The Fastest Jaws in Nature- the ultra-fast bite

Dr. Doug Booher, a collection associate with the Museum's Arthropod Collection is the lead author on the paper "Functional innovation promotes diversification of form in the evolution of an ultrafast trap-jaw mechanism in ants" just published in PLOS Biology. The article is available freely from this weblink http://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.3001031. Dr. Booher spent many an hour working on this project in the Arthropod Collection and has deposited many specimens of ants that support this work.

PLOS Biology Press Release-- 

The trap-jaw ants are famous for having one of the natural world’s fastest movements, but how did the latch-spring mechanism that drives their jaws evolve? According to a study published 2 March 2021 in the open-access journal PLOS Biology by Douglas Booher and Evan Economo of the Okinawa Institute of Science and Technology Graduate University, and colleagues, the core mechanism itself arose multiple times before going on to a spectacular diversification of mandible shape.

The ants need their fast jaws to catch their similarly fast prey, springtails, which themselves have a spring-loaded escape mechanism. The new findings may explain why the mechanism has evolved so many times independently around the world, eventually developing into the animal kingdom’s fastest-accelerating resettable part.

Evolutionary change is marked by occasional breakthroughs in the design of organisms, often involving the reorganization of parts into new functional systems. But understanding how transitions in function evolve when they require changes in multiple interacting parts remains a major challenge. While most agree the evolution of new complex features involves sequences of gradual changes, these transitional pathways are not yet well understood. In the new study, the researchers examined the evolution of an iconic biomechanical adaptation, the latch-spring mechanism of trap-jaw ants, to address general questions about the nature and repeatability of biomechanical innovations.

The researchers reconstructed the relationships among 450 species of Strumigenys ants, and surveyed mandible mechanisms using physical examination, X-ray microtomography, 3D modeling, and high-speed videography. The researchers recorded the fastest acceleration of a resettable animal movement. In addition, the findings suggest that the trap-jaw mechanism evolved independently 7–10 times in a single ant genus, resulting in the repeated evolution of diverse forms on different continents.

Most diversification of shape (from short triangular jaws to long slender ones) occurred after the evolution of latch-spring mechanisms, which can evolve through only very minor realignments of mouthpart structures. The finding that incremental changes in form lead to a change of function, followed by large morphological reorganization around that new function, provides a model for understanding the evolution of complex biomechanical traits, as well as insights into why such innovations often happen repeatedly.

Below are various animations of the Trap-jaw evolution and high-speed videography. 

Title: Animation showing evolution of trap-jaw
Caption: The animation shows the changes in form as the trap-jaw mechanism becomes more divergent from the ancestral form. The jaws (yellow) develop small projections that can latch onto the labrum (purple). The labrum changes from functioning as a sensor in the ancestral gripping jaw to acting as a latch in the trap-jaw. The muscles in the head that control the jaw and the labrum undergo large changes in structure.
Credit: Reproduced with permission from "Functional innovation promotes diversification of form in the evolution of an ultrafast trap-jaw mechanism in ants." by Booher DB et al. (2021). PLOS Biology, CC-BY

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

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

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

Title: The head structure of different Strumigenys species
Caption: Trap-jaw ants show remarkable diversity in the length of the jaw and how wide it opens.
Credit: Reproduced with permission from "Functional innovation promotes diversification of form in the evolution of an ultrafast trap-jaw mechanism in ants." by Booher DB et al. (2021). PLOS Biology, CC-BY

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

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

To that end, the research team now plans to sequence the genomes of representative Strumigenys species across the world. “We want to bridge the gap between the changes we see on the genetic and molecular level, and what we see on a morphological level. That’s the next big project.”

Title: A trap-jaw ant catches a springtail
Caption: A trap-jaw ant strikes at its springtail prey.
Credit: Reproduced with permission from "Functional innovation promotes diversification of form in the evolution of an ultrafast trap-jaw mechanism in ants." by Booher DB et al. (2021). PLOS Biology, CC-BY

It’s not yet known how all these different ant species hunt, but ants with shorter trap jaws are typically passive hunters, hiding in the leaf litter with their jaws waiting to snap shut on any unsuspecting prey that ventures too close. Meanwhile, the ants with longer trap jaws are active hunters, searching for prey to strike down.

The researchers believe that how the ants use their jaws to catch prey may help explain the surprising level of diversity in the shape of trap jaws. In every region of the world, both on a continental level and on the local community level, both long and short trap-jaws are found.

Title: High-speed videography captures the motion of gripping jaw and trap-jaw
Caption: The high-speed videography captures motion at a rate of 480,000 frames per second (fps) and plays it back at 30fps. The trap-jaw accelerates faster and reaches higher speeds than the simpler gripping mechanism.
Credit: Reproduced with permission from "Functional innovation promotes diversification of form in the evolution of an ultrafast trap-jaw mechanism in ants." by Booher DB et al. (2021). PLOS Biology, CC-BY

Strumigenys ants have a need for speed, using their ultrafast jaws to thwart the spring-loaded escape mechanism of springtails, their most abundant prey.