An Evolutionary Transition, In Vivo

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Among the most dazzling products of insect evolution are leafcutter ants, which cultivate an edible fungus on a compost of fresh vegetation. The ants’ digestive chemistry is so simplified that they can only eat the fungus that grows in the underground gardens.

The leafcutter/fungus system is complex enough to seem highly improbable, and indeed, it appears to have evolved only once in the 130 million year history of ants. How could such a complex system appear?

The system did not spring forth fully-formed, of course. I was reminded of the gradual evolutionary transition on our recent BugShot course in Belize, when I happened across Trachymyrmex intermedius.

Trachymyrmex intermedius

 

Contrary to appearances, T. intermedius is not a leafcutter ant.

At least, not technically. True leafcutters belong only to the genera Atta and AcromyrmexTrachymyrmex is instead the sprawling, paraphyletic genus from which the leafcutters arose. These ants also farm fungus, but they typically use dead vegetation, caterpillar frass, and other bits of detritus. Like so:

argentina1

Green vegetation is not the usual fare for Trachymyrmex, but T. intermedius and several others do take it on occasion. Seeing a few of these small ants trundling off with a harvest more fit for their larger cousins was just a reminder that animal behavior is naturally variable, and that variation is what allows the evolutionary process to explore new paths.

Little ant, big thoughts.

7 thoughts on “An Evolutionary Transition, In Vivo”

  1. Very interesting. I know next to nothing about leafcutter ant evolution. I expect people have looked for co-evolution of the ants and their fungi, so my question is if T. intermedius fungi is representative of a progenitor for the fungi cultivated by the Atta/Acromyrmex species.

  2. and Trachys in Tucson share colony founding and swarming behaviors with the local leaf cutter Acromyrmex. Just add in leaf cutting and they would be there.

    1. I leafcut because people in my neighborhood do not like it if there is a lovely natural prairie in the yard next door. This whole business of keeping a monoculture of short grass around our property is really pretty twisted.

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