Ants dynamically adjust liquid foraging strategies in response to biophysical constraints

Ant foragers need to provide food to the rest of the colony, which often requires food transport over long distances. Foraging for liquid is especially challenging because it is difficult to transport and share. Many social insects store liquid food inside the crop to transport it to the nest, and then regurgitate this fluid to distribute it to nestmates through a behaviour called trophallaxis. Some ants instead transport fluids with a riskier behaviour –holding a drop of liquid between the mandibles through surface tension– after which the ant shares this droplet with nestmates without ingestion or regurgitation in a behaviour called pseudotrophallaxis. Here, we hypothesized that ants optimise their liquid-collection approach depending on food quality and biophysical properties. Working with a ponerine ant that uses both trophallaxis and pseudotrophallaxis, we investigated why each liquid-collection behaviour might be favored under different conditions by measuring handling time and liquid viscosity and reaction to food quality (i.e., sugar concentration and viscosity) using a viscosity additive. We found that ants could collect more liquid food per unit time by mandibular grabbing than by drinking. At high viscosities, which in nature correspond to high sugar concentrations, ants switched their liquid collection method to mandibular grabbing in response to viscosity, and not to sweetness. In addition, mandibular grabbing of liquid food allowed ants to carry more sugar per unit time than drinking. Our results demonstrate that ants change not only their feeding preference but also their transport and sharing methods according to viscosity–a proxy for sugar concentration in nature–optimising the mass of sugar returned to the nest over time.