Low levels of hybridization in two species of African driver ants

Hybridization in ants can have consequences different from those observed in most other species, with many of the potential deleterious effects being mitigated due to haplodiploidy and eusociality. In some species where colonies are either headed by multiple queens or single queens that mate with many males, hybridization is associated with genetic caste determination, where hybrids develop into workers and purebred individuals develop into queens. A previous study suggested that hybridization occurs between two Dorylus army ant species with multiply mated queens. However, the extent and exact pattern of hybridization have remained unclear, and its possible effect on caste determination has not been investigated. In this study, we aimed to determine the extent and direction of hybridization by measuring how frequently hybrids occur in colonies of both species, and to investigate the possibility of genetic caste determination. We show that hybridization is bidirectional and occurs at equal rates in both species. Hybrid workers make up only 1–2% of the population, and successful interspecific matings represent approximately 2% of all matings in both species. This shows that, although interspecific matings that give rise to worker offspring occur regularly, they are much rarer than intraspecific mating. Finally, we find no evidence of an association between hybridization and genetic caste determination in this population. This means that genetic caste determination is not a necessary outcome of hybridization in ants, even in species where queens mate with multiple males.     

Not all ants are equal: obligate acacia ants provide different levels of protection against mega‐herbivores

In obligate ant–plant mutualisms, the asymmetric engagement of a single plant species with multiple ant species provides the opportunity for partners to vary in their behaviour. Variation in behaviour has implications for the interactions with third‐party species such as herbivores. This study assessed the effect of obligate ant‐mutualists (Crematogaster mimosae, Crematogaster nigriceps and Tetraponera penzigi) inhabiting the African ant‐acacia (Acacia drepanolobium) on three mega‐herbivore browsers: the Maasai giraffe (Giraffa camelopardalis tippelskirchi), the reticulated giraffe (Giraffa c. reticulata) and the black rhino (Diceros bicornis). Giraffes are abundant and wide‐ranging herbivores of the acacias, whereas black rhinos are localized and perennial herbivores of the acacias. Multiyear field studies comparing the ants’ aggressive behaviour and browsing by mega‐herbivores suggested differences between the tending abilities of the primary ant species inhabiting A. drepanolobium. Trees occupied by the aggressive ant species C. mimosae had significantly less browsing by giraffes and black rhino than trees occupied by other ant species. The results of this study provide evidence that ant‐mutualists on African acacias can serve as deterrents to mega‐herbivores and that different ant species vary in their tending abilities.     

Cadmium regulation by Lasius niger: A contribution to understanding high metal levels in ants

Ants (Formicinae, Hymenoptera) are considered efficient accumulators of Zn and Cd. In this study the relationship between Cd concentrations in soil and in Lasius niger workers was assessed for 20 colonies located along a gradient of Cd pollution. As soil Cd concentration increased from 1 to 8 mg/kg, ant body Cd concentration increased rapidly to about 40 mg/kg, and remained stable at this level, through a progressive increase of soil Cd concentration from 8 to 21 mg/kg. The phase of rapid increase confirms the high ability of L. niger for Cd accumulation, while the stabilization of Cd body concentration indicates either increasing elimination rate or decreasing rate of uptake. Therefore it can be concluded that Lasius niger has a high ability to accumulate Cd, but simultaneously an ability for Cd regulation was detected.     

Alate susceptibility in ants

Pathogens are predicted to pose a particular threat to eusocial insects because infections can spread rapidly in colonies with high densities of closely related individuals. In ants, there are two major castes: workers and reproductives. Sterile workers receive no direct benefit from investing in immunity, but can gain indirect fitness benefits if their immunity aids the survival of their fertile siblings. Virgin reproductives (alates), on the other hand, may be able to increase their investment in reproduction, rather than in immunity, because of the protection they receive from workers. Thus, we expect colonies to have highly immune workers, but relatively more susceptible alates. We examined the survival of workers, gynes, and males of nine ant species collected in Peru and Canada when exposed to the entomopathogenic fungus Beauveria bassiana. For the seven species in which treatment with B. bassiana increased ant mortality relative to controls, we found workers were significantly less susceptible compared with both alate sexes. Female and male alates did not differ significantly in their immunocompetence. Our results suggest that, as with other nonreproductive tasks in ant colonies like foraging and nest maintenance, workers have primary responsibility for colony immunity, allowing alates to specialize on reproduction. We highlight the importance of colony‐level selection on individual immunity in ants and other eusocial organisms.     

