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Quick bites and quirky adaptations
October 2006

Lions and great white sharks may boast the most famous jaws in the animal kingdom — but theirs are nowhere near the fastest. In August 2006, biologists Sheila Patek, Andy Suarez, and colleagues awarded that honor to Odontomachus bauri, a tiny trap-jaw ant native to Central and South America, whose mandibles (or jaws) can snap shut at a remarkable 145 miles per hour! That rapid-fire bite is quick enough to decapitate a soldier termite before it launches its own defense — a jet of sticky fluid that entangles and incapacitates predators. The ant's quick bite also does double-duty as an escape mechanism: when faced with a larger nest intruder, an ant may use its jaws to strike the invader, simultaneously flinging itself 8 or 9 inches away (known as the "bouncer defense") — or it may snap its jaws against the ground, propelling itself into the air and out of the way of such dangers as an anteater's tongue (known as an "escape jump").

The above movie, of the trap-jaw ant "bouncer defense", is 100 times slower than real time.

The above movie, of the trap-jaw ant "escape jump", is 100 times slower than real time.

Where's the evolution?
The record-breaking jaws of the trap-jaw ant have certainly made headlines, but the evolutionary story that underlies those mandibles is just as impressive as the jaws themselves.

The ant's jaws use a trigger system: small muscles in the head of the ant pull the mandibles back into position, where they are locked in place by a "latch" (part of the mandible joint). When the ant prepares to bite, it contracts large muscles in its head, which pull against the latch. Once those large muscles are contracted, the ant's jaws are cocked: when a trigger muscle releases the latch, the jaws snap together.

This movie, of the trap-jaw ant's mandibles snapping shut, is 1,667 times slower than real time!

How did this trap-jaw system evolve? The components of the system (the jaws themselves, the "latch," the trigger muscles, etc.) did not arise anew. Instead, they all evolved from basic body parts that all ants (even non-trap-jaw ants) have. All ants have mandibles, but in O. bauri, those mandibles evolved to be particularly long and sturdy. All ants have a mandible joint, but in O. bauri, the shape of this joint evolved to serve as a latch. All ants have muscles to close their mandibles, but in O. bauri, these muscles evolved to be unusually large to power a killer bite. In evolutionary terms, such features are known as exaptations. Exaptations are useful traits that were not originally produced by natural selection for their current use. In the case of the trap-jaw, traits that originally evolved to manipulate objects and process food became adapted for hunting and defense.

Odontomachus bauri Pogonomyrmex subdentatus
O. bauri (left) has some trap-jaws poised to strike. Pogonomyrmex subdentatus (right) has smaller, more typical mandibles.

Amazingly, O. bauri is not the only ant with killer jaws. Strumigenys ants, Mystrium ants, and Myrmoteras ants, to name a few, all boast trap-jaws with a trigger system much like O. bauri's.

Strumigenys Mystrium Myrmoteras
From left to right: Strumigenys, Mystrium, and Myrmoteras.

How did all these different trap-jaws evolve? One hypothesis is that a trap-jaw system evolved just once in the ants and that all these different ant species inherited their trap-jaws from that common ancestor. However, in this case, the evidence suggests otherwise. When researchers, used ants' DNA sequences to build a phylogenetic tree of ant relationships, they found that ant species with trap-jaws were scattered all across the tree — not clustered in a single tight-knit group (see tree below). Furthermore, anatomical studies of the mechanism behind the trap-jaws revealed subtle differences in the trigger systems: although all the ants use a trigger mechanism similar to O. bauri's, the parts of the system are built from different parts of the ants' mouths. For example, O. bauri's trigger is built from its mandible joint, but Strumigenys's trigger is built from its labrum — its "upper lip." The evidence strongly suggests that trap-jaws have independently evolved at least four times in ants' evolutionary history!

phylogeny showing 4 separate origins of trap-jaws

Similar features that independently evolved through convergent evolution (such as the trap-jaw) are known as analogies. Such cases of convergent evolution are certainly impressive, but once you are looking for them, they are not all that unusual. We find examples of analogies all across the living world — from the torpedo-shaped bodies of sharks and dolphins to the "thumbs" of pandas and primates — and now, in the powerful jaws of picnic pests.


Read more about it

Primary literature:

  • Gronenberg, W. (1996). The trap-jaw mechanism in the Dacetine ants Daceton armigerum and Strumigenys sp. The Journal of Experimental Biology 199(9):2021-2033.
    read it

  • Moreau, C. S., Bell, C. D., Vila, R., Archibald, S. B., and Pierce, N. E. (2006). Phylogeny of the ants: Diversification in the age of angiosperms. Science 312:101-104.

  • Patek, S. N., Baio, J. E., Fisher, B. L., and Suarez, A. V. (2006). Multifunctionality and mechanical origins: Ballistic jaw propulsion in trap-jaw ants. Proceedings of the National Academy of Sciences USA 103(34):12787-12792.
    read it


News articles:

Understanding Evolution resources:

Discussion and extension questions

  1. What is an adaptation? What is the main difference between an exaptation and an adaptation?

  2. What makes O. bauri's trap-jaws an example of an exaptation?

  3. Review the concept of homology here. Based on that reading, what are the main differences between a homology and an analogy?

  4. What evidence suggests that the trap-jaws of O. bauri and Strumigenys ants are analogies and not homologies?

  5. Review this page on convergent evolution. In a series of steps, as shown in the animation on that page, explain how O. bauri and Strumigenys ants might have evolved trap-jaws.

  6. Read this page on the evolution of flower petals. Compare and contrast the evolution of the "trigger" in O. bauri and Strumigenys ants to the evolution of the petal in Costaceae and poinsettia plants.


Related lessons and teaching resources

  • Teach about exaptations: In this interactive module for grades 9-12, students learn about exaptations, phylogenies, and reconstructing evolutionary history through an investigation of the evolution of flight in birds.

  • Teach about analogies: This interactive module for grades 6-8 helps students understand what homologies and analogies are, how to recognize them, and how they evolve. It also deals with the topic of convergent evolution. A version of this module for grades 9-12 is also available.

  • Teach about another example of convergent evolution: This research profile for grades 9-12 follows biologist Chelsea Specht as she uses phylogenetic and anatomical studies to investigate convergent evolution in tropical ginger plants.


References

  • Gronenberg, W. (1996). The trap-jaw mechanism in the Dacetine ants Daceton armigerum and Strumigenys sp. The Journal of Experimental Biology 199(9):2021-2033.

  • Moreau, C. S., Bell, C. D., Vila, R., Archibald, S. B., and Pierce, N. E. (2006). Phylogeny of the ants: Diversification in the age of angiosperms. Science 312:101-104.

  • Patek, S. N., Baio, J. E., Fisher, B. L., and Suarez, A. V. (2006). Multifunctionality and mechanical origins: Ballistic jaw propulsion in trap-jaw ants. Proceedings of the National Academy of Sciences USA 103(34):12787-12792.



Movie clips by Sheila Patek and her collaborators; Odontomachus bauri and Pogonomyrmex subdentatus photos © Alex Wild; all other ant photos from antweb.org, taken by April Nobile © California Academy of Sciences.



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