Ant pupae feed adult ants and larvae

Scientists and naturalists have been fascinated by the industry and organisation of ants for hundreds of years. And we thought we knew most of the basics about how they organise themselves and their colonies. So how did we overlook the fact that developing “baby” ant pupae feed the adult ant workers and other developing ants with a milky substance that appears to be critical for their survival? In an extraordinary reversal of the breast feeding process adopted many mammals like us, the pupae secrete this milky substance that other ants consume. Daniel Kronauer, at Rockefeller University, discovered it…

Daniel - So it turns out that this fluid is actually the malting fluid of the pupae. The moulting fluid is a fluid that all insects actually produce because they have to shed their exoskeleton, their skin in order to grow. Most solitary insects reabsorb and recycle this moulting fluid. But now it turns out that the ants actually secrete it. Normally in solitary insects it kind of accumulates in the space between the old exoskeleton and the new cuticle, the new exoskeleton. In the ants, it's secreted and it kind of leaks out of the rectal opening of the former cuticle. So it kind of accumulates a droplet at the abdominal tip of the ant pupae.

Chris - What happens then? The other ants come along and collect it?

Daniel - Exactly. That's what happens. So the adults, they drink it as it accumulates, and on top of that, they also bring the larvae of the ants, which in ants can't really move independently. The adults carry them and place them on top of the pupae. So the larvae also lap up this liquid as it gets secreted.

Chris - And you can prove that they're actually not just cleaning this up, because there's a danger obviously if you end up with this stuff accumulating and if it's rich in lots of potential food, then it's going to feed microbes and things and that could cause disease. So are you sure that the animals are eating this?

Daniel - You're absolutely right because the fluid is very rich in nutrients, it can lead to fungal growth and actually if the fluid does not remove the ant pupae die of fungal infections. And so the way we showed that the larvae and the adults actually drink it is we injected food colouring into the pupae into this fluid. And then we could show by tracing it into the adults and the larvae actually take up the fluid.

Chris - And do they actually incorporate it into their own bodies? They're not just taking it up in order to then regurgitate it and dump it somewhere. They're actually using it?

Daniel - Yes. So they're using it, they're digesting it. And we could actually show that the young larvae really depend on this fluid for growth and survival. So if the young larvae don't have access to this fluid, they don't grow well and the mortality really increases compared to a lot of that have access to this fluid.

Chris - Does it have any role beyond just nutrition? Does it have any role in terms of determining who is who because there is a hierarchy in these sorts of organizations of social insects, isn't there, where there's the queen who's laying eggs and dictates the behavior of other workers with various pheromones and so on. So does this have any role to play in that sort of role determination system?

Daniel - So that's something we're very interested in for future research. It's something that we haven't studied yet explicitly, but because this social fluid is really rich in hormones, which we know are important for the development of ants into queens and the workers for example. So for example, one compound that we found in this fluid is also very prominent in the royal jelly that honeybees feed to larvae that they want to wear into queen honeybees. And that seems to be important in determining queen development. And so it seems like a good hypothesis that we want to follow up on.

Chris - And you raise a good point about bees because they're the other major group of social insects that we think about a lot. Do they do anything like this?

Daniel - Yeah, this was an interesting question that we actually also asked ourselves and we decided in the end to get a few bees into the lab and took honeybee pupae and put them in social isolation to see whether they secrete the fluid. But it turns out that honeybees don't. So we don't know yet whether maybe other social insects do something similar, like wasps or so on, but at least the honey bees don't. And I think one reason might be that honey bees of course develop in these combs, so they're like enclosed in a cell. Maybe the adults have less of an access to the pupae in ants. So it's very different. Ants don't build combs like honeybees do.

Chris - Have you any idea how this evolved in the first place? One approach might be to look at the fact that there are thousands, I think I read somewhere, 15,000 different species of ant. And if they all did it then it must be a very ancient behavior that got going very early in their evolution. So where did they get this from?

Daniel - Yeah, that's another very good question. There's many, many ant species, as you said, about 15,000. We initially found this in our main lab model system, which is called the clonal raider ant. But that species is a little bit unusual. And so what we did is we got another four ant species that kind of covered this biodiversity of the ants phylogenetically. So they're all very distantly related ant species. And it turns out that they all secrete this pupal fluid. And so it seems like this evolutionary novelty really arose very early in ant evolution. And so it's a very ancient thing and we think it has probably been very important for the early evolution of social behavior and ants because it creates these dependencies across different developmental stages, right? And it really helps integrate this ant colony superorganism like structure where individuals are really dependent on other individuals in the colony.