we previously studied generalization of odorants in the carpenter ant camponotus aethiops . by using an associative learning protocol known as the maxilla - labium extension response ( maler ) , we conditioned individual ants by training them to for example substance a , and afterwards tested their response to a mixture of a and a novel substance b. ants trained to substance a treated a mixture of a and b as similar , but surprisingly individuals trained to b did not generalize toward the mixture of a and b , indicating that generalization between stimuli is not only influenced by the similarity of molecular structure of the odorants involved , but also by the animal s previous experience . while asymmetry in generalization itself has previously been found in many organisms , especially in the olfactory modality ( e.g. ) , most of these studies either used very few substances , or substances that greatly differed in their molecular structure or function ( e.g. , floral odour vs. alarm pheromone ) , making it impossible to draw conclusions about the molecular basis of the asymmetry . in order to address this question , our experimental setup allowed us to describe how molecular structure systematically plays a role in predicting whether the ants would generalize or not . the ants consistently generalized from molecules with a long carbon chain to mixtures containing this trained substance and a molecule with a shorter chain . however , when trained to a molecule with a short chain , no generalization to a mixture containing this trained substance and a molecule with a longer chain was found . additionally , we elucidated a hierarchy of stimuli regarding functional groups , where ants trained to alcohols generalized to aldehydes and ketones of similar or shorter length . when trained to aldehydes , ants would generalize to ketones of similar or shorter length , but not to alcohols . lastly , when trained to ketones ants would not generalize to molecules with other functional groups at all . this asymmetry thus follows an inclusion criterion ( a term adapted from guerrieri et al . ) , where substances with shorter chain lengths or certain functional groups are treated as if similar to the trained substance . this raises the question about whether the inclusion criterion is a general phenomenon in the animal world , or whether it is exclusive to ants . two pioneering studies by smith and menzel and guerrieri et al . in honeybees allowed for extensive analysis of asymmetry in generalization with regards to molecular structure , as both studies used different volatile floral odours varying in either chain length or functional group . although asymmetry in generalization was found in these studies , it could not systematically be predicted by molecular structure , thus not supporting the inclusion criterion . however , in these studies the test stimulus was always a single substance instead of a mixture of the conditioned stimulus and the test stimulus . in these conditions , ants also show no evidence of the inclusion criterion . as generalization to a mixture after training to a single substance has , to my knowledge , not been studied in a systematic way in other animals , it is currently impossible to draw conclusions on how widespread the inclusion criterion is . in ants , a similar phenomenon of inclusion appears to come into play in nestmate recognition , where non - nestmates ( enemies ) are aggressed only if their cuticular chemical profile contains additional substances in comparison to the profile of the discriminating individual , but are not recognized when they have less substances . indeed , some social parasites use this to their advantage , and are known to be chemically insignificant , having only a very small amount of substances on their cuticle , which ensures that they are not recognized by the host colony . this implies that the inclusion criterion might be a general phenomenon , at least in ants . now that we established that , at least in the carpenter ant c. aethiops , asymmetry in generalisation follows an inclusion criterion , the question remains about whether it arises from the perception or action component of communication . if inclusion arises at the perception component , the receiving individual does not notice the difference between the trained odorant and the substance or mixture that it generalizes toward . however , this does not necessarily mean the odour is not detected by the sensilla , as inclusion could potentially result from processes at different levels of the perception component . intuitively , one could predict that these proteins might also bind smaller substances due to the size of their binding pocket ( while not binding larger molecules than the one they are tuned to ) . in turn , generalization toward a molecule smaller than the conditioned stimulus might be expected , as these proteins would bind to both to the trained substance , as well as smaller ones . however , evidence points in the other direction , where sensory proteins tuned to a certain substance bind , next to the molecule they are tuned to , larger molecules better than smaller ones . if larger molecules indeed activate proteins tuned to smaller molecules but not the other way around , a binary mixture containing a large and small