Wood Ant Formica Research Articles

Individual variation in immune function in the ant Formica exsecta; effects of the nest, body size and sex

It has been generally thought that sex differences in the immune system are the result of the differing life history strategies of the sexes, although the available data are not entirely consistent with the hypothesis. In this study, we studied the variation in the immune function in the mound-building wood ant Formica exsecta. F. exsecta has two forms of males, distinguished by size: the small males (micraners) and the large males (macraners), which die after the mating period, whereas females live tens of years laying their eggs. We found that in general males have a lower encapsulation response against nylon monofilament (i.e. lower immune function) than queens. Among males, the micraners had a lower encapsulation rate than the macraners. However, in queens, there was no correlation between size and encapsulation rate. The origin nest had an effect on the encapsulation rate of males: males from the large nests had a stronger encapsulation rate than males from small nests. However, in queens, nest size did not have any effect on encapsulation response. The observed variation between sexes and individuals in the encapsulation rate is discussed in the context of reproductive strategies and parasite-mediated sexual selection.

Habitat-related aggressive behaviour between neighbouring colonies of the polydomous wood ant Formica aquilonia

Ant species of the late succession stages usually have several queens and a polydomous colony structure, where several colonies occupy the same territory without competing. The wood ant Formica aquilonia is a good example of such a species in boreal forests. In this species, the lack of intraspecific competition may be caused by the stable environment and abundant food resources (e.g. excretions of tree-living aphids). We studied how habitat destruction, in the form of clear-cutting, affects aggressiveness between neighbouring colonies in F. aquilonia. Intercolonial relationships were more hostile in clear-cuts than in forest interiors. This aggression may be the result of increased intraspecific resource competition or alienation of neighbouring colonies caused by the loss of visual orientation cues. We suggest that the original polydomous relationships between colonies decline and colonies may start aggressive competition for the remaining resources.

Proximate and ultimate control of sex ratios in Myrmica brevispinosa colonies.

The literature on sex ratio evolution in ant colonies is dominated by inclusive fitness arguments. In general, genetic theory makes good predictions about sexual investment in ant populations, but understanding colony-level variance in sex investment ratios has proven more difficult. Recently, however, more studies have addressed ecological factors that influence colony-level sex investment ratios. Food availability, in particular, has been manipulated because larval nutrition influences female caste determination, thus implying that resource availability should be of critical importance for colony sex investment ratios. However, results from food supplementation experiments are equivocal, and it is clear that ant colony response to food supplementation is dependent on the ecological background of the population. We presented field colonies of the ant Myrmica brevispinosa with two food types (proteins and carbohydrates), and assessed their relative impact on colony-level sex investment ratios. We show that colonies receiving carbohydrate enhancement invested in more female sexuals and produced more female-biased sex allocation ratios than protein-fed or control colonies. Thus, our study is the first, to our knowledge, to demonstrate that sex ratios in social insect colonies might be sensitive to resource quality. Male investment was not influenced by food treatment, but was positively correlated with colony size. Therefore, the shift in sex ratios in our study must have been mediated through nutritional influences on female caste determination rather than male brood elimination. We also used our data to evaluate evidence for sex ratio compensation by queenright colonies in response to male production by workers from orphaned colonies.

Deleterious Wolbachia in the ant Formica truncorum.

Wolbachia is a maternally inherited bacterium that may manipulate the reproduction of its arthropod hosts. In insects, it is known to lead to inviable matings, cause asexual reproduction or kill male offspring, all to its own benefit, but to the detriment of its host. In social Hymenoptera, Wolbachia occurs widely, but little is known about its fitness effects. We report on a Wolbachia infection in the wood ant Formica truncorum, and evaluate whether it influences reproductive patterns. All 33 colonies of the study population were infected, suggesting that Wolbachia infection is at, or close to, fixation. Interestingly, in colonies with fewer infected workers, significantly more sexuals are produced, indicating that Wolbachia has deleterious effects in this species. In addition, adult workers are shown to have significantly lower infection rates (45%) than worker pupae (87%) or virgin queens (94%), suggesting that workers lose their infection over life. Clearance of Wolbachia infection has, to our knowledge, never been shown in any other natural system, but we argue that it may, in this case, represent an adaptive strategy to reduce colony load. The cause of fixation requires further study, but our data strongly suggest that Wolbachia has no influence on the sex ratio in this species.

Foraging efficiency in the wood ant, Formica rufa: is time of the essence in trail following?

