Interactions with combined chemical cues inform harvester ant foragers' decisions to leave the nest in search of food

Abstract

Social insect colonies operate without central control or any global assessment of what needs to be done by workers. Colony organization arises from the responses of individuals to local cues. Red harvester ants (Pogonomyrmex barbatus) regulate foraging using interactions between returning and outgoing foragers. The rate at which foragers return with seeds, a measure of food availability, sets the rate at which outgoing foragers leave the nest on foraging trips. We used mimics to test whether outgoing foragers inside the nest respond to the odor of food, oleic acid, the odor of the forager itself, cuticular hydrocarbons, or a combination of both with increased foraging activity. We compared foraging activity, the rate at which foragers passed a line on a trail, before and after the addition of mimics. The combination of both odors, those of food and of foragers, is required to stimulate foraging. The addition of blank mimics, mimics coated with food odor alone, or mimics coated with forager odor alone did not increase foraging activity. We compared the rates at which foragers inside the nest interacted with other ants, blank mimics, and mimics coated with a combination of food and forager odor. Foragers inside the nest interacted more with mimics coated with combined forager/seed odors than with blank mimics, and these interactions had the same effect as those with other foragers. Outgoing foragers inside the nest entrance are stimulated to leave the nest in search of food by interacting with foragers returning with seeds. By using the combined odors of forager cuticular hydrocarbons and of seeds, the colony captures precise information, on the timescale of seconds, about the current availability of food.

Figure 1. Number of foragers leaving the nest along foraging trails for one representative colony.

Also shown are time periods during which data were collected and seed mimics were added to colonies.

Figure 2. Mean number of foragers per unit time (+/− standard error of mean) before and after addition of seed mimics to nest entrances.

A) Addition of blank seed mimics. B) Addition of forager cuticular hydrocarbon seed mimics. C) Addition of oleic acid seed mimics. D) Addition of seed mimics treated with both forager cuticular hydrocarbons and oleic acid. * indicate statistical significant differences among means (p<0.0125).

Figure 3. Mean interaction rate (interactions/second; +/− standard error of mean) in the nest entrance with A.

ants and mimics before and after addition of blank mimics (white) or combination of forager and seed odor mimics (gray); B. only with ants before and after addition of blank mimics (white) or combination of forager and seed odor mimics (gray); C. only with mimics in blank (white) or combination of forager and seed odor (gray) treatments. * indicates p-value<0.05.