Trap-Jaw Mechanism Tells an Ant Story of Natural Selection

The speedy mandibles of Strumigenys ants developed repeatedly throughout the world, explaining how evolution creates new abilities to help a species survive. 

Ants gather on a dewy peony bud. (Image courtesy of Alexandra from Pixabay via Courthouse News)

(CN) — Like something out of an Aesop fable, researchers recorded the fastest-ever acceleration of a resettable animal movement in a humble ant.

Thanks to specialized muscles in their trap-like jaws, ants in the Strumigenys genus use a latch-spring effect to strike prey before it hops out of reach.

Led by Douglas Booher, the team examined hundreds of species of ants in the Strumigenys genus to learn how these ultrafast mandibles developed, ultimately discovering that the trap-jaw mechanism evolved not once but 7 to 10 times, independently, across the world. 

The findings, published Tuesday in the journal PLOS Biology, tell a story of how small evolutionary changes in an animal’s body can lead to complex traits, which in turn give way to more species diversity across the globe. 

Underpinning the ants’ jaw development is an evolutionary process known as convergence, which is when unrelated organisms take on similar characteristics because they are exposed to similar environmental conditions. 

Convergence is not uncommon in the plant and animal kingdoms — but the authors of the new study say they were surprised to find so many convergent evolutions of such a complex, yet miniature, mandible system. 

“We found that not only did they evolve this mechanism in a repeated manner, but that each biogeographic region has a similar fauna containing a similar breadth of morphological variation seen globally,” Booher said in an email. 

A postdoctoral research associate at the Yale University Center for Biodiversity and Global Change, Booher said the results suggest that “Strumigenys ants are filling niches in a similar way around the world.” 

Curiously, over the course of evolution, not all Strumigenys ants have developed a trap-style jaw. 

Booher and the other researchers looked at ants with and without the feature to better understand how natural selection played a role in the ants’ evolution. The team reconstructed evolutionary relationships between 470 species, studying them using X-ray microtomography, 3D computer models and high-speed videography. 

Regardless of whether the ants had the trap jaws, they were still able to prey upon springtails, which are “named for their hydrostatic spring-escape mechanism,” Booher said. 

That’s because Strumigenys ant species without a trap jaw have mandible-closing muscles optimized to move quickly, placed parallel to one another. 

“We find that species without trap jaws still close their mandibles extraordinarily quickly,” Booher said, “faster than any other non-trap jaw ant from any other ant genera that had been measured and faster than their springtail prey can employ their escape mechanism.”

The trap-jaw mandibles, however, can be 100,000 faster than their non-trap-jawed counterparts, “providing damaging strikes that help immobilize their prey,” and outpacing a springtail’s attempted escape. 

The extra power comes from muscles positioned at oblique angles to one another, an arrangement that helps store power needed for the “mouse-trap like latch release.”

Out of 1,000 or so Strumigenys species, around 700 have trap jaws, and the remaining 300 don’t. The reason the trap jaw wins out is likely that it gave ants a “breakout innovation” in capturing prey. 

“I think of this innovation as an analogue to snapping one’s fingers,” Booher explained. 

“Without snapping your fingers, the speed at which you can move your fingers is limited to muscle movement. However, when you snap your fingers you are loading energy to be released in an instant and generating much greater speed than explained by muscle movement alone.”

Similarly, Booher continued, when non-trap-jaw ant species close their mandibles, they have to swing parts of their mouth — called labrum — down and away from their jaws. 

“If for some reason the mandibles locked onto this labrum and created a snap, then that might produce a higher performance action — and this is what we expect happened in species leading to the transition and evolution of trap jaws.”

The transition tells a tale of natural selection, and how species adapt as a useful innovation comes along. In Booher’s opinion, however, the species without a trap jaw still take one prize.

“Measuring success by numbers — trap jaws win,” Booher said. “That said, I find the non-trap-jaw species ‘cuter’ if there is an award for that.”

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