Central place foraging in ants is expected to maximize colony efficiency by minimizing the lengths of established trails. Short trails reduce both the time and energy costs of delivering food back to the nest, but which is most important? To examine trail route choice in the wood ant Formica rufa, we used a series of bridges, each with two alternative routes allowing the comparison of combinations of horizontal, vertical or time-expensive detours of differing lengths or delays. The ants preferred the shortest, quickest routes in all cases, although when vertical and horizontal deviations were equal in length they preferred the former, probably because this represents a reduced risk in terms of navigation errors. When comparing a range of bridge combinations, we were able to separate the relative differences in time versus energy saving represented within a series of successive choices. Contrary to earlier evidence, savings in energy explained significantly more of the variation in ant trail preferences than savings in time. We suggest that our results can be explained by wood ants adaptively adjusting their speed when walking in the vertical plane in response to the additional potential energy costs encountered.

Aphid‐mediated coexistence of ladybirds (Coleoptera: Coccinellidae) and the wood ant Formica rufa: seasonal effects, interspecific variability and the evolution of a coccinellid myrmecophile.

It is generally believed that most homopteran‐eating insects avoid ant‐tended colonies of Homoptera, due to the ant aggression they encounter there. However, because homopteran colonies which are ant‐tended often persist for longer than untended colonies, some homopteran‐eaters may utilise ant‐tended Homoptera when untended colonies are scarce. Furthermore, a few homopteran‐eaters are myrmecophilous, habitually coexisting with ants. To investigate these phenomena, a study was made of aphids and aphidophagous coccinellids (ladybirds) on Scots pine, Pinus sylvestris, growing in areas foraged and unforaged by the wood ant Formica rufa. The non‐tended aphid Schizolachnus pineti exhibited a marked population decline in late summer but persisted in both areas at very low density. Facultatively tended Cinara aphids exhibited higher population densities when associated with F. rufa, and remaining colonies of these aphids were only found associated with ants in late summer. Coccinellids exhibited considerable interspecific variability in their level of association with F. rufa, and there was some evidence of an increase in certain species’ frequencies of occurrence with the ant when Cinara aphids were all ant‐tended, in late summer. Coexistence with ants appears to be associated with either an intolerance of low aphid densities, in Coccinella septempunctata and Harmonia quadripunctata, or with extreme dietary specialisation, in Myzia oblongoguttata. Similar factors to those which bring C. septempunctata into contact with ants were probably of importance in the initial stages of the evolution of myrmecophily of its congener, Coccinella magnifica.

The size–distance relationship in the wood ant Formica rufa

Summary1. The size–distance relationship among honeydew‐collecting foragers of the red wood ant Formica rufa was investigated. Within the colony territory, the size (as measured by head width) and fresh weight of samples of foragers were determined for ants ascending and descending trees near, and farther from, the central nest mound.2. The mean size of the ants was significantly higher at far trees than at near trees in six out of the seven colonies investigated, confirming the general presence of the size–distance relationship.3. In three colonies, a load–distance relationship was also found. For a given head width, honeydew‐carrying ants descending far trees were significantly heavier than those descending near trees (i.e. they were carrying heavier loads from trees farther away from the central nest mound).4. This is the first time that both load–distance and size–distance relationships have been reported in foraging workers from the same ant colony.5. The combined effects of these characteristics suggest that colony foraging efficiency is enhanced by far trees being visited by the larger workers that then return with heavier loads of honeydew.

Costs and benefits of ant attendance to the drepanosiphid aphid Tuberculatus quercicola

The defensive effects of ants against aphid predators have been well documented in the mutualistic relationship of aphids and their attending ants. However, it is not clear whether ant attendance has any direct effect on the aphids’ growth and reproduction. Through field experiments, this study evaluates the benefits and, in particular, the costs of ant attendance to aphid colonies, focusing on the drepanosiphid aphid Tuberculatus quercicola which is associated with the Daimyo oak, Quercus dentata, and which is always attended by the red wood ant Formica yessensis. Ant attendance was clearly beneficial to the aphid; the exclusion of ants led to a significant increase in the extinction rate of aphid colonies. However, MANOVA and randomized block ANOVA indicated that in colonies continuously attended by ants, aphids had significantly smaller body size and produced a smaller number of embryos than in colonies isolated from ants when they were reared under homogeneous host conditions free from natural enemies. Thus, ant attendance had a negative influence on the growth and reproduction of the aphids, even though it contributed to the greater longevity of the aphid colonies. We hypothesize that ant‐attended aphids are under intense selective pressures that act against aphid clones which fail to attract many ants, so that aphids have developed an adaptive mechanism to allocate a larger fraction of resources to the honeydew when they are requested to do so by the ants in order to ensure the ants’ consistent visitation.

