For the Greater Good

Tom Ireland talks to Deborah Gordon, the first biologist to witness ant colonies' entire life histories – from formation, to reproduction and death – play out over decades in the wild

The Biologist 63(3) p24-28

Professor Deborah Gordon knows how ant colonies work better than the ants themselves. For the last 30 years, she has observed a population of around 300 colonies in the New Mexico desert, a study that has helped us to understand how these remarkable collectives function without any centralised control.

During this period, Gordon has watched ant colonies form, produce daughter colonies and die. She has even been able to watch the effects of natural selection in action as different foraging strategies led to different reproductive success for the colonies. She believes knowing how ants work together without any central control can help us better understand similar decentralised networks, such as the internet, groups of cells and even the human brain.

What are the common features of all ant colonies?

There are more than 14,000 species of ant, but we don't know much about most of them. Only about 50 have been well studied. Some are probably going extinct in a tropical forest somewhere while we have this conversation.

All ants make colonies, and all colonies consist of one or more reproductive females. The others you see walking around are the female workers, and the males are only around for a short time to mate and then they die. The queen doesn't issue orders or tell anybody what to do, she just lays eggs.

In all the ones we've seen, ants go out and get food and bring it back to a nest. They keep the eggs, larvae and pupae in the nest and take care of them until they emerge as adults. Some will make a nest from something that is there already, such as hollow trees or special structures that are grown by plants with which they have a mutualistic relationship.

There is a lot of variation in what ants do and the environments in which they live, but they all work without any central control. No ant ever tells another what to do; there is never the case that some ants are in charge of the others.

How is the behaviour of the colony regulated without any central control?

Most ants can't see very well, but they have a very well developed sense of smell. They smell with their antennae and when one ant touches the other with its antennae, it smells the other ant. There is a layer of grease on the ants' bodies, which is there to keep them from drying out. These cuticular hydrocarbons also carry odours and ants use them to identify who is a nest mate and who is not.

An ant decides what to do based on its recent experience of interactions with other ants. In the aggregate, this can regulate the behaviour of the whole colony, although no ant is making a decision about how much needs to be done.

In harvester ants, the desert sun changes the chemistry of the hydrocarbons on the ants that have been working outside and they smell different from those that have been working inside the nest. When one ant touches another, it can decide if it is a nest mate and what task it has been doing (see p25).

harvester ant r

If there are no ant species that live individually, how and when did this system evolve?

It is not really clear how ants evolved from wasps – there has been a lot of speculation about that. It is thought that the system evolved from an ancestral system that was more like wasps'. Existing wasp species show a range of systems: some take turns laying the eggs, or some lay eggs and then die while others raise the young. There are options that are like an ant colony, but not the same.

It seems that the evolution depends on seasonal changes in when females lay eggs. So that from a system where females only lay eggs some of the time evolved a system where only one female lays eggs all of the time. It's an evolution of the processes that turn egg-laying on and off in different seasons; how a group of females living together decides who lays the eggs and when.

When you began studying these ants in 1985, did you have any idea you'd still be studying them 30 years later?

No! If I did I'd have probably organised the way I did things very differently from the beginning. I had no idea that they were going to live for 20, 25, 30 years, because nobody knew anything about that. The oldest colony is 29 years old now, and I don't know how long they are going to keep going, although most of them die in their 20s.

This is the longest term study tracking a population of social insect colonies. I began studying them in the desert as a graduate student. I started to mark which colony was which and when I'd done this for a few years, I realised they were growing and that, of course, a colony has a life history like any living system. It begins, founded by a single queen, and grows into a very large colony of maybe 10,000 ants, and I started to see how their behaviour changes as they get older. So then I realised it would be worthwhile to identify many of them so I could follow them. Once I started, I was stuck with it – now that I have built up this amazing data set, of course I have to keep going.

So the colony outlives the individuals living within it many times over?

The queen lives for the whole lifetime of the colony. She mates only once, in the first few weeks of her life, and uses the sperm stored from that original mating session to keep producing ants year after year for the rest of her life. The ants all live for a year and she has to make them all over again each year.

