Worker-caste Differences in Army Ant Brains: Soldier Specialization

In my most recent publication with Drs. Sean O’Donnell, Susan J Bulova, and Christoph von Beeren entitled ‘Brain investment under colony-level selection: soldier specialization in Eciton army ants (Formicidae: Dorylinae)’, we discuss the differences in the brains of task-specialized workers in multiple species of army ants. 

Worker army ants come in a variety of shapes and sizes that generally correspond to the task that worker performs (form meets function). In the case of the Eciton burchellii cupiens to the left (a), the grey arrow points to a small ‘foraging’ worker ant who cares for brood, assembles the nest, forages for food, and cares for queen and soldier ants. The white arrow points to a larger ‘soldier’ worker ant in charge of colony defense (the mouth parts are so large that this ant can’t even feed itself!). Neither of these belongs to the reproductive caste, like queens, yet they look very, very different and complete very different tasks too!

Different tasks impose cognitive loads on the different regions of the brain that help the organism complete them (for example, flight requires an organism to bring in a lot of visual information quickly so it imposes a large load on the visual processing region of the brain called the optic lobes). Since brain tissue is expensive to produce and maintain, evolutionarily-successful organisms will invest more in the parts of the brain that help them deal with the cognitive load of their behaviors (and will not invest in more brain tissue than strictly necessary, to conserve those resources for other tissues like reproductive organs or muscle).

At the colony level, there may be selection across different worker types so that workers, like soldiers, with reduced sets of behavior have smaller brains than other workers, like foragers. In b, above, we see a foraging worker (A) has a much larger brain relative to body size than the soldier (B) – and a similar size brain, even regardless of the differences in body size!

Looking across eight species of army ants, we assessed the volumes of the total brain, mushroom bodies (centers of learning, memory, and sensory integration), optic lobes (visual information processing) and antennal lobes (olfactory information processing) in forager and soldier workers. We found:

  1. As body size (measured by head capsule volume) increases, the ratio of brain:body increases sharply in support of Haller’s Rule (the idea that there is a minimum size the brain can be and still allow the organism to function as that type of organism). In Fig 2a to the right, we see that as body size gets very, very small, brains take up significantly more space in the head because we are approaching the minimum brain size it takes to function as an ant of any kind!
  2. Soldiers, despite being significantly larger (open symbols in 2b) do not have significantly larger brains than workers of the same species that have a smaller body size (closed symbols) and their ratio of brain size relative to their body size was significantly smaller. Soldiers – larger bodies but relatively smaller brains (Figure 1b illustrates this well).
  3. Not all regions of the brain are equally affected (Figure 3; not shown but can be found here) by this forager-soldier difference. The relative volumes of the mushroom bodies and antennal lobes compared to the rest of the brain volume was smaller in soldiers of all eight species (meaning they de-prioritize sending their already limited resources to these regions).

In total, these results suggest that there can be consistent, colony-level selection on the brains of workers in army ants and that specialized soldiers have reduced investment in their brains (particularly in the mushroom bodies and antennal lobes) to correspond with their reduced set of behaviors.

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Caste Differences in Wasp Brains

Brain section within this photo credit to the O'Donnell lab and Drexel University.
Brain section within this photo credit to the O’Donnell lab and Drexel University.

My first co-author published paper came out this summer in Behavioral Ecology and Sociobiology, and I’m (no surprise) very excited. The paper, the foundation for the kind of work I want to do in my PhD, is titled: Caste Differences in the Mushroom Bodies of Swarm-Founding Paper Wasps: Implications for Brain Plasticity and Brain Evolution (Vespidae, Epiponini), authored by Sean O’Donnell, Susan J Bulova, Sara De Leon, Meghan Barrett, and Katherine Fiocca. If you haven’t left me after the long title, that’s a good sign – let’s take a quick jog back to our eusocial basics!

A eusocial colony has several different types of individuals (called castes) which perform different tasks in the colony. In the most advanced colonies, reproducing Queens complete few tasks besides laying eggs; they rarely leave their nests, have extensive social contact with workers, and do not gather food. In contrast, female Workers don’t reproduce, complete a variety of tasks inside and outside of the nest (gathering food, feeding larvae, maintaining the nest, defense), and often act alone. It would be hard for these two groups to act more differently!

In theory, we could see how some of these tasks could differ in the demands they place on the animal’s brain. For example, flying outside and searching for food while avoiding predators might require larger regions of the brain in charge of processing visual information (called optic lobes) when compared to sitting in a dark nest all day, laying eggs.

And this is where a key ecological theory comes in – called Neuroecology Theory. This theory predicts that, because brain tissue (like all tissue) is costly to produce and maintain, evolutionarily successful organisms won’t invest willy-nilly in brain tissue they don’t need since those resources can be spent elsewhere. Instead they will invest more in regions of the brain that help them deal with the cognitive demands their environment presents – for the flying wasp, this is the optic lobes.

Imagine we are building a two story house in a flood plain. Sure, we could invest in fire-resistant materials, nuclear-grade shutters, and the kinds of steel they use to make sky-scrapers sway just right in the wind. But if we have a limited amount of money, or resources, we know we will be most successful if we address the most pressing demand of our environment and build a house on stilts, to withstand floods.

The ‘mushroom bodies’ are one area of a wasp brain, involved in learning, memory, and integrating information from the various senses. Previous to our study, scientists have shown that the relative size of the mushroom bodies (mushroom body volume as compared to total brain volume) increases with foraging behavior, the job of the workers. But scientists had also shown that maintaining social dominance via aggressive social interactions can have positive relative mushroom body size effects. This leaves us in a bit of a pickle:

Do queens, with their limited task repertoire but high social dominance, or workers, with their many tasks but decreased social demand, have larger relative mushroom bodies?

Histological section and photo credit to the O'Donnell lab and Drexel University.
Histological section and photo credit to the O’Donnell lab and Drexel University.

To answer this, we sliced wasp brains at a thickness of 14 microns – or 1/7 the thickness of a sheet of paper (see left). After setting the slices on glass slides, we stained the brain tissue a deep blue and photographed each slice – allowing us to measure the area of each region of the brain every 14 microns. By stacking all the slices on top of one another, we get a complete 3D image of the brain – and thus the volume of each of the pieces, since we know the thickness of our slices! Then, we could compare the relative volumes of our mushroom bodies in queens and workers of several different eusocial wasp species. You can see a 3D brain below.

Brain reconstruction credited to the O'Donnell lab and Drexel University.
Brain reconstruction credited to the O’Donnell lab and Drexel University.

 

Figure credit to the O'Donnell lab and Drexel University.
Figure credit to the O’Donnell lab and Drexel University.

So what did we find? In a nutshell:

  1. for 13/16 species, Queens had greater relative mushroom body sizes than their workers
  2. species with larger colony sizes saw larger differences in the relative mushroom body sizes of their queens and workers

These results are consistent with the idea that maintaining social dominance increases relative mushroom body investment – and adds the additional caveat that it seems to increase investment more than foraging.

 

The fact that larger colonies saw larger queen-worker differences in mushroom body investment may be a result of increasing ‘caste specialization’ – this is the idea that, as colonies get larger, the gap in work performed by the queen and the workers gets larger as well.

This is just one kind of question we can answer using histological techniques – we can also look at the effects of light, sociality and other behaviors, disease, body size, etc. on animal brains. All of this gives us a better understanding of how evolution works, and how different brain region sizes are selected for – or against – in different conditions.

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