Trail pheromone modulates subjective reward evaluation in Argentine ants.

The Argentine ant, Linepithema humile, is native to South America but has become one of the most invasive species in the world. These ants heavily rely on trail pheromones for foraging, and previous studies have focused on such signals to develop a strategy for chemical control. Here, we studied the effects of pre-exposure to the trail pheromone on sugar acceptance and olfactory learning in Argentine ants. We used the synthetic trail pheromone component (Z)-9-hexadecenal, which triggers the same attraction and trail-following behavior as the natural trail pheromone. We found that pre-exposure to (Z)-9-hexadecenal increases the acceptance of sucrose solutions of different concentrations, thus changing the ants' subjective evaluation of a food reward. However, although ants learned to associate an odor with a sucrose reward, pheromone pre-exposure affected neither the learning nor the mid-term memory of the odor-reward association. Taking into account the importance of the Argentine ant as a pest and invasive organism, our results highlight the importance of pheromonal cues in resource evaluation, a fact that could be useful in control strategies implemented for this species.

Kin-informative recognition cues in ants.

Although social groups are characterized by cooperation, they are also often the scene of conflict. In non-clonal systems, the reproductive interests of group members will differ and individuals may benefit by exploiting the cooperative efforts of other group members. However, such selfish behaviour is thought to be rare in one of the classic examples of cooperation--social insect colonies--because the colony-level costs of individual selfishness select against cues that would allow workers to recognize their closest relatives. In accord with this, previous studies of wasps and ants have found little or no kin information in recognition cues. Here, we test the hypothesis that social insects do not have kin-informative recognition cues by investigating the recognition cues and relatedness of workers from four colonies of the ant Acromyrmex octospinosus. Contrary to the theoretical prediction, we show that the cuticular hydrocarbons of ant workers in all four colonies are informative enough to allow full-sisters to be distinguished from half-sisters with a high accuracy. These results contradict the hypothesis of non-heritable recognition cues and suggest that there is more potential for within-colony conflicts in genetically diverse societies than previously thought.

Dynamics and genetic structure of Argentine ant supercolonies in their native range.

Some introduced ant populations have an extraordinary social organization, called unicoloniality, whereby individuals mix freely within large supercolonies. We investigated whether this mode of social organization also exists in native populations of the Argentine ant Linepithema humile. Behavioral analyses revealed the presence of 11 supercolonies (width 1 to 515 m) over a 3-km transect. As in the introduced range, there was always strong aggression between but never within supercolonies. The genetic data were in perfect agreement with the behavioral tests, all nests being assigned to identical supercolonies with the different methods. There was strong genetic differentiation between supercolonies but no genetic differentiation among nests within supercolonies. We never found more than a single mitochondrial haplotype per supercolony, further supporting the view that supercolonies are closed breeding units. Genetic and chemical distances between supercolonies were positively correlated, but there were no other significant associations between geographic, genetic, chemical, and behavioral distances. A comparison of supercolonies sampled in 1999 and 2005 revealed a very high turnover, with about one-third of the supercolonies being replaced yearly. This dynamic is likely to involve strong competition between supercolonies and thus act as a potent selective force maintaining unicoloniality over evolutionary time.

Individual recognition in ant queens.

Personal relationships are the cornerstone of vertebrate societies, but insect societies are either too large for individual recognition, or their members were assumed to lack the necessary cognitive abilities . This paradigm has been challenged by the recent discovery that paper wasps recognize each other's unique facial color patterns . Individual recognition is advantageous when dominance hierarchies control the partitioning of work and reproduction . Here, we show that unrelated founding queens of the ant Pachycondyla villosa use chemical cues to recognize each other individually. Aggression was significantly lower in pairs of queens that had previously interacted than in pairs with similar social history but no experience with one another. Moreover, subordinates discriminated familiar and unfamiliar dominants in choice experiments in which physical contact, but not odor perception, was prevented and in tests with anaesthetized queens. The cuticular chemical profiles of queens were neither associated with dominance nor fertility and, therefore, do not represent status badges , and nestmate queens did not share a common odor. Personal recognition facilitates the maintenance of stable dominance hierarchies in these small societies. This suggests that the ability to discriminate between individual traits is selected for when it incurs net benefits for the resolution of conflict.

Worker policing by egg eating in the ponerine ant Pachycondyla inversa.

