Dead ants appear to attract other ants, but this activity is not for feeding or congregation. The attraction is a biological mechanism that signals colony members to begin a specific cleaning behavior. This response is triggered by chemical changes in the deceased individual’s body, flagging the corpse for removal rather than drawing the colony to a food source.
The Immediate Answer: Death Cues
The recognition of a dead nestmate is initiated by detecting specific chemical signals known as “death cues.” The most widely studied cues are two unsaturated fatty acids, oleic acid and linoleic acid, which appear on the ant’s outer cuticle after death. These compounds are decomposition products, released as cell membranes break down, a process that can take one to two days to produce detectable concentrations. Applying these fatty acids to a live ant or an inert object causes living ants to treat the object as a corpse, demonstrating the power of the chemical signal.
This chemical mechanism differs significantly from the pheromones ants use for daily communication, such as trail marking or alarm signaling. While communication pheromones are actively secreted signaling molecules, oleic and linoleic acids are passive byproducts of biological decay. In some species, like the Argentine ant, the trigger involves the absence of “life chemicals” like iridomyrmecin and dolichodial. These life-associated compounds mask the underlying death cue present on both living and dead ants. Once the ant dies, these volatile life chemicals dissipate rapidly, unmasking the latent death signal and triggering the removal response.
Necrophoresis: The Colony Cleanup Process
The behavioral response to these death cues is called necrophoresis, a specialized sanitation process. When a worker ant detects the fatty acid or the lack of life cues, it initiates actions aimed at discarding the deceased member. The worker uses its antennae to confirm the chemical signature before grasping the dead ant with its mandibles. This handling and transport makes it appear the dead ant is attracting others.
The intent of the behavior is not to gather but to dispose of the body, moving it away from the nest’s living chambers. The corpse is carried away and often deposited in a designated refuse pile, sometimes called a graveyard, external to the main colony structure. This organized removal is an example of the colony’s collective defense against contamination. The efficiency of this task highlights its importance for maintaining a healthy environment within the densely populated nest.
Evolutionary Purpose of Ant Hygiene
Corpse removal provides a substantial survival advantage for the entire colony. This hygienic behavior is an integral part of the colony’s “social immunity” system. By quickly eliminating dead individuals, the ants minimize the potential for pathogens, such as infectious fungi or bacteria, to spread throughout the nest. The high density of individuals within the nest makes the colony vulnerable to disease outbreaks.
Studies have shown that colonies allowed to perform necrophoresis have a significantly higher survival rate for adult workers compared to colonies where corpse removal is restricted. This rapid disposal reduces the overall parasitic load and helps prevent a localized infection from becoming a colony-wide epidemic. The evolutionary rationale behind this behavior is prophylactic, ensuring the long-term health and viability of the entire social unit.