Carpenter ants identify intruders based on the scent of cuticular chemicals not present on nest-mates.

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For colony-based social insects, distinguishing nest-mates from non-nest-mates is important for maintaining the evolutionary fitness of the colony. Colony members must help only nest-mates and ensure non-nest-mates aren't allowed to threaten the resources of the colony. Carpenter ants were long thought to distinguish nest-mates from non-nest-mates by comparing the chemical signature of suspect ants with their own. However, the biochemical mechanism is actually much simpler than that. Carpenter ants secrete a variety of hydrocarbons on their cuticles. The hydrocarbons include many complexly branched molecules that can form an almost limitless number of unique structures. The specific hydrocarbon structures and relative abundances thereof are unique to each colony (due to shared diet) and act as a shared chemical fingerprint for nest-mates. Ants detect the hydrocarbon signature of other ants using their antennae and use it to determine if they are friends or foes. Instead of both checking for the presence of friendly-type hydrocarbons and checking for the presence of foreign-type hydrocarbons, the ants simply respond to chemical signatures that contain new elements not previously habituated to. In other words, they do not identify nest-mates; they only identify non-nest-mates. Since nest-mates consume the same specific diet and share food with each other, they develop the same cuticular hydrocarbon fingerprint.

In a simple form of learning, the antennal lobes become desensitized by near-constant exposure to nest-mate fingerprints and response to it is reduced. However, the reception of a signature that contains additional compounds not previously habituated to (e.g., that of a non-nest-mate) initiates the olfactory receptor response fully and causes aggresive behavior. This mechanism explains why insects with practically no hydrocarbon fingerprint (i.e., certain colony parasites and young workers) do not elicit a response in guard ants. To put it simply, carpenter ants don't distinguish nest-mates by comparing fingerprint similarity, they do so instead by determining whether or not a fingerprint fits within the pattern of the habituated signature. The neurological basis is perhaps best understood by human analogy. When constantly exposed to an odor, one becomes accustomed to it and may not be able to detect its presence. Nonetheless, even after being habituated to that specific odor, it is possible to notice the presence of a new one.

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"Ants are good models for studying recognition mechanisms, because they are typically very efficient in discriminating 'friends' (nest-mates) from 'foes' (non-nest-mates). Recognition in ants involves multicomponent cues encoded in cuticular hydrocarbon profiles...They do not specifically recognize nest-mates, but rather recognize and reject non-nest-mates bearing odour cues that are novel to their own colony cuticular hydrocarbon profile...Ant bodies are covered by a layer of cuticular hydrocarbons...Cuticular hydrocarbon cues are perceived by other individuals by direct antennal contact or at a short distance. The pattern of cuticular hydrocarbons can be complex and dynamic, with many compounds varying both qualitatively (different species typically have different compounds) and quantitatively (different colonies of the same species have different relative proportions of the same hydrocarbons). A typical ant cuticular hydrocarbon profile is composed of linear and methyl-branched molecules (alkanes)...Molecules differing in their structure may carry different information." (Guerrieri et al. 2009:2461).

"More configurations per chain length are possible for methyl-branched alkanes and alkenes than for linear alkanes. Also, recent studies on ants have suggested that unsaturated or branched hydrocarbons contain more information than linear ones and are more often used in nest-mate recognition...individuals bearing an undesirable cue, i.e. a cue that is present on the cuticle of undesirable individuals and not present on the cuticle of desirable nest-mates, would be rejected." (Guerrieri et al. 2009:2462).

"Our results clearly demonstrate that discriminating ants reject (attack) an individual if it displays additional chemicals on its cuticle (alien-supplemented), but not if it displays less molecules (alien-deficient) than the discriminator...linear hydrocarbons might have little role in nest-mate recognition." (Guerrieri et al. 2009:2465-6).

"In Camponotus ants, trophallaxis (exchange of liquid food between nest-mates) is common within a nest, which causes all individuals to have a similar diet and hydro-carbon composition...olfactory receptor responses to biologically relevant cuticular substances are downregulated, which could be realized by sensory adaptation (desensitization). In this case, antennal receptors of the discriminator would not respond to substances that are around all the time, e.g. the cuticular pattern of nest-mates...central plasticity plays a major role, i.e. that ants learn about their nest-mates’ odours. The most parsimonious mechanism involves habituation, a non-associative form of elemental learning. In this case, the response to the supplemented substances would be reduced by synaptic changes in the brain, most likely in the antennal lobes...This would represent an efficient and economic way of updating the reference system (template) without involving the formation of long-term memory...for nest-mate recognition, odours are not judged by the similarity of the glomerular pattern that they elicit, but rather by an inclusion criterion: any odour that elicits a subpattern of the nest-mates’ odour will be acceptable." (Guerrieri et al. 2009:2466). 

"An added substance to the cuticle of an ant will elicit aggressive behaviour while a missing one will not, indicating that a comparison is made based on inclusiveness and not on odour similarity." (Guerrieri et al.,2009:2467).

Journal article
Ants recognize foes and not friends

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A Carpenter AntCamponotus herculeanusSpecies

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