Queen ants are a beautiful example of how form evolves with function

Thanks to expanded muscles in the front of the thorax, this Anochetus worker can hoist a heavy load.

Roberto Keller,  Christian Peeters, and Patrícia Beldade have published an intriguing new paper about ant thorax morphology. Worker ants lack wings, of course, so their thoraces are smaller. But this new study notes there is more to the thorax than mere reduction. Ant workers might not fly, but they do lift things with their heads, so the first segment- the pronotum- is enlarged to strengthen the neck. It’s essentially the factoid “ants can lift 50 times their own weight!!”, explained.

But I’m not blogging the paper for the worker necks. Workers are not the best part.

Rather, Keller et al also looked at queen ants, and here is where the study gets clever. Consider:

Queen ants of most species fall into one of two body types.

Queens that hunt need strong, weight-bearing necks, so the thoracic segment anchoring neck muscles is enlarged as it is for workers. Queens that stay in the nest and don’t gather food don’t need neck strength, so the corresponding segment is reduced. Meanwhile, the segment associated with muscles that are metabolized to raise early workers is enlarged.

Thoracic tergite #1 (T1), in blue, is much larger in queen ants that leave the nest to forage, while the second tergite (T2), in orange, is enlarged in queens that do not leave the nest. (Modified from Figure 2 of Keller et al).

While this pattern of claustral/non-claustral was generally known, Keller et al perform a more rigorous accounting. First, they quantified the phenomenon by measuring many species. Remarkably, the data did not form a continuum of values; rather, queens sort cleanly into one type or the other. Thus, queen ants really do come in two forms. It’s not just our imagination.

Then- and here’s the fun part- the authors tested for an evolutionary association of form and function. When thorax shape and colony-founding behavior are mapped on an evolutionary tree inferred from genetic data, we see two traits changing in concert rather than randomly. Ancestrally, queens forage and have an enlarged T1. Several times in evolution queens have shifted to claustral, non-foraging founding. When they do, the morphology also shifts.

That ant shape and ant behavior are intertwined should not be surprising, but I mention it because this study forms an especially nice example.

The form/function association has a practical use for ant-keepers, too. If you’ve caught a young queen and want to know if she requires feeding to raise her first workers, you can look at her thorax. A big T1 needs feeding, a small T1 is claustral and does not.

Source: Keller RA, Peeters C, Beldade P. 2014. Evolution of thorax architecture in ant castes highlights trade-off between flight and ground behaviors. eLife 2014;3:e01539 http://dx.doi.org/10.7554/eLife.01539


14 thoughts on “Queen ants are a beautiful example of how form evolves with function”

  1. Very practical for wild ant watchers too! I now know that ‘little’ Podomyrma queen (found wandering around on a pile of concrete blocks) had just flown and was looking for a home rather than foraging.

  2. Hi Alex,

    So are foraging queens always ancestral to non-foraging queens? Are non-foraging queens ever ancestral to foraging queens?

  3. Christian PEETERS

    Clear and insightful post Alex!
    So little is known about the colony-founding behaviour of most ants, presumably because time-consuming fieldwork is required at the appropriate time of year. Simply looking at the thorax morphology of winged queens can generate testable hypotheses.
    In addition, the occurrence of ergatoid (permanently wingless) queens in many ant species hints strongly that they cannot found without the help of nestmate workers, i.e. dependent colony foundation (known also as fission or budding).

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