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.

Identifying robustness in the regulation of collective foraging of ant colonies using an interaction-based model with backward bifurcation

“…In conclusion, our results support the evidence that a simple interaction-based recruitment strategy can provide a robust regulatory system for managing collective foraging in ant colonies [17,21]. Furthermore, our model provides useful insights into how internal and external variables can impact foraging dynamics, including identifying potential sources of inter-colony variability in activity patterns [24].…”

“…We have developed a simple model describing the interaction-based recruitment and regulation of collective foraging in harvester ant colonies [4,17,19,21]. Our main result is that depending on model parameters, foraging activity can either persist at high levels or may advance toward one of two coexisting attractors corresponding to an intermediate or non-foraging state.…”

“…4). In terms of biological significance, these estimates likely reflect the fact that harvester ant colonies are not very sensitive to foraging interruptions once they have reached high stable levels of activity [4,21,40].…”

“…Foragers leave the nest in search of seeds after reaching a threshold number of antennal interactions with successful returning foragers in the area just inside the nest entrance, called the vestibule [17,21]. Because foragers continue their search for food until a seed item is found, forager return rates may serve as a reliable proxy for external conditions such as seed availability, humidity, etc.…”

“…Antennal contacts inside the nest are used to sense cuticular hydrocarbons which provide information about an ant's task [19,25,26]. Furthermore, when encountering the combined odor of foragers and seeds at a particular rate, an inactive forager will leave the nest and look for food [17,21]. Thus, interactions among workers in the vestibule are fundamental for regulating foraging activity in response to environmental perturbations.…”