Turtle ant soldiers use their heads

Turtle ant soldiers (Cephalotes) are tree-dwelling insects with strangely oversized heads, which they use to block the entrances of their nests, essentially acting as living doors.

Not all heads are shaped alike: some turtle ant soldiers have ones that resemble manhole covers and perfectly sealed tunnel entrances. Others have square heads, which they assemble into multi-member blockades reminiscent of a Spartan army’s overlapping shields.

This variety in head shapes reveals more than just another of nature’s quirky oddities: it can also reveal how species evolve to fill ecological niches.

And that evolution, new research published in the Proceedings of the National Academy of Sciences shows, is not always a one-way street toward increasing specialization. Occasionally, it can take a species back to a more-generalist stage.

“Usually, you would think that once a species is specialised, it’s stuck in that very narrow niche,” says Daniel Kronauer, head of Rockefeller’s Laboratory of Social Evolution and Behavior. “But turtle ants are an interesting case of a very dynamic evolutionary trajectory, with a lot of back and forth.”

Like many other social insects living in colonies, turtle ants specialise for different functions, often evolving exaggerated features suited to their job. For the soldiers, this process has resulted in large heads that come in a variety of shapes.

And because the ants don’t dig the tunnels themselves, like many house hunters, they must find the place that suits them the best. Since a tunnel might be too big or too small, Kronauer says, the ants diversify rapidly to be able to occupy it.