October 2006

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.

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.

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.

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!

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.