Territorial strategies in ants

Functioning of developed (definitive) social systems in ants is impossible without formation of a complex of interrelated supporting structures and methods, regulating forager activity in the territory, in other words, without formation of the infrastructure. In dependence of the territorial strategy, different infrastructures are formed; each of them possesses specific features. Four types of infrastructure are described. The tunnel-based infrastructure is characterized by the use of tunnels (Solenopsis invicta is a typical representative). The road-based infrastructure is characterized by the use of constant roads (Formica s. str.). The nomadic infrastructure is characterized by a constant change of family habitats (army ants). The orbital infrastructure is characterized by the multivariant access of foragers to sites of the secondary subdivision of the territory (Formica exsecta). The types of infrastructure are compared according to the following parameters:cost of raising and maintaining, dynamics of elements, the degree of forager protection, level of territory exploitation, and capacity. An assumption on the influence of the type of infrastructure on ant behavior is made.     

Optimal cue integration in ants

In situations with redundant or competing sensory information, humans have been shown to perform cue integration, weighting different cues according to their certainty in a quantifiably optimal manner. Ants have been shown to merge the directional information available from their path integration (PI) and visual memory, but as yet it is not clear that they do so in a way that reflects the relative certainty of the cues. In this study, we manipulate the variance of the PI home vector by allowing ants (Cataglyphis velox) to run different distances and testing their directional choice when the PI vector direction is put in competition with visual memory. Ants show progressively stronger weighting of their PI direction as PI length increases. The weighting is quantitatively predicted by modelling the expected directional variance of home vectors of different lengths and assuming optimal cue integration. However, a subsequent experiment suggests ants may not actually compute an internal estimate of the PI certainty, but are using the PI home vector length as a proxy.     

Mandible movements in ants.

Ants use their mandibles for almost any task, including prey-catching, fighting, leaf-cutting, brood care and communication. The key to the versatility of mandible functions is the mandible closer muscle. In ants, this muscle is generally composed of distinct muscle fiber types that differ in morphology and contractile properties. Fast contracting fibers have short sarcomeres (2-3 microm) and attach directly to the closer apodeme, that conveys the muscle power to the mandible joint. Slow but forceful contracting fibers have long sarcomeres (5-6 microm) and attach to the apodeme either directly or via thin thread-like filaments. Volume proportions of the fiber types are species-specific and correlate with feeding habits. Two biomechanical models explain why species that rely on fast mandible strikes, such as predatory ants, have elongated head capsules that accommodate long muscle fibers directly attached to the apodeme at small angles, whereas species that depend on forceful movements, like leaf-cutting ants, have broader heads and many filament-attached fibers. Trap-jaw ants feature highly specialized catapult mechanisms. Their mandible closing is known as one of the fastest movements in the animal kingdom. The relatively large number of motor neurons that control the mandible closer reflects the importance of this muscle for the behavior of ants as well as other insects.     

