Parasite ants stimulate chemical production in host ants

Ants use chemical cues to recognise their own nest-mates and eggs, and avoid being tricked by parasite ant species. But when there are no parasites, hosts seem to stop producing so many cues and aren't so good at recognising their own mates.

20 July 2010, by Adele Rackley

Ants use chemical cues to recognise their own nest-mates and eggs, which is crucial when parasitic ant species invade and try to trick them into a spot of childcare.

Parasitic wood ant Formica lugubris

But when there are no parasite ants present, the host ant species seems to produce fewer chemicals and can't recognise their own nest-mates so well.

Just like cuckoos, some ants like to get others to bring up their brood. Parasitic wood ants are particularly fond of replacing a host colony's queen with their own, so the host ants rear the parasitic young and are eventually replaced by them.

Now it looks like the presence of parasites is actually making host ants better at recognising their own nest-mates, through the production of a greater number of 'chemical cues'.

Ants produce hydrocarbon compounds which vary in number and proportion between colonies but remain stable within colonies. The insects can detect very small chemical differences, so ants from out of town just don't 'smell' the same.

It has been suggested that having a diverse armoury of chemical signals also helps hosts defend themselves against parasites. This would mean that greater pressure from parasites could produce better recognition abilities among hosts.

'The biggest step forward in this study was pulling together the chemistry' Dr Stephen Martin, University of Sheffield

A team of researchers from the UK and Finland, led by Dr Stephen Martin of the University of Sheffield, decided to test the hypothesis.

'We knew that the chemical cues used by Finnish ants vary between colonies,' says Martin. 'When we found that the cues don't vary very much at all between English ant colonies we decided to find out why.'

The team looked at two populations in each country of the pioneering ant Formica fusca. This species quickly colonizes areas of cleared forest or disturbed ground, and is a favourite victim of parasitic wood ants.

The aim of the study was to compare the behaviour and the range of chemical cues used by F. Fusca colonies in southern England, where parasitic wood ant species are rare, to those in southern Finland where parasitism by wood ants is common. The results are published in the Proceedings of the Royal Society B.

The ants' chemical cues act like colony bar codes

First the researchers looked at how readily the ants accepted or rejected eggs from their own nests and eggs from another colony. Next, using a petri dish for an arena, they recorded how individual ants behaved when confronted by their own nest-mates or by strangers.

When the scores were in, the Finnish ants had proved much better at recognising eggs from other colonies – the UK ants accepted almost as many 'foreign' eggs as they did their own, suggesting they just couldn't distinguish well between them. And the Finns were significantly more aggressive towards non-nest mates, while once again the UK ants didn't seem to be particularly good at telling the difference.

'We've long known that some ants are aggressive towards each other,' says Martin, 'but this is the first time we've been able to compare the behaviour of the same species from colonies in different countries.'

The scientists then analysed the chemicals the ants were using for recognition, to see how they varied between the two populations.

'The biggest step forward in this study was pulling together the chemistry,' says Martin.

The team found that the Finnish ants used a larger number of chemicals and greater variety of chemical combinations. This more diverse armoury appears to go hand-in-hand with being better at distinguishing between adults and eggs from their own and other colonies.

But the various populations both need to recognise their own nest-mates, so why do they have different chemical cues? Having ruled out genetic diversity as an explanation, the obvious difference between the two countries is the pressure they are under from parasites.

It appears that when parasites aren't present, F. fusca divert their energies into other things and the range of different chemical cues they employ is diminished. They can still recognise different species, but their abilities are reduced to the point where they can't distinguish between colonists of their own species very well – because if there are no parasites around they don't need to.

'We haven't conclusively proved these are the compounds used for nest-mate recognition,' says Martin, 'but this is the first strong evidence that this is what's happening.'

The implication is that the presence of the parasites actually drives the development of cue diversity in host ant species. The results have strong parallels in bird studies which show that in the absence of cuckoos, the eggs of host species become much more uniform in appearance.

'What we need now is for other teams to do work on the chemical cues of ant colonies in other parts of the world, to add to the data and test the hypothesis further,' Martin concludes.