A team of researchers from George Washington University and The Rockefeller University studying the morphology of turtle ant heads found that the critters undergo reverse-evolution in which their head shape—which is used as armor—changes.

The reverse-evolution is designed to help the ants belonging to the soldier caste properly close off burrow entrances in order to protect the colony.

Evolution is usually a forward-moving process, honing every single living thing to perfectly fit its environment. But even this steady march of biological progress isn't a universal truth, at least not for the humble turtle ant, which experiences what scientists call "reverse evolution."

A paper published in the Proceedings of the National Academy of Sciences (PNAS) studied the turtle ant caste system, the natural order these ants maintain to create functioning social order. Researchers found that those belonging to the soldier caste have developed a “head shape and size…specialized for nest entrance defense.”

What’s especially interesting about this evolutionary development is that the turtle ants belonging to the soldier caste don’t have a uniform size or shape to their heads. Researchers found that there were four morphotypes within the turtle ant population: those with square-, domed-, disc-, and dish-shaped heads.

The researchers say this discovery “demonstrate[s] that caste evolution can be highly dynamic at the species level” and that traits such as the head size and shape evolution seen in turtle ant soldiers “are extensively reversible [and] repeatable.”

Curiously, the ants don’t dig their own nests—instead, they move into the burrows of wood-boring beetles. According to Futurity , since the ants don’t make these tunnels themselves, sometimes they’re oversized or too small. This means that blockades of varying sizes are needed to protect their new home; this is where those morphotypes come in. Having soldiers with different-sized head armor provides a solution to openings of various shapes and sizes.

The findings highlight how a need can facilitate reverse-evolution within a species when usually, it’s the other way around. In this case, turtle ant colonies needed a way to protect their hills and the members of the soldier caste underwent an evolutionary change to fulfill that need.

Daniel Kronauer, one of the study authors and head of Rockefeller University’s Laboratory of Social Evolution and Behavior, notes that while it’s typical for a species to become specialized and stay within that specialization, turtle ants present an “interesting case of a very dynamic evolutionary trajectory, with a lot of back and forth.

The researchers analyzed 89 species of turtle ants—including one group that didn’t have any soldiers at all—and found that the oldest common ancestor they could trace “likely had a square head.” The theory is that this ancestor paved the way for a range of species within the Cephalotes genus including the ones with and without soldiers. From there, some specialized species evolved to have more generalized head shapes and sizes.

“The finding nicely shows just how surprisingly flexible nature can be in fitting the shape of an organism to the context of the environment they occupy,” says lead study author, Scott Powell.

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