Ants communicate using pheromones to coordinate nearly every aspect of their colony life, from marking foraging trails to signaling danger. These chemical messages ensure the smooth functioning of the highly organized society. When an ant dies, the event does not pass unnoticed by its nestmates, as the deceased member quickly becomes a subject of chemical scrutiny. The body of a dead ant releases a signal that prompts an automatic response from the rest of the colony.
The Chemical Signal of Death
Ants do not actively secrete a “death pheromone” at the moment of passing; instead, the chemical signal is a product of post-mortem decomposition. This signal is primarily composed of fatty acids, with oleic acid being the most researched and effective compound. Linoleic acid also contributes to this chemical signature of decay. These fatty acids are naturally contained within the ant’s body tissues and are released onto the outer shell as the cellular membranes begin to break down after death.
The presence of oleic acid on the cuticle is what other ants detect with their antennae, which essentially functions as a “dead” marker. Research has shown that a living ant painted with just a small amount of oleic acid will be treated as a corpse and carried away by its nestmates. This decomposition process takes time, meaning a newly deceased ant may not immediately be recognized. In some species, it can take hours or even a couple of days for the fatty acid concentration to reach the threshold that triggers a response from other workers.
The Purpose: Corpse Removal (Necrophoresis)
The detection of these decomposition products triggers a programmed, hygienic behavior in the colony known as necrophoresis, which literally translates to “corpse carrying.” Necrophoresis is a sanitation behavior observed in many social insects, including ants, bees, and termites. Worker ants, sometimes specialized “undertaker” workers, use their mandibles to grasp the dead body and carry it away from the living areas.
This systematic removal of dead colony members is necessary for maintaining the health of the densely populated nest. The biological advantage of necrophoresis is to prevent the spread of disease, as a decomposing body is a breeding ground for pathogens like fungi and bacteria. By taking the corpse to a designated refuse pile, often called a midden, the ants significantly reduce the risk of infection to the rest of the colony, including the vulnerable queen and developing brood.
Distinguishing Death from Alarm Pheromones
The death signal is distinct from the alarm pheromones that ants use to communicate immediate danger, although both are chemical messages. Alarm pheromones, which can include highly volatile compounds like formic acid in some species, are actively secreted by a threatened or injured ant. They are designed to be fast-acting and short-lived, dissipating quickly to prevent a long-term panic response. These signals elicit an immediate behavioral shift, causing nearby workers to become aggressive, run toward the source of the danger, or sometimes flee in panic.
In contrast, the oleic acid “death signal” is a non-volatile, passive cue that is a byproduct of decay. It is long-lasting and only prompts a cleanup response. Furthermore, some researchers have proposed that for certain species, the initial cue for necrophoresis is the absence of “life signals,” which are specific volatile hydrocarbons that disappear rapidly from the ant’s cuticle upon death. This dual system ensures the colony is both quickly alerted to a fresh corpse and provided with a clear, persistent cue for its eventual removal.