molecule could activate the same sensory proteins as the large molecule alone , potentially explaining our results . alternatively , not the binding pocket , but the chemical attributes of the odorants might cause asymmetry in generalization . for example , there is a difference in the capability of alcohols and aldehydes / ketones to form hydrogen bonds . the oxygen in all of these classes of molecules can build a hydrogen bond with the hydrogen atoms of the sensory protein , but only alcohols can also build a hydrogen bond with the protein s oxygen through the hydrogen in the alcohol s -oh group . this might cause alcohols to bind better than aldehydes and ketones , possibly explaining the hierarchy of stimuli found in our study , where generalization occurred from alcohols to aldehydes and ketones , but not the other way around . every odour elicits a unique pattern ( much like a barcode ) in the antennal lobe by activating a specific subset of glomeruli . if higher chain length molecules elicit the same pattern as their lower chain length counterparts , but activate some additional glomeruli in the process , this could give rise to asymmetry in generalisation , as the activation of additional compounds seems to enable discrimination , whereas the deactivation of many glomeruli does not . however , smaller molecules do not necessarily activate less glomeruli , at least in the honey bee . furthermore , mixtures are not always represented in the antennal lobe as the sum of their parts due to mixture interactions . because of this , odour blends can acquire a unique quality , where sometimes not all information about the individual components is perceptually available anymore . a map of the antennal lobe , including activation patterns of many different odorants and their mixtures would be required to test this hypothesis in ants . unfortunately , due to the complex morphology of the ant antennal lobe , full mapping remains extremely challenging . another potential cause for the inclusion criterion arising at the perception component could be inhibition due to overshadowing . overshadowing occurs when an individual encounters a mixture , but only perceives a subset of that mixture . for example , if ants trained to e.g. , 2-hexanone are tested with a mixture of 2-hexanone and octanol , reaction might be inhibited if octanol overshadows 2-hexanone ( i.e. , the ant mainly perceives octanol , even though 2-hexanone is also present in the mixture ) . however , this seems to be an unlikely cause of our results , as we also conducted an overshadowing experiment by training ants to a mixture and afterwards testing their reaction to the individual components , which suggested that overshadowing is rare in c. aethiops . instead of the perception component , the inclusion criterion could arise at the action component . this would imply that the animal receives all the information needed in order to discriminate between different substances , but follows certain decision rules resulting in asymmetrical generalisation . our experiment showed that ants could perceive each tested substance , making it likely that the action component is the cause for the inclusion criterion . however , as previous experience to a single substance can adjust processes in the perception component ( through for example sensory adaptation ) , more experimental data are needed in order to tease apart the two components from each other . more species should be studied in order to find out whether our study species ( camponotus aethiops ) is unique with respect to the inclusion criterion or not . as learning paradigms are well established for many different animals , a systematic investigation , including many substances of different chain - lengths and functional groups , of the reaction of individuals to a mixture of the conditioned stimulus and a novel odorant would elucidate whether the inclusion criterion is widespread or not . if the ant is indeed the exception and one of the only groups of animals that uses the inclusion criterion , this would suggest ( but not prove ) that it arises from the action component , as the olfactory pathway ( which is part of the perception component ) is well preserved between different groups of animals . whether the inclusion criterion is adaptive or not is not clear . one potential benefit for ants could be during recognition of nestmates vs. non - nestmates . as their cuticular chemical profile consists of many different substances , the inclusion criterion could compact this profile into something simpler to interpret by treating multiple different substances as similar . this hypothesis is supported by recent evidence in the argentine ant ( linipithema humile ) . ants were exposed to a filter paper containing their own nestmate odour , to which they were not aggressive . when one of eight synthetic hydrocarbons ( differing in branch position , chain length , or both ) was added to this filter paper , the ants started to be aggressive . the authors found functional homologs , where hydrocarbons differing in chain length ( but not in the position of the functional group ) received equal levels aggression , whereas this was not the case when comparing aggression levels against hydrocarbons differing in the position of the functional group .