Sundström, L. Seppä, P. & Pamilo, P. 2005: Genetic population structure and dispersal patterns in Formica ants — a review. — Ann. Zool. Fennici 42: 163–177.

Human impact on boreal forests has been extensive during a fairly short evolutionary time scale. Character species of boreal forests, such as Formica ants, may face loss of genetic diversity, increasing inbreeding, and decreasing gene flow among extant habitat fragments owing to habitat loss and fragmentation. Here we review the genetic data on old-world boreal species of the genus Formica. In Formica ants colonies can have one or several queens (mono- and polygyny respectively) and this trait is often assumed to be linked with dispersal propensity, such that monogyne species disperse well and polygyne species disperse less well. Our analysis of the available data reveals three important aspects of the social and dispersal biology of Formica. First, the traditional division in mono- and polygyne species is too simple and we propose a population-based division into highly polygyne, weakly or moderately polygyne, and monogyne populations. Second, there is indeed an association between colony kin structure and dispersal in the predicted direction, i.e. restricted dispersal in polygyne species. However, this only holds for between-population differentiation, not within population genetic viscosity. When genetic viscosity within populations was examined most species nevertheless showed a negative relationship between F_IS and relatedness, indicating that low relatedness (many queens) is associated with reduced dispersal also locally. Only one species (F. exsecta) showed a significant positive relationship. Finally, we predict that sex-biased dispersal may be a common trait in Formica species, although data on more species are needed to confirm this.

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Pamilo, P., Zhu, D. Fortelius, W., Rosengren, R., Seppä, P. & Sundström, L. 2005: Genetic patchwork of network-building wood ant populations. — Ann. Zool. Fennici 42: 179–187.

Genetic organization of colonies and populations of the ant Formica aquilonia were studied at the edge of the urban area of the city of Helsinki within an area of about 400 km². Over six thousand old queens and workers were sampled from a total of 288 nest mounds from 14 populations (patches of forest) for an allozyme study, and workers from 13–15 nests in each of three populations were also characterized by microsatellite genotyping. Genetic relatedness among nest mates within populations was close to zero for both queens (estimates ranging from 0.02 to 0.13) and workers (from 0.01 to 0.22), with some of the estimates being significantly greater than zero. These results supported the view of a high level of polygyny within the nests. The populations showed significant genetic differences both at the allozyme loci (overall F_ST = 0.17) and at the microsatellites (F_ST = 0.24). The estimates of F_ST between pairs of populations varied from 0.01 to 0.61, the largest values being associated to reduced genetic variation and an apparent bottleneck within one population. The results showed that the local populations of this highly polygynous (multiple queens in a nest) and polydomous (multiple nests in a colony) ant can be differentiated genetically within potential dispersal distances, suggesting restricted dispersal and possible bottlenecks when colonizing new patches of forest.

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Bernasconi, C., Maeder, A., Cherix, D. & Pamilo, P. 2005: Diversity and genetic structure of the wood ant Formica lugubris in unmanaged forests. — Ann. Zool. Fennici 42: 189–199.

Wood ant species show differences in their social structure, especially in the level of polygyny (number of laying queens per nest) and polydomy (number of nest per colony), both within and between species. We demonstrate here for the first time that Formica lugubris displays two different social forms in close proximity in alpine unmanaged forests of the Swiss National Park. The genetic data (7 microsatellite loci) and field data indicate that one population is mostly monogynous to weakly polygynous (r = 0.438) and monodomous, the second one being polygynous (r = 0.113) and polydomous. Within this latter population new nests are founded by budding, leading to the observed high density of nests. These two different social structures, possibly being two expressions of a same continuum, could be explained by several ecological or environmental factors (e.g. habitat saturation, resource competition) and also historical effects.

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Maeder, A., Freitag, A. & Cherix, D. 2005: Species- and nestmate brood discrimination in the sibling wood ant species Formica paralugubris and Formica lugubris. — Ann. Zool. Fennici 42: 201–212.

Formica lugubris and F. paralugubris are sympatric sibling species of wood ants, both of which are widely distributed in Switzerland. Until 1996 they were considered the same species, F. lugubris. To investigate whether the two species can be distinguished based on discrimination cues used by the workers we used the pupa-carrying test first introduced by Rainer Rosengren. In this test workers of discriminator colonies are faced with two kinds of pupae and their preferences for one of the types are recorded based on differential retrieval. Interspecific comparisons showed that ants preferred conspecific worker pupae to those of the sibling species regardless whether the pupae were con-colonial or hetero-colonial. Hence, this test can be used as a taxonomic tool to identify wood ants hardly distinguishable by morphological characters. In intraspecific comparisons the highly polygynous (many queens per colony) F. paralugubris, the polygynous form of F. lugubris and the monogynous (single queen per nest) to weakly polygynous form of F. lugubris expressed different trends in their preference behaviour (with nestmate recognition in 14%, 20% and 31% of replicates, respectively). Only F. paralugubris presented no significant nestmate recognition.

