A blog post highlighting the article written by Burchill and Moreau in Insectes Sociaux

Written by Andrew Burchill

Imagine that you’re a social insect scientist with musical aspirations. After many long years, you finally have enough free time to turn your persistent rock ‘n roll day-dreams into reality. But before you rock out as the most righteous post-hardcore, grungewave, electropunk band ever seen, you’ll need to recruit a few more members.

A social media shout-out to your colleagues garners more replies than you expected. Like ants at a spilled soft drink, the number of interested applicants begins to sky-rocket: it seems your entire department has been secretly harboring rock-star fantasies.

You are now faced with the problem of deciding how large the group should be. Traditionally, it seems that four members is the golden mean. For example, the Beatles followed the common pattern of two guitars, bass, and drums. (In your case this would be one accordion, a guitar, bass, and bongos.) But in a flashback from the late 60s, you remember that the successful psychedelic rock group Jefferson Airplane had seven members at one time.

Yet the options don’t stop there: a visiting researcher from Japan points out that AKB48—an all-girl pop group with 48 original members—is wildly popular overseas right now and has expanded to include more than 120 individuals. It’s clear that group size is an important consideration for any burgeoning rock band, but the question remains: what is the optimal size, and what factors determine this?

For biologists interested in social groups, this is not a new problem. Although there are many reasons that cause organisms to cooperate, it is still unclear what number of individuals works “best” in any given situation. Recent theory suggests that in complex environments, smaller groups should end up making better decisions (Kao and Couzin 2014), but Sasaki et al. (2013) found that when it comes to making difficult decisions like choosing the best nest, larger colonies of ants outperform smaller ones. With conflicting theories and little experimental data, we seem to be at an impasse. Where do we find the answer? If only we could run thousands and thousands of experiments, each with slightly varied environmental conditions…

All leading, rhetorical questions aside, we have arrived at our favorite subject: ants. As is oft-repeated by social insect enthusiasts, ants dominate the terrestrial biomass. Working and cooperating as a group is the key to their success; social cohesion between many individuals allows them to access and even create ecological niches that other species cannot. Naturally, the size of these groups (the colony) is a vitally important factor in social insect ecology—it affects traits such as foraging strategies, social organization, colony defense, and colony-wide immune responses. On one hand, we have species living in the leaf litter with colony sizes as small as the Fab Four from Liverpool. On the other hand, some species style themselves after the ever-growing AKB48, blurring the definition of what it even means to be “a” colony: the Argentine ant can form super-colonies that span thousands of kilometers. The 13,000+ living species of ants can thus be seen as a collection of “natural experiments,” with evolutionary forces “tweaking” colony sizes in response to changing environmental and ecological conditions.

But in order to understand the role of colony size in ant evolution, we first need to acquire a basic understanding of the patterns of colony size change over their 130 million year evolutionary history. Have average colony sizes gotten steadily larger over time? Do changes happen in little baby steps or in leaps and bounds? Once the average colony size gets very big, does it ever decrease in size?

A solid evolutionary study generally needs two things: data on lots of species (in our case, average colony sizes) and the evolutionary relationship between these species (the phylogenetic tree detailing which ants are most closely related to one another). Fortunately, Moreau & Bell (2013) had just published the most complete phylogeny of ant species ever seen, giving us the perfect foundation to begin our study. Unfortunately_, there was no way a single empirical study could gather enough data on the average colonysizes from hundreds of species. We were left with one option: combing through the previously published literature for size estimates.

This is where yours truly spent months slogging through data from controlled experiments, field measurements, and anecdotal observations, desperately trying to find estimates for as many species as possible. (We won’t directly describe such actions as ‘heroic,’ but we will leave the word for you to apply as you see fit.)

With an evolutionary tree in one hand and colony size estimates in the other, we decided to use Multiple State Speciation and Extinction (MuSSE) analysis to investigate our data. This analysis simultaneously estimates how frequently species with given traits—small, medium, or large colony sizes—transition from one trait to another and how frequently ants in such categories speciate into two daughter species. By constraining some of these transition rates, we can emulate popular hypotheses proposed in the literature and then compare these models.

We found that colony size change seems to undergo a kind of threshold event. After the colony grows large enough over evolutionary time, it seldom decreases backwards in size. In a somewhat tortured metaphor, imagine your hypothetical rock band from earlier. With only a few members, it’s relatively easy to add or lose new musicians. But suppose that you accept all those hopeful applicants from your department to form an AKB48-esque conglomeration. Now that you’ve been branded as a “mega-group,” it’s going to be almost impossible to eject enough members to play at the local drinking establishment. Reverting back to your typical four or five person band is not really an option anymore.

Additionally, our investigation suggests that changes are usually the result of incremental “tinkerings” with the number of workers in the colony. Large, exponential changes in average colony size were rare. Again, imagine our still-nameless rock band: musicians join or leave the group one at a time instead of in big cliques. Taken with the above-mentioned threshold-like activity, we suggest that as colony sizes grow larger, they may fall into a feedback loop. Workers may be able to specialize in certain tasks within a large colony, which could increase overall efficiency, allowing the colony to grow larger, etc., etc.

So what IS the optimal group size? We still don’t know. But now researchers have an empirical groundwork for further studying colony size evolution in ants. Do particular clades or groups of ants exhibit unusual changes in colony size? Before our work, myrmecologists wouldn’t have even been able to say what “unusual” was; now we can pinpoint clades that exhibit unique patterns, ideal of future investigation. Our results also corroborate previous theoretical (Bourke 1999) and mathematical models (Guatrais et al. 2002) of social insect evolution. Can we further refine these models or explore why the alternative models failed? We believe that the most promising route of inquiry should involve adding ecological data into this study: perhaps we can find how the number of queens a colony has, the type of food the ants eat, and/or the environment a species inhabits will affect the group size. Although the issue is by no means settled, we believe our work is a good first step in the right direction.

 

References:

Bourke AFG (1999) Colony size, social complexity and reproductive conflict in social insects. J Evol Biol 12:245–257. doi: 10.1046/j.1420-9101.1999.00028.x

Gautrais J, Theraulaz G, Deneubourg J-L, Anderson C (2002) Emergent polyethism as a consequence of increased colony size in insect societies. J Theor Biol 215:363–373. doi: 10.1006/jtbi.2001.2506

Kao AB, Couzin ID (2014) Decision accuracy in complex environments is often maximized by small group sizes. Proc Biol Sci 281:20133305. doi: 10.1098/rspb.2013.3305

Moreau CS, Bell CD (2013) Testing The Museum Versus Cradle Tropical Biological Diversity Hypothesis: Phylogeny, Diversification, And Ancestral Biogeographic Range Evolution Of The Ants. Evolution (N Y) 67:2240–2257. doi: 10.1111/evo.12105

Sasaki T, Granovskiy B, Mann RP, et al (2013) Ant colonies outperform individuals when a sensory discrimination task is difficult but not when it is easy. Proc Natl Acad Sci 110:13769–13773. doi: 10.1073/pnas.1304917110