May 16, 2013
Natural Selection Shapes Ant Behavior Colony Evolution
April Flowers for redOrbit.com - Your Universe Online
City-states in ancient Greece that waited until their own harvest was in before attacking and destroying a rival community´s crops often experienced better long-term success. Ant colonies that show similar selectivity when gathering food have similar results, according to a new study from Stanford University.Deborah M. Gordon, Stanford biology professor, conducted a long-term study of harvester ants. Her findings, published in the journal Nature, reveal that the colonies that restrain their foraging except in prime conditions also experience improved rates of reproductive success.
Gordon´s study provides the first evidence of natural selection shaping collective behavior.
There has been a long-held belief in biology that the amount of food an animal acquires can serve as a proxy for its reproductive success. For instance, hummingbirds that drink the most nectar probably hold the best chance of surviving to reproduce.
There isn´t always such a linear connection between the two, however. The ants that Gordon studies in the desert of southeast Arizona have to spend water to obtain water. The ants lose water while foraging and then obtain more from the fats in the seeds they eat.
To regulate foraging activity, the ants use simple positive feedback interactions with the foragers waiting near the opening of the nest to bump antennae with the ants returning with food. The faster outgoing ants meet with ants returning to the nest with food, the more they go out to forage. In a previous study last year, Gordon and her colleagues showed that the ants´ “Anternet” algorithm follows the same rules as the protocols that regulate data traffic congestion in the Internet.
The way colonies use these interactions to regulate foraging is not always the same from one colony to the next. Some colonies are likely to forage less when conditions are dry, but are more likely to forage more steadily when the conditions are good.
The findings suggest it is more important for the ants to not waste water than to forage for all the available food. This strategy has no survival cost for the colony, even though they will sometimes forgo foraging for an entire day. The colonies that hunker down on bad days not only live just as long as other colonies, but they have more offspring colonies as well.
"Natural selection is not favoring the behavior that sends out the most ants to get the most food, but instead regulating foraging to hold back when conditions are bad," Gordon said. "This is natural selection shaping a collective behavior exhibited by the entire colony."
The researchers are still trying to determine how the ants gauge humidity. They have determined, however, that the collective response of the colony to environmental conditions is heritable from parent to offspring colonies. The offspring colony resembles the parent colony in their sensitivity to conditions even if the daughter-queen establishes the new colony so far away that the two never interact.
The behavior of the parent and offspring colonies don´t entirely match up on all days, however, they were similar on extreme days, making similar judgments about when to lie low or take advantage of ideal conditions.
The region has experienced 15 years of extended drought, making it possible that the more restrained a colony is, the better its survival and reproductive future looks. However, Gordon can´t say yet whether the emphasis on sustainability evolved in response to climate change pressures.
"What's evolving here are simple rules for how ants participate in a network that regulates the collective behavior of the colony," she said.