How Does an Ant Know Who Belongs? Smelling the Difference
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
Ants distinguish nestmates from strangers by a chemical signature carried on the body surface, and an individual carrying the wrong one is attacked. That single mechanism organises everything a colony does.
What the signature is
The outer surface of an insect is coated in a waxy layer of long-chain hydrocarbons, whose original function is preventing water loss and which turns out to carry information. The exact mixture of compounds varies between species, between colonies of the same species and to some extent between castes and individuals within a colony, and ants detect that mixture with receptors on their antennae during the brief contact that occurs whenever two ants meet. Recognising a nestmate is therefore a chemical comparison performed constantly, and it works as a gate on everything, since an individual accepted into the nest receives food, is groomed and is tolerated while one rejected is attacked immediately.
How the shared signature is maintained
The colony's odour is produced and kept uniform by several processes:
- •A genetic component, since related individuals produce similar compounds
- •Environmental contributions from diet and from nest material
- •Constant transfer between individuals through grooming and through exchanging food mouth to mouth
- •Contact with the queen and with the nest surfaces, which distributes the mixture
- •The result is a blend that no individual carries alone and that the colony holds collectively
- •The signature drifts over time, so isolated groups from one colony eventually stop recognising each other
What exploits it
A recognition system this important is a target, and a great many organisms have found ways past it. Parasitic insects including beetles, flies and other ants produce or acquire the host colony's compounds and live inside the nest unmolested, in some cases being fed and groomed by the ants they are consuming. Some acquire the odour passively by contact and others synthesise it. Caterpillars of several butterfly species are carried into ant nests after producing the right chemical cues and then eat the brood. Slave-making ants raid other colonies and the captured workers, having emerged among the raiders' odour, work for them. Each of these is an evolutionary response to a system that cannot easily be made more selective without rejecting genuine nestmates.
The other chemical signals
Recognition is one of several chemical systems running in a colony at once. Trail pheromones laid by returning foragers mark a route and evaporate at a rate that makes a trail fade unless it is reinforced, which is what allows a colony to abandon an exhausted source without any decision being taken. Alarm pheromones released when an ant is disturbed recruit others and are volatile enough to spread fast and disappear fast. Queen substances suppress the development of ovaries in workers and signal the queen's presence, so a colony learns of her death within hours by the absence of a signal. Death recognition operates on compounds released as a corpse decays, which triggers its removal. Together these coordinate a colony with no central instruction of any kind.
The colonies that stopped fighting
A striking consequence appears where the signature stops varying. Several invasive ant species have formed supercolonies in their introduced ranges, in which individuals from nests hundreds or thousands of kilometres apart recognise each other as nestmates and do not fight, which removes the territorial conflict that limits population density in their native range and allows enormous continuous populations. One such supercolony of a Mediterranean species extends along thousands of kilometres of European coastline, and individuals taken from opposite ends are mutually tolerant while attacking ants from a separate nearby supercolony. The usual explanation involves a genetic bottleneck during introduction reducing the variation the recognition system depends on, which is an unintended consequence of moving a few individuals a long way.
The takeaway
Waxy hydrocarbons on the body surface carry a mixture that varies between colonies, and ants compare it at every meeting, admitting or attacking accordingly. The blend is maintained collectively by grooming and food exchange rather than held by any individual. Parasites that acquire or produce the right compounds live inside nests unmolested, and invasive supercolonies arise where the variation has been lost.