genetic analysis.

They just drop the queen inside and then go back to their home nest. But I'm not sure the workers know if they found the right place.

How does she feel about that? Does anyone ask her?

When I see them, a future queen is being dropped somewhere in the field. Sometimes the worker drops the queen because of the winds or because of other reasons. And then the queen is totally disorientated. She doesn't know where she goes. She doesn't know where she is. She is looking for a worker to carry her again. And she's completely lost. 

I think she's pretty OK being carried around because apparently she can't walk well on her own.

But wait a second, you said she had wings. Doesn't she know how to use those?

No, she doesn't.

We think that maybe [an] ancestor species that had the wings used them. And then at some point [they] just lost the ability but still somehow kept the wings. 

OK, so now once she's found a place where she's not going to be with any kissing cousins and then she has sex with them, does she stay there? Does that become a place where she puts down some roots?

She stays there for a while, but then eventually she has to move because there is only going to be one queen in each nest. 

The mother queen is going to be only one laying all the eggs. And of the [female] ones that are made during the summer, [the queens are] going to mate with brothers and then with unrelated males. 

They're going to stay over the winter, just in order not to die, and then when the spring comes the next year, they have to go out because only one queen can lay all the eggs. 

So the new one will just have to go away and find another nest to build.

Written by Mehek Mazhar. Interview produced by Ryan Chatterjee. Q&A edited for length and clarity.