Extended family structure in the ant Formica paralugubris : the role of the breeding system

In populations of various ant species, many queens reproduce in the same nest (polygyny), and colony boundaries appear to be absent with individuals able to move freely between nests (unicoloniality). Such societies depart strongly from a simple family structure and pose a potential challenge to kin selection theory, because high queen number coupled with unrestricted gene flow among nests should result in levels of relatedness among nestmates close to zero. This study investigated the breeding system and genetic structure of a highly polygynous and largely unicolonial population of the wood ant Formica paralugubris. A microsatellite analysis revealed that nestmate workers, reproductive queens and reproductive males (the queens' mates) are all equally related to each other, with relatedness estimates centring around 0.14. This suggests that most of the queens and males reproducing in the study population had mated within or close to their natal nest, and that the queens did not disperse far after mating. We developed a theoretical model to investigate how the breeding system affects the relatedness structure of polygynous colonies. By combining the model and our empirical data, it was estimated that about 99.8% of the reproducing queens and males originated from within the nest, or from a nearby nest. This high rate of local mating and the rarity of long-distance dispersal maintain significant relatedness among nestmates, and contrast with the common view that unicoloniality is coupled with unrestricted gene flow among nests.

Mating frequency of ant queens with alternative dispersal strategies, as revealed by microsatellite analysis of sperm

In social Hymenoptera (ants, bees, and wasps), the number of males that mate with the same queen affects social and genetic organization of the colony. However, the selective forces leading to single mating in certain conditions and multiple mating in others remain enigmatic. In this study, I investigated whether queens of the wood ant Formica paralugubris adopting different dispersal strategies varied in their mating frequency (the number of males with whom they mated). The frequency of multiple mating was determined by using microsatellite markers to genotype the sperm stored in the spermatheca of queens, and the validity of this method was confirmed by analysing mother–offspring combinations obtained from experimental single‐queen colonies. Dispersing queens, which may found new colonies, did not mate with more males than queens that stayed within polygynous colonies, where the presence of numerous reproductive individuals ensured a high level of genetic diversity. Hence, this study provides no support to the hypotheses that multiple mating is beneficial because it increases genetic variability within colonies. Most of the F. paralugubris queens mated with a single male, whatever their dispersal strategy and life history. Moreover, multiple mating had little effect on colony genetic structure: the effective mating frequency was 1.11 when calculated from within‐brood relatedness, and 1.13 when calculated from the number of mates detected in the sperm. Hence, occasional multiple mating by F. paralugubris queens may have no adaptive significance.

Seasonal Metal Accumulation Patterns in the Red Wood ant Formica pratensis (Hymenoptera) at Contaminated and Reference Sites

Elucidating details of the ecophysiology of metals in ants and assessing their potential application in biomonitoring programmes of environmental metal contamination requires knowledge of the general variability of metal accumulation in these organisms at different times and sites. Temporal fluctuations of metal concentrations were measured in worker ants of Formica pratensis, collected at sites with different contamination histories. The metal concentrations in the ants were found to differ significantly between the measured months (maximally 6-fold for lead) as well as between the sites (maximally 350-fold for lead). At all sites, a common seasonal pattern was found. The pattern revealed lowest metal concentrations in spring and autumn, resulting in an inverse U-shaped distribution. Significant weight changes in similarly sized workers occurred over the year, following a U-shaped distribution. with the minima in summer. In some cases, weight was negatively correlated with the metal concentrations, indicating that seasonal weight changes contribute to the temporal fluctuations of metal concentrations. The weight increase of workers towards winter may be associated with the storage of nutrient reserves in their fat body for the production of sexuals in early spring and also contributes to the temporal metal accumulation pattern. Other cases showed weight-independence and indicated a regulative capacity, e.g. for copper. Monthly maximum/minimum ratios were higher for the non-essential metals lead and cadmium, than for the essential iron, copper, zinc and manganese. While fluctuations of zinc increased with the level of site contamination, the opposite was true for lead and cadmium, demonstrating the impact of contamination history on temporal accumulation patterns. When individuals of mound-building ant species are included in biomonitoring studies, it should be borne in mind that not only ant activity and nutrition, but also their weight follows seasonal patterns and contributes to the metal dynamics of the colony. If between-site comparisons are conducted, identical collecting intervals are recommended.

