Powerful, deadly, super-fast bite of a trap-jawed ant

Small changes in shape can lead to innovative functional breakthroughs, new study finds.

• Researchers have revealed how an ant’s trap jaw – a powerful, lightning-fast and complex evolutionary trait – evolved from a simpler ancestral jaw mechanism.
• The study found that the jaw-trap mechanism independently evolved 7-10 times across the world
• The function of the trap jaw may have evolved with only minor changes in the shape of the jaw, followed by the evolution of various forms of trap jaws.
• Researchers have seen the same variations of trap jaws evolve independently on different continents, showing the repeatability of the evolution of this complex trait.
• High-speed videography has shown that the trap jaw is the part of the animal’s body that accelerates the fastest and can be reset.

Powerful and deadly, the bite of a trap-jawed ant is renowned throughout the animal kingdom. Unlike normal gripping jaws, which depend on muscles to open and close, the trap jaw locks to open, storing energy like a stretched spring. When released, the ant’s jaws close on their prey in a single, super-fast blow.

Jaw traps are a record-breaking evolutionary innovation, but scientists still don’t understand how this complex mechanism evolved from simpler ancestors. Now published today (March 2, 2021) in PLOS biology, a research team led by Professor Evan Economo of the Okinawa Institute of Science and Technology Graduate University (OIST) and Dr Douglas Booher of Yale University, New Haven CT, with an international team of collaborators, has shown how jaw traps were born and then diversified several times around the world.

The animation shows the shape changes as the jaw trap mechanism becomes more different from the ancestral shape. The jaws (yellow) develop small projections which can cling to the labrum (purple). The labrum changes from being a sensor in the ancestral grasping jaw to that of a lock in the trap jaw. The muscles of the head that control the jaw and the labrum undergo great structural changes.

“One of the central questions in biology is: how does something complex arise from something simple?” said Professor Economo, who heads the Biodiversity and Biocomplexity Unit at OIST. “Structures like the trap jaw depend on several interacting parts to function properly. At first, it can be difficult to see how such complexity can arise through the gradual and gradual changes in evolution. Nonetheless, when we look closely, biologists can discover evolutionary pathways to complexity. “

Many species of ants with trap jaws are part of the genus Strumigenys – an extremely diverse group with over 900 species found in the tropics and subtropics around the world.

“This genus contains many closely related species where the trap jaw is present or absent, which gives us a truly unique opportunity to understand how it originated,” said Professor Economo.

High-speed videography captures motion at a speed of 480,000 frames per second (fps) and plays it back at 30 fps (16,000x slow motion). The trap jaw accelerates faster and reaches higher speeds than the simpler gripping mechanism.

At OIST, in collaboration with Professor Alexander Mikheyev’s Ecology and Evolution Unit, the research team extracted and sequenced DNA of 470 Strumigenys ant species worldwide, including those with ancestral gripping jaws and those with modified trap jaws.

The researchers reconstructed a tree showing the evolutionary relationships between species. They then analyzed the mechanism of the jaw using micro-CT scanners to create 3D images and models of the ants.

The researchers found that the jaw-trap mechanism had evolved 7 to 10 times independently across the world.

Importantly, only a very small change in shape was needed to radically change the function of the jaw from the gripping mechanism to the trap mechanism. After the change in function, the head of jaw-trap ants began to undergo massive muscle restructuring and diversified into jaw length and open jaw width.

Jaw-trap ants exhibit remarkable diversity in jaw length and opening width.

“Previously, we thought that all trapezoidal jaws had both divergent shape and function, so it was much less obvious whether the change in function could occur early or if many shape changes were occurring first. required. a prerequisite, ”said Professor Economo. “But it turned out that there are many intermediate forms of the jaw-trap mechanism that people just hadn’t identified before, some that differ only slightly from the ancestral form.

The researchers collaborated with Andrew Suarez’s lab at the University of Illinois, which used high-speed videography to capture the jaws of moving Strumigenys ants. They found that the trap jaws have the fastest known acceleration of any part of the animal’s body that can return to its original position.

“The acceleration of the mandible of the trap jaw is one hundred thousand times greater than that of the standard mandibles,” said Professor Economo. “And they close thousands of times faster than a human wink.”

Strumigenys ants crave speed, using their lightning-fast jaws to thwart the spring-loaded escape mechanism of springtails, their most abundant prey.

It is not yet clear how all of these different species of ants hunt, but ants with shorter trap jaws are usually passive hunters, hiding in the leaf litter with their jaws waiting to close on any unsuspecting prey that ventures too close. Meanwhile, ants with longer jaws are active hunters, looking for prey to cut down.

Researchers believe that the way ants use their jaws to catch prey may help explain the surprising level of diversity in the form of trap jaws. In all parts of the world, both at the continental level and at the local community level, there are long and short trap jaws.

“It was really striking to see how we saw the same variations evolve over and over again on different continents. It illustrates how reproducible evolution can be, finding similar solutions to life’s challenges, ”said Professor Economo.

What is less clear is whether the underlying genetic changes needed to build the trap jaw are the same, or whether the ants achieved these similar results in different ways.

To this end, the research team now plans to sequence the genomes of representative Strumigenys species around the world. “We want to bridge the gap between the changes we see at the genetic and molecular level and what we see at the morphological level. This is the next big project. “

Reference: “Functional innovation promotes shape diversification in the evolution of an ultra-rapid jaw-trap mechanism in ants” by Douglas B. Booher, Joshua C. Gibson, Cong Liu, John T. Longino, Brian L. Fisher, Milan Janda, Nitish Narula, Evropi Toulkeridou, Alexander S. Mikheyev, Andrew V. Suarez and Evan P. Economo, March 2, 2021, PLOS Biology.
DOI: 10.1371 / journal.pbio.3001031

Funding: Okinawa Institute of Science and Technology Graduate University, Japan Society for the Promotion of Science, National Science Foundation, Tinker foundation at the UIUC Center for Latin American and Caribbean Studies