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Fortelius, W. 2005: Mating behaviour in the polygynous/polydomous wood ant Formica aquilonia. — Ann. Zool. Fennici 42: 213–224.

The behaviour of reproductive ant females and males was examined in mating experiments in laboratory conditions. The aims were to identify mechanisms affecting sexual selection in ants in general, and to explore more specific issues applicable to the reproductive strategies present in ants like F. aquilonia characterized by a multinest/multiqueen (polydomous/polygynous) organisation of colonies. The mating process was described from video recordings. Multiple mating by both sexes was demonstrated and the observed mating frequency was in good agreement with results for the effective number of matings based on allozyme mother–offspring analyses. For females and males random mate choice was indicated. In choice experiments neither differences between the frequencies of intra- and interpopulation copulations occured, nor did the mating status of males and females affect the pattern of random mating. However, the number of preceding copulations affected the duration of different parts of the mating process in both males and females. Thus, female mating resistance tends to increase after several matings, whereas in males the duration of genital contact increases with the number of previous copulations. Also, male mating experience reduces the time used to get into genital contact, except when multiple copulations occur within a short time. The operational sex ratios in the experimental chamber did not affect the pattern of random mate choice. Spermathecal sperm counts correlated with the number of female matings and were comparable with both the estimated number of sperm for virgin males, and the spermathecal counts for old and young queens collected in the field. The findings are discussed in relation to general patterns of sexual selection in ants. Selection acting on male mating behaviour might drive both female and male multiple mating in F. aquilonia.

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Johnson, C. A., Sundström, L. & Billen, J. 2005: Development of alary muscles in single- and multiple-queen populations of the wood ant Formica truncorum. — Ann. Zool. Fennici 42: 225–234.

Formica truncorum is an ant species that maintains populations dominated by either single-queen (monogyne) or multiple-queen (polygyne) colonies. New queens (gynes) from monogyne colonies disperse after mating whereas gynes from polygyne colonies are philopatric. Gyne physical condition frequently reflects the overall dispersal propensity of the maternal colony, with dispersers heavier and with larger wings than philopatric morphs. Little, however, is known about the musculature that powers flight dispersal. Our histological examination of alary muscle tissue revealed that mitochondrion and sarcomere numbers did not differ in newly emerged gynes from monogyne and polygyne populations. Significantly greater numbers of mitochondria and sarcomeres were, however, detected in mature virgin gynes from monogyne nests. Within both populations, gynes that had shed their wings had significantly fewer mitochondria than winged gynes. Wingless mated gynes from monogyne nests also had greater numbers of mitochondria. As the sites of ATP production, mitochondria are critical to any biochemical process and reduced numbers are likely to influence the success of flight dispersal. Differential provisioning of new gynes post-emergence in the two populations may account for the variation in flight musculature, which may provide a reliable mechanism by which current dispersal conditions can be assessed. As flight capability generally represents a trade-off with ovarian growth for many insect species, gynes able to forgo flight dispersal and redirect resources from flight muscle development into oocyte development may gain an early reproductive edge compared to their dispersing counterparts.

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Jurgensen, M. F., Storer, A. J. & Risch, A. C. 2005: Red wood ants in North America. — Ann. Zool. Fennici 42: 235–242.

Red wood ants of the Formica rufagroup are present in many conifer and mixed-conifer forests of northern Europe and Asia. These six species are part of the Formica s. str. subgenus, and build large above-ground organic mounds. In contrast, the taxonomic usage of the F. rufagroup in North America seems to have a much broader meaning than in Europe and Asia. Twenty-four species and subspecies are placed in the North American F. rufagroup, but only a few build large mounds. Our survey of the literature indicates that very little is known on the abundance and distribution of North American red wood ants, under what forest conditions they are found, and what role they may have in forest ecosystems. Environmental conditions (temperature, moisture), disturbances (fire, human), predation, and competition with other ant species are all possible factors that may explain why red wood ants predominate in many Eurasian but not in North American forests. Detailed studies on the distribution and ecology of red wood ants in North America are needed, especially the interaction and possible competition from carpenter ants (Camponotusspp.) in limiting their distribution. Finally, studies on the taxonomic relationships of the North American F. rufagroup to the Eurasian Formica s. str. subgenus are needed to help understand the origin and development of red wood ants in North America.

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Kilpeläinen, J., Punttila, P., Sundström, L., Niemelä, P. & Finér, L. 2005 : Forest stand structure, site type and distribution of ant mounds in boreal forests in Finland in the 1950s. — Ann. Zool. Fennici 42: 243–258.

