Heikki Helanterä

Unicolonial ants: where do they come from, what are they and where are they going?

Unicolonial ant populations are the most extensive cooperative units known in nature, forming networks of interconnected nests extending sometimes hundreds of kilometers. Within such a supercolony, worker altruistic behavior might be maladaptive, because it seems to aid random members of the population instead of relatives. However, recent genetic and behavioral data show that, viewed on a sufficiently large scale, unicolonial ants do have colony boundaries that define very large kin groups. It seems likely that they are family groups that continue to express their kin-selected behavior as they grow to extreme sizes. However, at extreme sizes, kin selection theory predicts that these behaviors are maladapted and evolutionarily unstable, a prediction that is supported by their twiggy phylogenetic distribution.

Colony-specific Hydrocarbons Identify Nest Mates in Two Species of Formica Ant

The possession of a colony identity is a fundamental property that underlies much animal behavior. In insect societies, it is widely accepted that nest-mate recognition cues are encoded within the cuticular hydrocarbons. Despite numerous studies over the past 30years, the identification of these nest-mate specific signatures is only just starting to occur. In this paper, we show two different methods by which nest-mate-specific signatures can be encoded within the hydrocarbon profile of two species of Formica ants. In F. exsecta, nest-mate-specific signatures rely on the distribution of chain lengths of a single type of hydrocarbon, various (Z)-9-alkenes, which are present in colony-specific proportions. In F. fusca, variation in nine different positional isomers of C25–dimethylalkanes is sufficient to produce unique colony profiles. By using this information alone, we correctly assigned 97 F. exsecta workers into their respective 20 colonies and 111 F. fusca workers into their respective 30 colonies. These two systems or variations of them may be expected to occur in many insect societies that have a strong colony identity.

Worker Reproduction in Formica Ants

A potential tragedy of the commons arises in social‐insect colonies where workers are fertile if egg‐laying workers decrease their contribution to other tasks. We studied worker ovary development and egg laying in relation to kin structure, colony size, and the presence of a queen in nine species (11 populations) of Formica ants. Workers were highly fertile and laid eggs in the presence of a queen in five out of the seven species where egg samples were obtained. Worker fertility correlated neither with colony size nor with kin structure, which suggests that colony‐level costs and efficiency of policing precede relatedness as the most important conflict determinant. We conclude that careful quantification of the costs of worker reproduction and policing is essential for inferences about the tragedy of the commons.

Worker policing and nest mate recognition in the ant Formica fusca

A conflict over male production arises in social insects where workers are able to lay unfertilized male eggs. This happens because each female (queen or worker) is most closely related to her own sons and is thus predicted to reproduce. The conflict is modulated by worker policing where workers prevent each other from reproducing by aggression or egg cannibalism. In this study, we show that in the ant Formica fusca, worker policing occurs by egg cannibalism rather than by overt aggression among workers. Furthermore, we show that, contrary to bees, wasps and other ant species, egg discrimination in F. fusca is not based only on a universal queen signature chemical and that nest mate recognition of eggs occurs.

Two independent mechanisms of egg recognition in worker Formica fusca ants

Informational constraints can be an important limitation on the accuracy of recognition. One potential constraint is the use of recognition information from the same sources in multiple discriminatory contexts. Worker wood ants, Formica fusca, discriminate eggs based on their maternal sources of origin in two main contexts: recognition of eggs laid by nestmate versus non-nestmate queens and recognition of worker-laid versus queen-laid eggs. We manipulated the experience of F. fusca workers in laboratory colonies to both worker-laid and queen-laid eggs by transferring eggs between colonies in order to investigate whether these two contexts of egg discrimination are independent. Experience of non-nestmate queen-laid eggs significantly increased worker acceptance of both familiar (18% accepted) and unfamiliar (10%) queen-laid eggs compared to control workers without experience of eggs other than those laid by their own colony’s queen (2%). In contrast, worker acceptance of worker-laid eggs was not affected by variation in the egg experience of workers (14% in workers from control colonies exposed only to eggs from their own colony’s queen versus 19% and 17% in workers from colonies which had received eggs laid by either a non-nestmate queen or nestmate workers, respectively). Our results suggest that these two recognition contexts do not strongly constrain each other and are different in their ontogeny. In particular, worker-laid eggs are universally discriminated against by workers from colonies with a queen whatever the egg experience of the workers, while non-nestmate queen-laid eggs are strongly discriminated against only by workers without experience of eggs laid by more than one queen.

