Colonies of the red harvester ant (Pogonomyrmex barbatus) forage in the desert for seeds that provide both food and water. Foragers lose water while out in the desert Sun, and the rate of water loss is higher in dry conditions. To manage the tradeoff between food accumulation and water loss, colonies adjust foraging activity to changes in ambient conditions, especially humidity. In a new study, Stanford University researcher Daniel Friedman and co-authors examined the neurophysiological basis of variation among colonies in how they regulate their collective foraging behavior.

“An ant colony acts almost like a multicellular organism; the colony is the evolutionary unit,” Friedman said.

“We wanted to see if collective behavioral variations among colonies were associated with differences in individual forager brain chemistry.”

First, Friedman and his colleagues from Stanford University, the University of Virginia, the Universities of California, Los Angeles and San Diego collected foragers from six previously studied colonies of red harvester ants in the Arizona desert.

Half of the colonies foraged often in dry weather, while the others kept their foragers home on drier days.

When the researchers dissected the foragers’ brains and measured gene expression with RNA sequencing, they found differences between the two groups in the expression of genes related to neurotransmitter signaling and metabolism.

Seeing these results, they wondered whether manipulating brain levels of a neurotransmitter called dopamine would influence forager behavior in the field.

The team decided to study a new set of nine colonies in Arizona.

For each of these colonies, the scientists treated some nest-mates with dopamine and some with a control solution. They color coded the ants by group with paint.

The day after administration, they observed that dopamine-treated ants went on more foraging trips than their control-treated nest-mates.

They also found that the dopamine-affected colonies seemed naturally more sensitive to humidity, foraging more on muggier days and staying home on drier days.

To verify their results, the study authors repeated the experiment but added a group treated with 3-iodo-tyrosine, a chemical that inhibits dopamine.

They saw the opposite effect: the inhibitor of dopamine synthesis caused treated ants to forage less.

“The increases in forager brain dopamine seemed to increase individual ant foraging. That supports the idea that behavioral differences between nest-mates might be related to differences in brain dopamine levels,” Friedman said.

“We know that the individual risk that the foragers take relates to the collective decision-making of the colony, but there’s a lot more to learn there.”

The study was published in the journal iScience.

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Daniel A. Friedman et al. The Role of Dopamine in the Collective Regulation of Foraging in Harvester Ants. iScience, published online September 27, 2018; doi: 10.1016/j.isci.2018.09.001