Economo Lab

One of the most striking features of ant diversity is the sheer variety and ingenuity of mandibles. Mandibles are a primary way that ants interact with the world, and are used as diverse tools for many tasks and purposes.  Ant mandibles are cutters and carriers, but also trap-jaws, snap-jaws, millipede shavers, seed millers, wood borers, and beyond.  There are truly a huge number of mandible shapes and functions to explore from ecological, evolutionary, biomechanical, and developmental perspectives.  We have a number of projects with collaborators ongoing about these interesting structures, here’s a little tour:

First up here’s a broad look at the diversity of ant mandibles, segmented from micro-CT scans by our PhD student Julian Katzke.  Julian is doing his thesis surveying the diversity of mandibles, building a massive dataset across the tree of life, and analyzing their evolution.  How many morphotypes are there, how many times have they evolved?  Are there macroevolutionary pathways for their evolution (i.e. certain shapes tending to evolve into other shapes)?  Julian is working on it.

A related topic is the structure of the muscles inside the head and its supporting endoskeleton. In scanning ants, we have found there is at least as much diversity in these structures inside the head as the mandible outside. Julian is working on analyzing that too.

Within that diversity, there are some really interesting innovations and adaptations. A major project in the lab in the past years is the evolution of Strumigenys trap jaw ants.  Strumigenys are a globally distributed, hyperdiverse group with a variety of mandible types. 

We found the trap-jaw mechanism has evolved between 7-10 times, and identified functional stepping stones that could explain how it may have evolved.  Read the paper (Booher et al. 2021, PLoS Biology) and see Evan’s twitter thread to dig into this more.

PhD student Gaurav Agavekar is following this up by investigating the genomic and developmental basis of this convergent evolution.

Another recent collaborative study focused on the wood-boring ant, Melissotarsus, the only ant to live completely in tunnels it carves in living wood, and one of the wildest ants out there. We found a number of adaptations to help chew through living wood, including changes to head musculature and interesting conical mandibles. This study was led by Adam Khalife and the late Christian Peeters, was published in Frontiers in Zoology. 

And there are other mysterious species, such as Protanilla lini which has a very odd mandibular articulation with the head.  Some think it may have a power-amplifying function, however this is still unclear.  We recently looked at this species in a study led by Adrian Richter in collaboration of our lab.  Julian made an animation of head movement to try to understand it.

Aside from these more exotic examples, we still lack a basic understanding of what axes of morphological and functional variation underlie normal triangular mandibles.  How does shape vary with ecology and performance?  Two recent studies help shed light on this.  

First, our now postdoc Ale Casadei-Ferreira and colleagues analyzed ecology-mandible correlations and integration in the evolution of mandibles and head shape, focusing on Pheidole majors and minors.

Second, Christian Klunk and colleagues used finite element analysis to analyze Pheidole mandibles, helping to understand how different mandible shapes reflect tradeoffs in force vs. speed, and how stress moves through the mandible shapes.

For more general discussion of the mandibles in context of the head, recent papers by Adrian Richter and various collaborators surveying the head anatomy of different ants are a must-read.

Of course there is lots of other great work going on out there by other labs, for example a terrific recent study by David Labonte’s lab on bite force allometries in Atta leafcutter ants.

There’s much more to come from our lab and others on the morphology, functional ecology, evolution, and development of these diverse tools ants use to interact with the world.

If you look hard enough in the field, you are guaranteed to uncover cool stuff.  Such is the case in Madagascar, where Brian Fisher and colleagues have been surveying the island intensively for 20+ years, uncovering a huge and mostly endemic ant fauna.  Georg Fischer has been working for some years on the collections of Malagasy Pheidole, and, in the process found something very strange.  There is an extremely rare, and unusual group of social parasites with workers that seem to look like their hosts.  Working with Sasha Mikheyev’s lab and other colleagues, we found they belong to a single clade, a mini-radiation of parasites within the broader endemic Malagasy radiation, rather than (as is typical) each parasite being closely related to their host.  Using micro-CT and a comparative analysis (led by Nick Friedman), we found they have evolved size and shape to match their hosts.  Parasites more generally evolve to resemble their hosts because they aim to fool predators (Batesian mimicry) or they aim to fool the hosts themselves (Wasmannian mimicry).  It is hard to invoke Batesian mimicry here, which leaves Wasmannian as the putative mechanism, and this is not known for ants that parasitize other ants (although is known for other insects like beetles to parasitize ants).  If true, it implies ants may sense morphology to some degree to distinguish friend from foe, and these abilities are very sensitive.   There are other potential explanations too, as we discuss in the paper, just out in Current Biology. This is definitely a study that raises as many questions as answers, hopefully it will inspire some follow up experiments.

Birds use their beaks for foraging, but also for regulating their temperature, singing and a variety of other functions. Do beaks and other multifunctional traits have characteristics that are specialized for each function, or are they an evolutionary compromise? To test this, we examined beak shape and size in a family of Australian songbirds, and compared the influence of each function on the evolution of species differences. We found that foraging behaviour and climate both have an effect on the evolution of bird beaks and their characteristics, and that this also affects features of the songs these birds perform.

Read the paper led by Nick here.
Read the article OIST published on this paper: English / Japanese.

Listen to the call of Philemon corniculatus (commonly known as the Noisy Friarbird) below!

This year, our lab co-organised and hosted SWARM 2019: The 3rd International Symposium on Swarm Behavior and Bio-Inspired Robotics, bringing together 160 biologists and engineers from around Japan and the world. The theme of this conference, which has been held in Kyoto twice before, is to promote interdisciplinary interaction between biology and engineers within the realms of collective behavior and bioinspired design.

We co-organized (with Christian Peeters) a symposium within the conference, “Engineering Insect Morphology by Natural Selection” to highlight recent research from understanding how insects work to inspire robotics. From our lab, Evropi Toulkeridou presented her research on the automated segmentation of micro-CT images by deep learning. In the same session, Adam Khalife, a former intern (now a PhD student at IEES-Paris), talked about his work on the muscular and skeletal structure of worker ants.

Other talks included Christian Peeters (ant thorax), from Yuko Ulrich (collective behavior) and Adria Labeouf (social circulation), and Hitoshi Aonuma (trap jaw ant mechanics). Evan wrapped up the session by asking how the endless engineering solutions of nature, currently locked up in museum collections, can be utilised to its maximum potential to inspire human innovation.

We hope that the symposium provided a fertile ground for biologists and engineers to exchange ideas and develop collaborations.

Thank you and otsukaresama to all the volunteers and especially to Chisa for her hard work organising the logistics!