20 November 2012

Dying young? Better live fast — if you're an ant.

Myrmica scabrinodis. Photo by Myrmecophilie.
Cross-posted from Nothing in Biology Makes Sense!

Among the many things I hope you're thankful for — whether you're U.S.-based and celebrating Thanksgiving this week, or you're feeling generally grateful regardless of geography and time — you can add to the list the fact that you're not an ant. Worker ants are essentially enslaved to the task of helping their mother, the queen, reproduce. Any individual worker is disposable, in support of that broader task of the whole colony.

And it's not as though the workers don't seem to be aware of this — to the extent that a worker ant can be "aware" — at some level. An experiment described in the current issue of The American Naturalist demonstrates pretty clearly that, when workers are injured, they take greater risks — as you'd expect if they're trying to give the colony the greatest possible benefit from their shortened lives.

The study itself sound logistically tricky, and maybe a bit mean-spirited, but as an experiment it's elegant. A team of Polish researchers started by going out into the forest to collect ants — Myrmica scabrinodis, a common European species — and setting them up in artificial colonies. In each case, the team set up pairs of artificial colonies with equal numbers of workers collected from the same natural source colony, and supplied these transplanted workers with a queen and some larval ants to tend. Which, apparently, the captured workers were perfectly happy to do after about a day of acclimation.

Within each artificial colony, then, the researchers injured half of the workers. They did this either by exposing the ants to carbon dioxide for an hour and a half, or by breaking off their propodeal spines — pointy projections from the rear-ward part of the ants' thoraxes. Neither of these treatments left the workers unable to work, but they both have the effect of shortening their lifespans.

Within each pair of artificial colonies, then, the team chose one colony to present with a "risky" condition. All the colonies were connected via a PVC passageway to a small foraging arena where the ants could gather food. Risky conditions, in this context, were one of three possible variations on the basic colony design. First, there could be a much longer passage to the foraging arena; since, in the wild, more time spent outside the nest means more time vulnerable predators. Second, the passage could be heated up to a temperature that would be uncomfortable to ants. Third, the foraging arena might contain workers of a competing ant species — which were kept from actually attacking by a mesh barrier, but still able to interact with the ants from the experimental colony.

So, for each pair of colonies, one had a riskier path for workers to take to collect food. And in each colony within the pair, half the workers had been gassed or maimed. The collaborators allowed the ants a couple days to acclimate to the artificial colonies, then closed off the foraging arenas to capture the ants that were out collecting food, and tallied up how many were injured (individual ants were identifiable by dots of paint).

And, consistently across colonies and the three different kinds of risk, the foraging arena in the riskier colony of each pair contained a larger proportion of injured workers.

How this might actually work is beyond the scope of the experiment. It seems unlikely that an individual worker ant has anything like the train of thought: "Hmm. I seem to be injured. I guess I can walk through this hot tunnel to go find food, since the Colony needs me to, and I don't have much productive life left anyway."

So maybe the injuries the researchers inflicted on the workers made them poorer judges of risk — less able to detect the risky conditions, or less able to respond when they did. But the end result is the same, from the perspective of a whole colony: workers who are closer to death are more expendable, and they act accordingly.◼

Reference

Moroń D., Lenda M., Skórka P., Woyciechowski M. 2012. Short-lived ants take greater risks during food collection. The American Naturalist. 180:744–750. DOI: 10.1086/668009.

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