How some ants feed without drowning

Animals have been known to develop clever tricks to ensure a full stomach. Some dolphins, for example, use empty shells to catch fish, and chimps use rocks to open nuts and hard-shelled fruit.  

Now, black imported fire ants (Solenopsis richteri) have been observed using an unlikely tool to avoid drowning when foraging for liquid food – sand grains.

Researchers from China and the US report that when the risk of drowning increased, the ants created a sand structure that acted as a syphon, drawing liquid food out of a container instead of directly foraging on the water surface.

The ants adjusted this strategy in response to different foraging risks. The researchers suggest this is the first time such sophisticated tool use has been reported in animals. 

“This exceptional tool-making skill not only reduced the drowning risk of ants, but also provided a larger space for them to collect sugar water,” says lead author Aiming Zhou, from China’s Huazhong Agricultural University. 

Black imported fire ants have hydrophobic exoskeletons that allow them to float on water with minimal risk of drowning. However, when the surface tension is reduced, that risk increases.

To test how they respond, researchers provided small containers of sugar water, on which the ants could normally float, then altered the surface tension by adding surfactant. With this increased risk of drowning, the ants started positioning sand grains on the inside of the container leading out of it. 

These structures were so efficient the ants could syphon almost half the sugar water out within five minutes. 

Ants are among the few invertebrates that use specific tools when foraging for food. However, little is known about their flexibility in using tools under environmental pressures, says co-author Jian Chen from the US Agricultural Research Service. 

“We knew some ant species can use tools, particularly in collecting liquid food; however, we were surprised by such remarkable tool use displayed by black imported fire ants,” he says. “Our findings suggest that ants and other social insects may have considerable high cognitive capabilities unique for foraging strategies.”

The researchers acknowledge that to date the experiments have only been conducted in the laboratory and limited to the black imported fire ants. “The next steps will be to determine how widespread this behaviour is in other ant species,” says Zhou. 

The findings are published in the British Ecological Society journal Functional Ecology.