Evolution at work in ants

The body shapes of queen ants and worker ants have evolved in different ways to reflect their different roles. A careful dissection of their necks can tell us about howtheir queens behave. Roberto Keller from the Gulbenkian Institute in Portugal told us more...

Roberto - These insects are very strong. They're able to carry things around that are much bigger and heavier than them - 30 times their own weight. So, what is it in their anatomy that allows them to do this? Of course, the other question was, well if you have a system where you are producing queens and workers, and each of them are doing different behaviours within the colony, how is it that their anatomy is optimised to the task that they are going to be doing?

Chris - So tell us a bit about the experimental method. What did you do to try to explore this question?

Roberto - So, it was two parts. We wanted to describe what are the differences in the anatomy between the queens and the workers? And for that, we looked for different species. We collected in the field, we also went to museums, and we'll basically do dissections and take photographs of the different muscles, and tried to see which muscle corresponds to which one in the worker and the queen and see how are they different. We wanted to quantify the differences. So we went and measured the different thickness in the thorax of queens and workers of many different species. Once we established that there is a pattern of differences, we wanted know if these differences is something that all ants have. So, we carefully chose species that will represent all the different groups of ants that are known, and try to see if the differences we first established were present in all the species.

Chris - What were the differences that you saw?

Roberto - One the one hand, the fact that the segments in the thorax of the queen will match their behaviour, meaning the first is not very large because they are not lifting things around. The second segment is very large because they are flying away, and they have all these wing muscles. And in workers, the first segment is always very enlarged because they will be using their head as a crane to carry things around and so they need these very, very strong neck and shoulders. Whereas, they all have a very reduced second segment. While we start doing the survey, we find out other things that we were not expecting. And that's that queens in ants across species are not all the same. The first segment, the one that has the neck muscles, in some queens is very, very reduced. The second queen's first segment was enlarged, it was never as big as a worker's, but it was certainly much more than in the other type of queens.

Chris - Is it something in the behaviour of those queens that would be a reason for that?

Roberto - By the time I was doing this work, I have a collaboration already established with Christian Peeters, and he specialises in ant behaviour. So one day when I start showing these patterns of the different queens, he suddenly jumped and realised that it appears that the difference of the anatomy of the queen was a reflection also of a certain difference we already knew that existed in the behaviour of the queen. And the majority of the queens you will see around, when they are virgin, they will fly away from the nest, mate with a male, then go to the ground, shed their wings, and burrow themselves, and start a new colony by themselves. Now what the queen does is that it never leaves this nest to look for food, what it does is it metabolises it's huge wing muscles that she will no longer use and she will use that to feed the first generation of workers. Now, the other type of behaviour that is more primitive is that the queens do something similar but then they will go out and hunt and look for pieces of food to feed the first generation of workers.

Chris - So, can I speculate then that the ones that do this more primitive hunting behaviour, don't have this enhanced musculature that they're then going to use to feed the colony. So they have a smaller section compared with the ones that do feed that colony off their own muscles.

Roberto - Yeah. That's exactly the case. They won't have to go and hunt. They have a slightly bigger first segment that will help them hunt and carry things around, but because they will not metabolise their wing muscles, their second segment is not very large. Where the ones that do metabolise the wing muscles have a huge second segment with huge wing muscles that are not only used for flight but of course they are proteins that can be used for function in the colony.

Chris - So, by studying the neck structure of the queen, you can work out whether she is one of the hunting type species or a non-hunting type which can spare you the need to do sometimes very laborious field studies.