Conditional Manipulation of Sex Ratios by Ant Workers: A Test of Kin Selection Theory

Variable queen mating frequencies provide a unique opportunity to study the resolution of worker-queen conflict over sex ratio in social Hymenoptera, because the conflict is maximal in colonies headed by a singly mated queen and is weak or nonexistent in colonies headed by a multiply mated queen. In the wood ant Formica exsecta, workers in colonies with a singly mated queen, but not those in colonies with a multiply mated queen, altered the sex ratio of queen-laid eggs by eliminating males to preferentially raise queens. By this conditional response to queen mating frequency, workers enhance their inclusive fitness.

Effects of ants on the foraging of birds in spruce trees

I experimentally excluded ants from randomly selected spruce trees Picea abies near colonies of the wood ant Formica aquilonia. Foraging activity of birds in these trees was then compared to the foraging activity of birds in neighboring spruce trees, where ants were allowed to continue foraging. Birds which foraged in the foliage showed the effects of competition with ants: they visited the trees without ants more frequently, and for longer periods. In addition, the insects and spiders that they utilized as food were more abundant in the foliage of trees without ants. Cone-foraging birds, however, which fed on seeds in cones at the tops of the trees, did not show a preference for trees without ants. The differences of tree usage between foliage-gleaning and coneforaging birds can be explained by alteration of the birds' food supply by wood ants: ants did not feed on seeds in cones, and so did not compete with cone-foraging birds. However, foraging wood ants did feed on arthropods living in the foliage, thus reducing the amount of food available to birds there.

Genetic population structure and sociogenetic organisation in Formica truncorum (Hymenoptera; Formicidae)

The genetic population structure and the sociogenetic organization of the red wood ant Formica truncorum were compared in two populations with monogynous colonies and two populations with polygynous colonies. The genetic population structure was analysed by measuring allele frequency differences among local subsets of the main study populations. The analysis of sociogenetic organisation included estimates of nestmate queen and nestmate worker relatedness, effective number of queens, effective number of matings per queen, relatedness among male mates of nestmate queens and relatedness between queens and their male mates. The monogynous populations showed no differentiation between subpopulations, whereas there were significant allele frequency differences among the subpopulations in the polygynous population. Workers, queens and males showed the same genetical population structure. The relatedness among nestmate workers and among nestmate queens was identical in the polygynous societies. In three of the four populations there was a significant heterozygote excess among queens. The queens were related to their male mates in the polygynous population analysed, but not in the monogynous ones. The data suggest limited dispersal and partial intranidal mating in the populations with polygynous colonies and outbreeding in the populations having monogynous colonies. Polyandry was common in both population types; about 50% of the females had mated at least twice. The males contributed unequally to the progeny, one male fathering on average 75% of the offspring with double mating and 45–80% with three or more matings.

Polyandry and allele frequency differences between the sexes in the ant Formica aquilonia

Genetic mother-offspring analyses based on six enzyme gene loci show that about 60 per cent of the females of the mound-building red wood ant Formica aquilonia mate with several males. The number of matings inferred from the offspring genotypes range from one to six, the arithmetic mean being 1.94. The mates do not contribute equally in the inseminations; in the case of two matings, one male is estimated to inseminate on average 77 per cent of the offspring. The average related-ness among the offspring of a single female is 0.6, corresponding to the effective number of matings of 1.43. Three of the six loci show remarkable allele frequency differences between the sexes. At each of these loci the males virtually lack the alleles present in females with frequencies 0.18 at Me, 0.25 at Pgk and 0.27 at Gpi. Segregation analyses indicate normal Mendelian inheritance at these loci and the difference between the sexes seem likely to result from selection.

Competition between Ants and Birds in a Swedish Forest

I studied the effects of ants on bird foraging in trees near colonies of the wood ant Formica aquilonia. Ants were experimentally excluded from half the trees I watched by applying a chemical repellent to their lower trunks. Birds which foraged for insects on the bark or foliage of trees (tits, warblers, woodpeckers and treecreepers) visited the trees without ants more frequently, and for longer time periods, than the trees with ants. Pied flycatchers Ficedula hypoleuca, which used the trees solely as perches for sallying away after flying insects, visited the trees with ants more frequently. Food resources were found to be lower in the trees with ants, indicating that ants partially excluded birds by means of exploitative competition.