We investigated worker policing by egg eating in the ponerine ant Pachycondyla inversa, a species with morphologically distinct queens and workers. Colonies were split into one half with the queen and one half without. Workers in queenless colony fragments started laying unfertilized male eggs after three weeks. Worker-laid eggs and queen-laid eggs were introduced into five other queenright colonies with a single queen and three colonies with multiple queens, and their fate was observed for 30 min. Significantly more worker-laid eggs (range of 35-62%, mean of 46%) than queen-laid eggs (range of 5-31%, mean of 15%) were eaten by workers in single-queen colonies, and the same trend was seen in multiple-queen colonies. This seems to be the first well-documented study of ants with a distinct caste polymorphism to show that workers kill worker-laid eggs in preference to queen-laid eggs. Chemical analyses showed that the surfaces of queen-laid and worker-laid eggs have different chemical profiles as a result of different relative proportions of several hydrocarbons. Such differences might provide the information necessary for differential treatment of eggs. One particular alkane, 3,11-dimeC27, was significantly more abundant on the surfaces of queen-laid eggs. This substance is also the most abundant compound on the cuticles of egg layers.

Does she smell like a queen? Chemoreception of a cuticular hydrocarbon signal in the ant Pachycondyla inversa.

Primitive ant societies, with their relatively simple social structure, provide an opportunity to explore the evolution of chemical communication, in particular of mechanisms underlying within-colony discrimination. In the same colony, slight differences in individual odours can be the basis for discrimination between different castes, classes of age and social status. There is some evidence from correlative studies that such inter-individual variation is associated with differences in reproductive status, but direct proof that certain chemical compounds are detected and recognized by ants is still lacking. In the ponerine ant Pachycondyla inversa, fertile queens and, in orphaned colonies, dominant egg-laying workers are characterized by the predominance of a branched hydrocarbon, 3,11-dimethylheptacosane (3,11-diMeC(27)) on the cuticle. Using electroanntennography and gas chromatography with electroantennographic detection, we show that the antennae of P. inversa workers react to this key compound. 3,11-diMeC(27) is correlated with ovarian activity and, because it is detected, is likely to assume the role of a fertility signal reflecting the quality of the sender.

Reconhecimento da prole por operárias companheiras e não companheiras de ninho em Acromyrmex laticeps nigrosetosus Forel, 1908 (Hymenoptera, Formicidae)

This study investigated the behavioral response (acception or rejection) of Acromyrmex laticeps nigrosetosus to their brood and to brood from different colonies of this subespecies. The four colonies used in the bioassays came from Paraopeba, MG, Brazil. Workers accepted either brood from their colonies or from different colonies. There was no significant difference on the time for brood acceptance (transport to the interior of the nest) among nestmates and non-nestmates.

Estudou-se a capacidade de discriminação de formas jovens de Acromyrmex laticeps nigrosetosus por operárias adultas da mesma subespécie. Eram oferecidas, na área de forrageamento, larvas e pupas companheiras e não companheiras de ninho, sendo quantificado o comportamento frente a essas formas jovens. Foram utilizadas colônias oriundas do município de Paraopeba, MG, Brasil, mantidas em condições de laboratório. Os resultados evidenciaram que essa subespécie não é capaz de discriminar formas jovens companheiras e não companheiras de ninho, ou seja, transportaram indiscriminadamente as formas jovens oferecidas para o interior do ninho. Também não se observou diferença significativa para o tempo de resposta de aceitação de prole companheira e não companheira de ninho.

Reconhecimento da prole por operárias companheiras e não companheiras de ninho em Acromyrmex laticeps nigrosetosus Forel, 1908 (Hymenoptera, Formicidae)

This study investigated the behavioral response (acception or rejection) of Acromyrmex laticeps nigrosetosus to their brood and to brood from different colonies of this subespecies. The four colonies used in the bioassays came from Paraopeba, MG, Brazil. Workers accepted either brood from their colonies or from different colonies. There was no significant difference on the time for brood acceptance (transport to the interior of the nest) among nestmates and non-nestmates.

Estudou-se a capacidade de discriminação de formas jovens de Acromyrmex laticeps nigrosetosus por operárias adultas da mesma subespécie. Eram oferecidas, na área de forrageamento, larvas e pupas companheiras e não companheiras de ninho, sendo quantificado o comportamento frente a essas formas jovens. Foram utilizadas colônias oriundas do município de Paraopeba, MG, Brasil, mantidas em condições de laboratório. Os resultados evidenciaram que essa subespécie não é capaz de discriminar formas jovens companheiras e não companheiras de ninho, ou seja, transportaram indiscriminadamente as formas jovens oferecidas para o interior do ninho. Também não se observou diferença significativa para o tempo de resposta de aceitação de prole companheira e não companheira de ninho.