Friday, August 26, 2011

Some ants are more equal than others

Individually coded ants (photo from JEB).
A study published in the September 15 issue of the Journal of Experimental Biology (and reported on the BBC web site) concludes that ants recognize the decision-makers in their midst.

Coordination of biological processes in groups that lack the ability to communicate globally among themselves, such as insect societies, has long been thought to be due to self-organization. That there is division of labor in these societies has long been known, but it has been thought that within strata, every individual must know the same rules, and be equally able to follow them.  And the general assumption has been that they are basically chemical automata in this respect--all molecular recognition rather than open cognition.  Indeed, human exceptionalism is so widespread in science that one often sees it viewed as surprising that any other species can think (much less has 'mind').  So, this experiment, while quite basic, is interesting.

It's starting to look as though ants not only learn, but recognize that some of their peers may have more knowledge than others.  That is, some may play more influential roles in key behaviors, including decision-making.  Stroeymeyt et al., at the University of Bristol, looked at this question in the house-hunting ant, Temnothorax albipennis.  These ants live in precarious places such as under tree bark or in rock crevices, and their homes are frequently destroyed, and they thus need to rebuild at a new site.

When it's time to move, scouts go out from the nest, or hive in the case of bees, to find new sites.  Behavior was always focused on the colony previously, not the individual, so it has always been thought that every scout had an equal chance of happening upon the best new site, and thus being followed by the rest of the group.  But Stroeymeyt et al. have discovered that Temnothorax ants seem to acknowledge that some scouts are more experienced than others, and thus should be allowed to make the decision as to where to move.

Temnothorax ants investigate their environment preemptively, collecting information about potential nest sites before they must move, which apparently increases their chances of choosing a good site (though it does sound as though none of their choices are all that good, or they wouldn't have to be moving all the time!).
Information about suitable sites is initially gathered by the workers that discover and explore the sites. It could then potentially be stored in two, non-exclusive forms: in a common repository of information, such as pheromones (social information) marking the nest itself or leading to it, or in the memories of informed workers and/or individual-specific chemical marking (private information), frequently used by Temnothorax ants. In the latter case, informed individuals accessing this information could play a key role during later emigrations.
To look at the role individuals play in this decision,  Stroeymeyt et al. painted tiny uniquely colored dots on the backs of all the workers in colonies they brought into the lab.  The idea was to determine "the role of specific individuals and the relative importance of private versus social information".

They brought 30 colonies into their lab, and moved them into nests consisting of 5 interconnected Petri dishes.  They were given a week to explore the area.  Their nest was then destroyed, inducing them to relocate.  They had the choice of a nest just like the one that had been destroyed, that some of their number had previously investigated, and an identical one none of them had ever seen before.  A new nest was considered chosen if all brood items were moved to that site, but not if items were split between two nests.

Exploration throughout the week was captured on webcams, so that it was known which individuals explored, and thus collected 'private' information about new nest sites.  These were considered 'informed' workers.  More informed workers were those that visited the site most, naive workers never visited it.

They also investigated "the relative importance of navigational memory and individual-specific chemical trails in the early discoveries of the familiar nest by informed workers".  They did this by laying sheets of acetate between two nests, then rotating them, thus changing the location of previously laid chemical trails. The idea was to test the relative importance of chemical versus visual orientation clues.

As it turned out, familiar nests were discovered first, and seemed to be preferred over unfamiliar nests.  Naive workers were as likely to find the unfamiliar nest as experienced workers, and took an equal amount of time to find either.  Informed workers, however, were much more likely to find the familiar nest quickly.  So, the researchers conclude that information collected by individual ants -- private information -- is important in leading them to new sites.

And they discovered that the ants do seem to follow chemical clues, since colonies emigrated significantly faster when the acetate under them, containing their chemical trail, was not moved, although they still preferred the familiar nest site over the unfamiliar, indicating that they were in fact following ants with private personal knowledge, which seems to include visual cues.  And, the existence of private knowledge seems to be recognized by other ants in the colony, which speeds emigration to the site preferred by the experienced workers.

So, yes, the choice of a new site seems to be a collective decision, but some individuals have more influence over that decision than do others.  We've blogged before about ants, and whether they think (we concluded that they do).  We noted then what Darwin said in Descent of Man (1871) about ants, and it's worth repeating:
...the wonderfully diversified instincts, mental powers, and affections of ants are notorious, yet their cerebral ganglia are not so large as the quarter of a pin's head. Under this view, the brain of an ant is one of the most marvelous atoms of matter in the world, perhaps more so than the brain of man.
The general idea about  cognition and consciousness (often implicitly equated) is that it's a phenomenon of large-scale interactions among synapsed neurons: you need zillions of these to have 'thoughts' or make cognitive, evaluative judgments that go beyond mecahnical responses to, say, odorant detection.  But if the pinheads of ants can do this, perhaps our ideas about what makes this possible has been too closely associated with our own high encephalization (big heads relative to body size) and egotism.  Maybe quality rather than quantity in neuronal function is more important than we've thought, or in ways we yet haven't thought of.


James Goetz said...

Ants do amazing feats, but do they know that they do amazing feats. :)

Ken Weiss said...

A good question. They don't know it in the way we do about ourselves, or about them, because they don't have language like ours. Whether a couple of them observe another and, in ant-speak, comment on the feat, is something to think about it, though.

Anne Buchanan said...

We can always find ways that humans are different from other animals. But we aren't as different as we like to think. And, if evolution works as we think it does, this is exactly as expected.