To clarify the association of the occurrence and density of ant mounds (Formica rufa group and other mound-building ants) with forest habitat attributes, such as stand structure and successional stage, we reanalyzed the data of the third Finnish National Forest Inventory from 1951–1953. Mound occurrence (presence vs. absence) on the sample plots was highest in medium-rich and medium-dense spruce- and birch-dominated forests. In southern Finland, mounds occurred more frequently in older than in younger forest development classes. Mound density (numbers counted on 0.1 ha circular sample plots) was best associated with latitude, with decreasing density towards the north, and averaged 0.25 mounds per 0.1 ha. In addition, density was highest on fine sandy soils. In sum, the habitat attributes related to climate, productivity, light conditions and food resource availability seemed to be key factors determining the distribution of ant mounds. For future surveys, we suggest that additional information on ant species, mound size and colony vitality should be included to explain mound distribution in more detail.

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Sorvari, J. & Hakkarainen, H. 2005: Deforestation reduces nest mound size and decreases the production of sexual offspring in the wood ant Formica aquilonia. — Ann. Zool. Fennici 42: 259–267.

Little is known about how anthropogenic changes in nature affect reproduction in social insects such as ants. We studied the effects of clear-cutting on the production of sexual offspring and on the mound size in Formica aquilonia, using neighbouring uncut forests as controls. Nest mounds were smaller in clear-cuts, apparently because they tended to be new bud nests. The basal area of nest mounds decreased towards forest edges. The production of sexual offspring increased with nest mound size. Clear-cutting reduced the production of sexual offspring also when the effect of nest mound size had been factored out. Our results show that anthropogenic changes do have the potential to modify the production of sexual offspring in social insect.

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Frouz, J., Kalcik, J. & Cudlin, P. 2005: Accumulation of phosphorus in nests of red wood ants Formica s. str. — Ann. Zool. Fennici 42: 269–275.

Here we summarize recent data on the accumulation of phosphorus in nests of wood ants as compared with that in the surrounding forest soil, and discuss mechanisms that cause this phenomenon. Moreover, we also indicate questions for future research. Various species of red wood ants (Formica s. str.) in a wide geographic area show an apparent increase of phosphorus content in their nests. Remarkably, this increase mainly entails the forms of phosphorus which are potentially available for plants or microflora, whereas the increase in total phosphorus content is less pronounced. This increase in available phosphorus is mainly attributable to release from food brought to the mound, release by decomposition of organic matter in the nest and a shift in pH of the nest material from acidic to neutral, which supports the stability of available forms of phosphorus. The effect of increased availability of phosphorus in wood-ant nests in the surrounding forest ecosystem is not quite clear and requires further attention.

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Domisch, T., Finér, L. & Jurgensen, M. F. 2005: Red wood ant mound densities in managed boreal forests. — Ann. Zool. Fennici 42: 277–282.

Clear-cutting can greatly impact red wood ant (Formica rufa group) colonies by changing microclimate, altering forage routes and eliminating aphid colonies as a food source. However, changed forestry practices in Finland, such as smaller clear-cut areas, leaving residual trees, and less intensive site preparation methods used during the last decade may result in less detrimental effects on red wood ants. Therefore, we studied how the number and size of red wood ant mounds varied between mature, 140-year-old, Norway spruce, Picea abies(L.) Karst., dominated mixed stands, and in 20-year-old and 5-year-old stands planted with Scots pine (Pinus sylvestrisL.) in eastern Finland, all originating from similar mature forest stands. No red wood ant mounds were found in the 20-year-old Scots pine stands, which seems to be due to the cutting practices and heavy site preparation used at that time. The mound density in the 5-year-old stands (2.7 mounds per ha) was not different from that in the mature stands (2.9 mounds per ha), which could be a result of lighter harvesting and site preparation practices. However, the mounds in the 5-year-old clear cut areas were smaller in volume and flatter in shape than those in the mature stands. This could be due to accelerated organic matter decomposition in the mounds after timber harvest, or large mounds may have split into smaller ones as a result of changed environmental or food supply conditions. However, five years after clear-cutting and site preparation could be too short a time period to observe the degeneration and eventual disappearance of the still existing red wood ant mounds.

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Risch, A. C., Schütz, M., Jurgensen, M. F., Domisch, T., Ohashi, M. & Finér, L. 2005: CO₂ emissions from red wood ant (Formica rufagroup) mounds: Seasonal and diurnal patterns related to air temperature. — Ann. Zool. Fennici 42: 283–290.

Red wood ant (Formica rufagroup) mounds release high amounts of carbon dioxide (CO₂). As red wood ants and other invertebrates living in mounds are poikilothermal organisms, their metabolism and therefore CO₂ emissions are affected by changes in temperature. Thus, seasonal or diurnal changes in air temperature could affect CO₂ emissions from mounds. We found that seasonal mound CO₂ emissions and air temperature were correlated, both peaking in mid-summer. In contrast, diurnal CO₂ emissions and air temperature were inversely correlated, as we observed highest C fluxes during the night when air temperature was lowest. This CO₂ emission pattern can likely be explained by higher metabolic rates of ants resulting from their clustering, and increased numbers of ants in the mound when outside air temperature drops at night. Changes in microbial decomposition of mound organic matter or thermal convection of warm CO₂-rich mound air to the colder surface at night likely do not play a major role in the diurnal C fluxes observed in our study.

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