Ants Seem to Defy Biology: They Lay Eggs That Hatch into Another Species

Iberian harvester ant queens produce offspring of their own species and of the builder harvester ant, seemingly by cloning males

Two brothers of different species, produced by the same mother: Messor ibericus (left) and Messor structor (right). Jonathan Romiguier

Iberian harvester ant queens have a unique superpower: They can lay eggs that hatch into an entirely different species.

This discovery, described in a new paper published September 3 in the journal Nature, defies a fundamental principle of biology and may cause scientists to reconsider how they define a species.

“The classic concept says that [a species] is a group of organisms with similar physical and genetic characteristics that can reproduce with each other in nature and produce fertile offspring,” says Xim Cerdá, an ecologist at Doñana Biological Station in Spain who was not involved with the research, to Miguel Ángel Criado at El País. “But it turns out that’s not the case; two species are needed here. We’re going to have to rethink the concept.”

Scientists recently discovered that Iberian harvester ant queens (Messor ibericus) mate with males of another species, the builder harvester ant (Messor structor). When they do, the M. ibericus queens store the M. structor male’s sperm, then use it to fertilize some of the eggs they lay. Researchers think the M. ibericus queens remove their own genetic material from the eggs’ nuclei, so that when those eggs hatch, they effectively turn out to be M. structor male clones.

The queens produce males of both M. ibericus and M. structor, and all the worker ants in M. ibericus colonies are female hybrids of the two species.

“It’s an absolutely fantastic, bizarre story of a system that allows things to happen that seem almost unimaginable,” says Jacobus Boomsma, an evolutionary biologist at the University of Copenhagen who was not involved with the research, to Nature’s Max Kozlov.

'How is this possible?' - two species birthed by one mother

Even more perplexing is the fact that M. ibericus and M. structor are not closely related, evolutionarily speaking. The two species diverged more than five million years ago, according to the paper. For comparison, scientists think humans and chimpanzees split from a common ancestor that lived between six million and eight million years ago.

Proving the relationship between M. ibericus and M. structor was challenging. The scientists dug up various M. ibericus colonies they found along the sides of farm roads near Lyon, France, looking for male ants. But among a colony of 10,000 ants, there might be only a few males, writes Science’s Erik Stokstad.

In the end, they found 132 males from 26 M. ibericus colonies. Of those, about half were nearly hairless—a hallmark of M. structor—while the others were covered in dense hair, a trait typically found in M. ibericus. DNA testing confirmed their hunch: The hairy males were M. ibericus, and the bald ones were M. structor.

Even more intriguing, the males of both species shared M. ibericus mitochondrial DNA, which is inherited from the mother, suggesting they had all been born from M. ibericus queens.

Iberian harvester ant queens lay eggs that can hatch into males of two differen species. Jonathan Romiguier

This discovery is so novel and so unusual that the researchers had to come up with a new term to describe the behavior exhibited by M. ibericus queens: “xenoparity,” which essentially means “foreign birth.”

The team also wanted to go beyond genetic evidence: They hoped to observe births of M. structor ants from an M. ibericus queen. So, they reared colonies in their laboratory. Then, they waited.

“It was very difficult, because in lab conditions, it’s nearly impossible to have males,” says co-author Jonathan Romiguier, an ecologist at the University of Montpellier in France, to New Scientist’s Tim Vernimmen. “We had something like 50 colonies and monitored them for two years without a single male being born. Then we got lucky.” Observing the births of M. structor males was another key piece of evidence in describing the ants’ strange biology.

Scientists suggest the relationship must be beneficial for both species. Jonathan Romiguier

As for the M. ibericus males in the colony, the queens mate with them to produce the next generation of M. ibericus queens.

But why do M. ibericus queens clone M. structor males? Scientists aren’t totally sure, but they say the partnership must be beneficial to both species.

For M. ibericus, this adaptation ensures they have plenty of workers, which are responsible for many important tasks in a colony, including building the nest, gathering food and raising the larvae. The arrangement also keeps M. structor males around for future M. ibericus queens to mate with, even in places without M. structor colonies. Shockingly, M. structor colonies are only found in mountainous areas across a small range. But by transporting the M. structor male clones around, M. ibericus has allowed that species to spread to new places.

However, the unique setup might not last forever. Because the M. structor males are clones and do not appear to be mating with members of their own species, they are probably accumulating harmful genetic mutations, which makes them more vulnerable in the long run, reports New Scientist. But, for now, the relationship seems to be working.

“Every step in this coevolutionary game makes perfect sense and uses the entire toolbox of reproductive tricks that we know ants are capable of employing,” says Sara Helms Cahan, an evolutionary ecologist at the University of Vermont who was not involved with the research, to Science. “The end result is fantastical but incredibly successful, with one species carrying another in its pocket, as it were, all over southern Europe.”

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