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amnhhealiz1The Jaws That Jump

Trap-jaw ants have smashed the record for fastest predatory strike on Earth, with mandibles that double as spring launchers.

Story by Adam Summers - Illustrations by Tom Moore

Dressed in shorts and wetsuit booties, I was out for a late evening walk in south Florida. Suddenly I felt a dozen little stabbing pains in my foot. Thinking I must have stepped in a stinging nettle, I carefully backed one foot out of range, but the stinging got worse, not better. A flashlight revealed the true source of my misery: my foot had landed squarely in a nest of Argentine fire ants. To this day I bear the scars of their assaults.

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Trap-jaw ants have two distinct jumps. The vertical jump, known as an escape jump (red), works as shown in the diagram given separately below. The horizontal jump, or "bouncer defense," is triggered when the ant’s jaws close on a predator, such as a gecko (at left). The recoil throws the ant away from the predator (blue). Both aerial moves send the ants tumbling rather haphazardly, but don’t seem to do any damage.

Recently I was reminded of just how powerful ants can be when inflicting damage on intruders. A team of biomechanists has studied the incredibly speedy bite of a group of Central and South American ants. The team clocked the bite as the fastest on the planet—and discovered that it also gives the ants the unique ability to jump with their jaws, adding to an impressive array of already known defenses.

Trap-jaw ants nest in leaf litter, rather than underground or in mounds. There they often feed on well-armored and elusive prey, including other species of ants. As they stalk their dinner, the trap-jaws hold their mandibles wide apart, often cocked open at 180 degrees or more by a latch mechanism. When minute trigger hairs on the inner edge of the mandible come in contact with something, the jaws snap shut at speeds now known to reach 145 miles per hour. No passerby could outrace that. The astoundingly high speed gives the jaws, despite their light weight, enough force to crack open the armor of most prey and get at the tasty meat inside.

The key to the jaws’ speed (and their even more amazing acceleration) is that the release comes from stored energy produced by the strong but slow muscles of the jaw. Think how an archer slowly draws an arrow in a bowstring against the flex of a bow: nearly all the energy from the archer’s muscles pours into the flexing of the bow. When released, the energy stored in the bow wings the arrow toward its target much faster than the archer could by throwing the arrow like a javelin. The biomechanics of energy storage is the bailiwick of Sheila N. Patek and Joseph E. Baio, both biomechanists at the University of California, Berkeley. They teamed up with two ant experts, Brian L. Fisher of the California Academy of Sciences in San Francisco and Andrew V. Suarez of the University of Illinois at Urbana–Champaign, to look at the trap-jaw ant Odontomachus bauri.

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Trap-jaw ant propels itself nearly straight into the air by hitting its mandibles—which are already cocked open—against the ground. The action triggers a latch mechanism that suddenly releases the energy stored in the open mandibles; they close against the ground with a force 400 times the ant’s body weight, launching the ant forcefully upward.

Fisher, Suarez, and other field biologists had already noted that catching O. bauri was like grabbing for popping popcorn—and very hot popcorn at that, because a painful sting goes with an ant’s trap-jaw bite. The insects bounced around in a dizzying frenzy and propelled themselves many times their body length when biologists or smaller intruders approached them. Patek and Baio made high-speed video images of their movements, and discovered that the secret of their self-propulsion was the well-executed “firing” of their mandibles. They also observed that mandibles started to decelerate before they meet—possibly to avoid self-inflicted damage. Most important, the ants had two distinct modes of aerial locomotion.

In the so-called escape jump, an ant orients its head and jaws perpendicular to the ground, then slams its face straight down. That triggers the cocked mandibles to release with a force 400 times the ant’s body weight, launching the insect ten or more body lengths nearly straight into the air. The ant doesn’t seem to go in any particular direction, but the jump is presumably fast and unpredictable enough to help the insect evade, say, the probing tongue of a lizard. Not only can the jumping ant gain height and sow confusion, but it may also get to a new vantage point from which to relaunch an attack.

The second kind of jaw-propelled locomotion is even more common than escape jumping. If an intruder enters the ants’ nest, one of the ants bangs its jaws against the intruder, which triggers the trap-jaw and propels the interloper (if small enough) in one direction, out of the nest, and the ant in the other. Often the force sends the ant skimming an inch off the ground for nearly a foot. The attack, for obvious reasons, is known as the “bouncer defense.” In the wild, gangs of defending ants team up to attack hostile strangers, sending them head over heels out of the nest.

From an evolutionary point of view, the trap-jaws are an intriguing story. The ants clearly evolved an entirely new function, propulsion, for a system that was already useful—chewing up prey. Several lineages of trap-jaw ants have independently hit on the tactic of storing energy in their jaws to penetrate well-defended prey. In Odontomachus, the horizontal, bouncer-defense jump could have arisen out of attempts to bite intruders, but the high, escape jump—with jaws aimed directly at the ground—must have arisen from a different, perhaps accidental kind of behavior. Such a serendipitous event would have been a rare instance in which banging one’s head against the ground got good results.

O. bauri is a member of a large group of trap-jaw ants whose bodies and trap-jaws come in a variety of sizes. That should enable Patek and Suarez to determine the evolutionary history of the jumps, as well as which body parts make for the best jumping. Fortunately, no one has any plans to allow these little beauties to gain a foothold in south Florida. Otherwise my next set of scars might go a lot farther up my leg.

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