So in one ant colony in New Mexico, a queen is still producing eggs with sperm from a mating session 29 years ago?

That's right. I don't know how fresh it is, but it's alive.

Why might a colony die?

We don't know. It's conceivable that the queen just gets worn out, or perhaps that store of sperm runs out. What we see is that when the queen dies the ants keep on going, but there's no one there to make new ants, and so the colony dies after about a year, when all the workers have died.

How do you count the number of individuals in a colony?

We dug up a whole colony, put all the ants in boxes and sat in the lab putting ants from one box into another, just counting them. It took a very long time. To do that, you have to destroy the nest, so I don't do that often or with the colonies we're tracking.

What have you been able to observe thanks to your unique study?

It has been possible to learn about the demography – how long a colony lives, how many offspring they have – which allowed us to make a life table we can use to estimate how the population is likely to change.

The most interesting thing is that we've been able to track the action of natural selection as it is happening. We've been able to see which colonies are the offspring of which – a first for social insects.

We can say out of this population of colonies which one is the mother colony that produced the daughter queen that started a new colony. That way, we can measure reproductive success. And that means we can ask whether colonies that behave in a certain way end up being more successful. Natural selection requires that there are differences in the way they act that lead to differences in the number of offspring they have. That's what we found. The differences were in the way they use interactions to regulate how much foraging a colony does.

What we see is that natural selection is acting on the way that colonies use feedback, based on simple interactions, to adjust foraging to food availability and the weather.

It surprised me to read that colonies have large numbers of ants that appear to be doing nothing. What is the purpose of these 'reservists'?

It's possible they are just there for emergencies of a type that has never happened all the time I've been watching, but I think there are good arguments to be made for having ants doing nothing. Inactive ants might function in a network of interactions – for example, when there is very high positive feedback. A forager goes out when it has enough interactions with ants coming back with food.

The more ants come in the more go out. That could become a runaway process and it would be useful to have some ants that just don't respond or dampen the feedback, like a buffer.

If an ant in the nest can distinguish between contact with active and idle ants then that is information about how well the active ants are doing – if they're all busy outside, it's going to meet lots of inactive ants. So the presence of a group of ants tagged as idle gives everybody information about what's going on.

What other systems function in a similar way?

There are many obvious analogies in engineered systems – we see a lot of distributed processes or processes without central control in the internet and the cloud. Very simple interactions in the aggregate produce a regulated outcome. So I think that we may be able to apply what we learn from ants to robotics and data networks. Ants show us ways of doing things that may seem messy, and often won't be as tidy or efficient as systems that we build. But these methods are working very well for ants and in some cases could work for us.

Another application that I think will take more work to develop is that there are probably analogies to other natural systems like brains and the way groups of cells interact, such as cancer cells, which may be using a process that we also see in ants but we haven't yet explored. I think there is growing interest among biologists in looking at cells interacting as individuals. I'm interested in questions about how evolution has shaped systems that appear to be very different, like cells or ants, but because of similar environmental constraints, they interact in similar ways.

Do you see a colony of ants as a kind of organism?

Yes – everything an ant does happens in the context of the colony; an ant never functions on its own. It's fascinating to think about how ants work together. The same question comes up with brains – we tend to think that our brains equal 'me', but just as an ant colony is just a bunch of ants working together, so a brain is just a bunch of cells working together. There is the same philosophical question about how to understand that for any collective behaviour.

After 30 years of studying the same colonies, how do you feel about ants?

My favourite moments watching ants are when they surprise me. They often remind me how little I know. Of course, there are some kinds of ants I like and others I don't. The ants are not aware of me – so it's not a two-way relationship.

Ant-i gravity

Deborah Gordon’s lab is looking for schools to participate in the Ant Colony Search, a simple, inexpensive experiment that aims to help understand how different ant species search.

“The more searchers there are, the more thoroughly they search,” says Gordon. “But if there are few searchers in a large space, they have to spread out their paths to cover all the ground.”

The experiment was even done in microgravity on the International Space Station. “I think that the struggle to hold on to the surface in microgravity interfered with their collective search.”

Professor Deborah M Gordon is a biologist at Stanford University.