Ants are remarkably successful insects, forming dense colonies that can house millions of individuals. These underground nests are often warm, humid, and crowded, creating an environment that should be a perfect incubator for infectious pathogens. Despite living in a microbial hot zone, ants rarely suffer from devastating epidemics that wipe out their populations. Their defense against disease is not a single mechanism, but a sophisticated, multi-layered system operating at the individual biological level and the collective social level. This investment in disease prevention is the secret to their survival.
The Ant’s Internal Immune System
An ant’s first biological defense is its tough, external exoskeleton, or cuticle, which functions as a formidable physical barrier against most microbes. This hard chitinous layer prevents fungal spores and bacteria from reaching the soft tissues inside. Should a pathogen successfully breach this outer defense, the ant relies on a robust innate immune system common to all insects.
The insect equivalent of blood, called hemolymph, contains specialized cells known as hemocytes that patrol the body cavity. These hemocytes are responsible for cellular defense responses, which include two primary actions against invaders. Through phagocytosis, hemocytes directly engulf and destroy smaller foreign particles, such as bacteria.
Against larger invaders, particularly the spores of parasitic fungi, the hemocytes initiate encapsulation. Multiple hemocytes aggregate around the foreign body, completely encasing it and walling it off from the rest of the ant’s system. This capsule is then often melanized, chemically destroying the trapped pathogen.
The humoral response is another powerful internal defense, involving the production of potent Antimicrobial Peptides (AMPs). These peptides are synthesized primarily by the fat body and secreted directly into the hemolymph when an infection is detected. AMPs function as broad-spectrum antibiotics, actively targeting and disrupting the cell membranes of bacteria and fungi.
Individual Hygienic Behaviors
Beyond their internal biology, individual ants are proactive in maintaining a sterile surface, engaging in self-care and disinfection routines. Self-grooming is a frequently observed behavior, where an ant meticulously cleans its antennae, legs, and body with its mouthparts. This mechanical action is highly effective at physically removing fungal spores and other contaminants adhered to the cuticle.
A specialized structure called the metapleural gland contributes significantly to individual hygiene. These glands, located on the sides of the thorax, secrete a cocktail of antibiotic compounds, often organic acids like formic acid in some species. The ant selectively grooms the opening of this gland with its forelegs, transferring the secretion onto its body to create a personal layer of chemical disinfectant.
Ants increase their grooming of the metapleural gland opening immediately following exposure to fungal conidia, demonstrating a targeted, threat-specific response. This application of natural antibiotics acts as a protective shield, preventing pathogens from germinating on the ant’s surface. Furthermore, ants use their mouthparts to collect and compress contaminants into a small, sterile packet called an infrabuccal pellet, which is then safely discarded outside the nest, preventing internal contamination.
Social Immunity: Colony-Level Defense
The collective, colony-level defense strategy is known as social immunity. This system treats the entire colony as a single organism, where workers perform specialized, hygienic tasks for the benefit of the group.
Necrophoresis
One urgent task is necrophoresis, the prompt removal of dead or sick nestmates. If an ant dies or exhibits chemical cues indicating infection, workers swiftly carry the contaminated individual far away from the nest entrance or to a specialized garbage pile. This action prevents the pathogen from spreading its infectious spores within the close quarters of the brood chambers and food stores. Workers specializing in waste management are often older or less genetically valuable, concentrating the risk onto an expendable group.
Allogrooming
Allogrooming, or communal cleaning, is a widespread behavior where ants clean one another. This is an especially important process for high-value individuals like the Queen and developing larvae. Cleaning removes fungal spores from nestmates and aids in the distribution of the metapleural gland secretions across the colony population. The sharing of these antimicrobial chemicals helps to create a collective chemical barrier for the entire nest.
Chemical Disinfection and Isolation
Chemical disinfection is also applied directly to the nest structure and to infected brood. Many ant species, particularly those in the subfamily Formicinae, use formic acid, a potent antiseptic, as a venom. Workers spray this venom onto contaminated nest materials and sometimes onto infected pupae in a process called “destructive disinfection.” When an ant becomes infected, it may behaviorally self-isolate, moving to the periphery of the nest or leaving entirely, a sacrifice that protects its close nestmates.