Interference by wood ant influences size selection and retrieval rate of prey by Formica fusca

Dominance relations between the territorial wood ant Formica polyctena and the submissive F. fusca generate testable predictions on the mechanisms of their coexistence. Here I tested the influence of interference competition by the dominant F. polyctena on the foraging of F. fusca. In the presence of F. polyctena, the activity and the retrieval rate of items of F. fusca decreased significantly. When F. fusca were given a choice between small chironomids and flies a hundred times heavier they selected nearly always flies in the absence of wood ants; when disturbed F. fusca took proportionately more chironomids. In nature, irrespective of distance from the wood-ant mound the size distribution of potentially available food items was the same. F. fusca collected smaller items close to the wood-ant mound, where the density of interfering wood ants was high, than far from it. Also, F. polyctena carried larger protein items from the outskirts of its territory than from the center. The items of F. fusca were on average smaller than those of F. polyctena although the item-size overlap was substantial. When encountering a F. polyctena, those F. fusca workers carrying a fly always lost their booty to the dominant but always managed to bring the chironomids to the nest. F. fusca and F. polyctena were equally efficient in detecting single chironomids placed on the surface of ground, but the presence of either species decreased the discovery rate of the other. This implies mutual exploitation competition between the species for the locally most abundant protein resource. F. polyctena found single flies faster than F. fusca. In an earlier study I showed that close to the wood-ant mound the colony size and production of sexual offspring of F. fusca were reduced. I suggest that the suppressed colony success of F. fusca is attributable to the diminished size and decreased retrieval rate of prey items close to the woodant mound abounding with interfering wood-ant foragers.

Colony success of the submissive ant Formica fusca within territories of the dominant Formica polyctena

Abstract. 1. Twenty‐three nests of the submissive ant Formica fusca L. were sampled in two adjacent territories of the dominant wood ant Formica polyctena Först. The nests were dug up at different distances from the wood‐ant mounds. Distance is assumed to be inversely related to the extent of disturbance of F. fusca by F. polyctena.2. Colony success of F. fusca was assayed by counting the numbers of workers (colony size), and worker and sexual offspring in a nest. For individual size, the head width and dry weight of fifty workers per nest were measured.3. Colony size correlated positively with the numbers of worker and sexual offspring in one territory, but only with worker offspring in the other.4. Distance from the wood‐ant mound correlated positively with colony size and numbers of worker and female offspring in one territory. In the other territory distance correlated positively with number of females, but negatively with number of worker offspring. No significant correlations between distance and the size measurements were obtained.5. Because of its nonaggressive behaviour F. fusca may nest fairly close to a wood‐ant mound but is likely to show reduced abundances in terms of nest density and forager number, and, ultimately, lowered fitness.

How do red wood ants orient during diurnal and nocturnal foraging in a three dimensional system? II. Field experiments

In order to investigate the orientational mechanisms involved in the ants' three dimensional movements workers of the red wood antFormica nigricans were enticed to forrage, both during the day and in night, in an artifical tree set up in their natural environment. Celestial cues were found to be irrelevant in the ants' directionnality. In contrast, the results stress the importance of terrestrial and chemical landmarks both in the orientation on the vertical axis and on the horizontal plane. When a discrepancy was experimentally induced in the directional information provided by the two types of landmarks, terrestrial cues were preferred over the chemical ones. In addition, kinaesthetic information is used as a back-up orientating system at the cross-road. On the other hand, nocturnal orientation was found to be essentially based on chemical cues. These results support therefore the assumption of a simultaneous utilization of all the available directional information apart from alternative use of single cues during competitive situations and rule out the concept of an absolute hierarchie of cues.

The dynamic nature of the activities of the wood ant Formica aquilonia foraging to static food resources within a laboratory habitat

ABSTRACT. A laboratory colony of the wood ant Formica aquilonia (Yarrow) was maintained under strictly controlled feeding regimens. The number of foragers at a particular site is related to the previous provisioning of the colony and the familiarity and accessibility of the resource to foragers. Under a standard regimen a predictable number of ants forage to a known site and display a typical pattern of daily activity. This pattern is examined under stable and changing circumstances. Following food‐deprivation individuals increase their foray‐frequency and carry larger volumes of sugar solution per foray. On days of critical food‐deficit supernumerary workers are recruited. How foragers are directed to a novel food source in unfamiliar territory is examined. Following‐behaviour is not involved, neither it appears are pheromones.