Karyotype evolution in Australian ants

105 Australian ant species, including members of the important primitive genera Amblyopone and Myrmecia, were karyotyped using a C-banding air-drying technique. The observed haploid numbers in this survey ranged from 2n=84 (the highest known in the Hymenoptera) to 2n=9. Seven types of chromosome rearrangement were detected, namely: Robertsonian rearrangements, pericentric inversions, saltatory changes in constitutive heterochromatin, simple reciprocal translocations, complex translocations accompanied by significant loss of euchromatin, supernumerary (B-) chromosome variation, and chromosome deletion. Most ant karyotype evolution is explicable in terms of the first three of these. No evidence was found for polyploidy or centric dissociation being of evolutionary significance in ants. The C-band analysis supports a model in which pericentric inversions converting acrocentrics to other types greatly predominate over those with reverse effects. There appears to be little, if any, correlation between whether a species is morphologically primitive or advanced and its karyotype organization. The data provide little support for the ancestral chromosome number in ants having been high with subsequent reduction (fusion hypothesis), but rather suggest that the ancestral number was either very low with subsequent increase (fission hypothesis) or coincident with the present mode (modal hypothesis). Moreover, for these ant data, the modal hypothesis is interpretable as a subset of the fission hypothesis.     

Task partitioning in leafcutting ants

A task is said to be partitioned when it is split into two or more sequential stages and material is passed from one worker to another; for instance, one individual collects a material from a source and passes it to another for transportation back to the nest. In this study, we review the existence of task partitioning in leafcutting ants (Attini) and find that, across species, this form of work organisation occurs in all stages of leaf collection, leaf transportation, and leaf processing within the nest; in the deposition of refuse (leaves and spent fungal garden) to internal or external dumps; and in colony emigration. Thus, task partitioning is shown to be a very important component of work organisation in leafcutting ants. Examples mostly concern Atta but task partitioning is also known in Acromyrmex. The costs and benefits of task partitioning of the various stages are discussed and suggestions for further research are highlighted. Electronic Publication     

Polydomy in red wood ants

Polydomy, a single colony spread between multiple nests, is a widespread life history strategy in ants. The mechanisms by which a polydomous colony functions, and the fitness benefits this nesting strategy provides, are poorly understood. Here we review what is known about polydomy in the well-studied and ecologically important Formica rufa group. We focus particularly on the ecological fitness benefits polydomy may provide to members of the F. rufa group. We discuss the well-documented association in this group between polygyny (multiple queens in a colony) and polydomy, and how this relationship may favour colony reproduction by budding. We argue that although polygyny and reproduction by budding may drive a colony to spread between multiple nests, the maintenance of prolonged communication between these nests needs further explanation in terms of fitness benefits. The potential benefits of polydomy in the F. rufa group are discussed, specifically how polydomy may help a colony: exploit resources, dominate spaces, or lower the cost of stochastic nest destruction. The potential consequences of polydomy for the social organisation of a colony are explored. We also highlight gaps in current knowledge, and suggest future research directions.     

Male parentage in army ants

In most social insects workers do not mate, but have retained the ability to produce haploid eggs that can develop into viable male offspring. Under what circumstances this reproductive potential is realized and how the ensuing worker-queen conflict over male production is resolved, is an area of active research in insect sociobiology. Here we present microsatellite data for 176 males from eight colonies of the African army ant Dorylus (Anomma) molestus. Comparison with worker genotypes and inferred queen genotypes from the same colonies show that workers do not or at best very rarely reproduce in the presence of the queen. Queens of D. (A.) molestus are known to be highly multiply mated. This implies that workers are on average more closely related to queen sons than to other workers' sons, so that our results are consistent with predictions from inclusive fitness theory. It remains unknown, however, whether worker sterility is maintained by active worker policing or by self-restraint.     

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.     

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.     

Symmetry breaking in escaping ants

The phenomenon of herding is a very general feature of the collective behavior of many species in panic conditions, including humans. It has been predicted theoretically that panic-induced herding in individuals confined to a room can produce a nonsymmetrical use of two identical exit doors. Here we demonstrate the existence of that phenomenon in experiments, using ants as a model of pedestrians. We show that ants confined to a cell with two symmetrically located exits use both exits in approximately equal proportions to abandon it in normal conditions but prefer one of the exits if panic is created by adding a repellent fluid. In addition, we are able to reproduce the observed escape dynamics in detail using a modification of a previous theoretical model that includes herding associated with a panic parameter as a central ingredient. Our experimental results, combined with theoretical models, suggest that some features of the collective behavior of humans and ants can be quite similar when escaping under panic.