Ant highway repair


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By Elsa Youngsteadt
Drawing by Nathalie Escure

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Army ants live in large nomadic colonies, conducting spectacular foraging raids during which thousands of ants run together, killing and collecting any prey that don't get away fast enough. In the species Eciton burchellii, these raids take place daily or every other day, but they never go to the same place two days in a row-- so the thousands of foraging ants never manage to tamp down a smooth highway through the leaf litter and fallen branches of their rainforest home. This uneven surface would slow the ants down, except that some ants form a public works department, filling the uneven surfaces with their own bodies. Ants can run over surfaces with ant-plugged holes almost as fast as they can over smooth surfaces. Once their nestmates have run on top of them, the plug-ants get up and follow the trail back home.

Biologists at the University of Bristol studied this pothole-plugging behavior in army ants in Panama; their findings are published in the June, 2007 issue of the journal Animal Behaviour. By forcing ants to run over planks of different widths drilled with different size holes, they showed that the formation of living pothole plugs increases the rate at which the ants can run to and from the foraging area to the colony's home base. This increased running speed means the ants can carry more food to the colony's queen and developing larvae. More food for the queen and larvae, in turn, translates to an overall higher reproductive rate for the colony. Colonies that manifest the hole-plugging behavior will therefore reproduce more often than those lacking the behavior, and hence the trait should spread in the population.

Social insects like army ants are unusual among animals in that they live in large colonies in which only one or a few individuals reproduce; the rest are sterile workers-- all daughters of the queen. Rather than mate themselves, the workers feed their mother and help rear her additional offspring. Thus, from the evolutionary perspective, the entire colony is an individual, and traits of workers should only evolve if they benefit the colony. Individual benefits to a sterile worker would be invisible to natural selection. The pothole-filling study is an example of how a specialized behavior performed by a few individuals improves colony fitness by getting more resources to the queen and her offspring.

Animal Behaviour for original article (limited access)
University of Bristol news release
Reuters news story
BBC news story
Webpage of author Nigel Franks
Background army ant biology

Questions for review and discussion

1. What specialized behavior do army ant workers perform that helps the colony to forage faster?

2. How would faster foraging improve colony fitness?

4. What is a social insect? How does natural selection work differently on social insect colonies than on other animals?

5. This study describes a self-sacrificing behavior that worker ants perform for the benefit of the colony. Can you think of other examples of altruisim in social insects? In other animals? How could natural selection favor these behaviors in individuals that do not reproduce? In individuals that do?

Curriculum Materials

Group Selection from Laboratory Exercises in Evolution from the University of Virginia

Zimmer, C. "From Ants to People, an Instinct to Swarm"
The New York Times, November 13, 2007

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