Worker policing in the common wasp Vespula vulgaris is not aimed at improving colony hygiene

Abstract.In insect societies, eggs laid by workers are frequently killed by other workers – a behaviour known as “worker policing”. The traditional explanation of worker policing is that it is a mechanism to resolve intracolony conflict, and maintain the reproductive monopoly of the queen. Recently, Pirk et al. (2004) proposed that worker policing instead is aimed at removing unviable worker-laid eggs and is ultimately just another example of hygienic behaviour. Here we test this hypothesis for the common wasp Vespula vulgaris, a species with highly effective worker policing. We show that worker-laid eggs from queenless colonies have a lower hatch rate (68%) than queen-laid eggs (82%). Analysis of egg laying rates of queens and workers, however, shows that the difference is not big enough to explain the apparent absence of adult worker-derived males in this species.

Prior experience with eggs laid by non-nestmate queens induces egg acceptance errors in ant workers

We studied the effect of prior experience to eggs laid by nestmate and non-nestmate queens on the acceptance of queen-laid eggs by worker wood ants, Formica fusca. We transferred eggs from a non-nestmate queen into colonies during early spring, when their own queen was recommencing egg laying. A few weeks later, workers from these “experienced” colonies accepted eggs of both familiar (44% acceptance) and unfamiliar (40%) non-nestmate queens much more than workers from control colonies (2%) that had only had previous contact with their own queen’s eggs. Thus, prior exposure to eggs laid by a non-nestmate queen induces much greater acceptance of all non-nestmate queen-laid eggs. Mechanistically, we hypothesize that exposure to eggs from several queens may increase acceptance by causing a highly permissive acceptance threshold of non-nestmate queen-laid eggs rather than by widening the template for acceptable queen-laid eggs. These novel results show that egg-discrimination behaviour in F. fusca is flexible and that workers respond to the diversity of eggs experienced in their colony.

Do unicolonial wood ants favor kin

Vast supercolonies of interconnected nests formed by unicolonial ant species are the largest cooperative groups of animals known. Research published recently in BMC Evolutionary Biology reveals that a supercolony can be more genetically structured than previously thought, comprising several extended families. Surprisingly, the families coexist peacefully, even though they seem to recognize each other as non-kin.

Alarm Pheromones Do Not Mediate Rapid Shifts in Honey Bee Guard Acceptance Threshold

Honey bee (Apis mellifera) guards discriminate nestmates from non-nestmates at the hive entrance. The acceptance threshold of guards is known to change adaptively, for example becoming less permissive when the number of intruder bees from other colonies increases. These adaptive shifts can occur within minutes. What is unknown is the mechanism behind this rapid shift. It was hypothesized that alarm pheromones released by guards may cause the adoption of a less permissive acceptance threshold. Here, we tested this hypothesis on five discriminator hives by using a behavioral assay. We used three amounts each of iso-pentyl acetate (IPA) and 2-heptanone (2H), which are the major components of the pheromones from the sting and the mandibular glands, respectively. Biologically relevant levels of chemicals were delivered to the hive entrance platform via an air pump. We found no effect of either IPA or 2H: there was no change in guard acceptance of either nestmate (on average, 91% accepted) or non-nestmate (on average, 30% accepted) under any of the pheromone treatments compared to the pentane control (98% nestmates accepted and 32% non-nestmates accepted). Therefore, we reject the hypothesis that the presence of IPA or 2H causes a rapid shift of guard acceptance threshold.

Worker–Worker Conflict and Worker Policing

Policing is a means by which conflicts within insect societies are resolved by coercing group members into altruism. Policing is especially important in resolving the conflicts over male parentage and caste fate. The evolution of policing is explained by the inclusive fitness theory. Policing is selected for through relatedness differences within a colony, colony efficiency, and interactions with other kin conflicts. Comparative evidence supports the inclusive fitness theory predictions concerning policing. First, worker policing is more common in societies where workers are more closely related to the sons of the queen than to the sons of other workers. Second, societies in which policing is more effective have fewer selfish workers.