The use of mark-recapture to measure worker number in the rock nesting ant species, Formica neorufibarbis Emery

Summary: I examined the use of a mark-recapture technique to measure colony size and colony growth in the ant species Formica neorufibarbis. I addressed three questions: 1) Is the method reasonably accurate?, 2) is the method precise?, and 3) how many workers does the method kill? I found that estimates of colony sizes based on mark-recapture were similar to those estimated by colony excavation. The error in estimates of worker and cocoon number due to the binomial nature of the mark-recapture method was relatively small, with a mean coefficient of variation of twelve percent for workers and nine percent for cocoons. I estimated that the method killed less than two percent of the workers in a nest.     

Testing the importance of the distribution of worker sizes to colony performance in the ant species <Emphasis Type=Italic>Formica obscuripes</Emphasis> Forel

We examined the relationship between colony performance and the distribution of worker sizes within colonies in the ant species, Formica obscuripes. We manipulated the distribution of worker sizes within colonies and found that experimental colonies whose distributions mimicked the natural distributions retained a larger percentage of colony biomass over three weeks when fed on honeydew, relative to colonies composed of only large or only small workers. In natural colonies most of the variation in worker sizes was found within, as opposed, to between colonies, suggesting that homeostatic mechanisms within colonies regulated the distribution of worker sizes. While there were no disjunctions in the distribution of worker sizes within colonies, the distribution tended to be bimodal. This study demonstrates that the distribution of worker sizes within colonies is important even for ant species that lack discrete worker castes. Received 31 October 2006; revised 26 December 2006; accepted 4 January 2007.     

Between a rock and a hard place: impact of nest selection behavior on the altitudinal range of an alpine ant, Formica neorufibarbis

Formica neorufibarbis Emery is a dominant alpine ant in North America. Above timberline, colonies nest under rocks, placing brood against the rock surface to warm during the day. We tested the hypothesis that nest rock preference represents a mechanism of behavioral thermoregulation and used a demographic model to explore its impact on the species' altitudinal range. Under sunny conditions rocks provide warm microclimates. Across a 400-m altitudinal gradient from below timberline to high alpine tundra, nest rocks warmed on average 5°C above ambient at mid-day, but cooled to near ambient before dawn. Soil was warmer at mid-day under rocks occupied by ants than under randomly chosen unoccupied rocks in the same area. Occupied nest rocks were painted white or black to manipulate temperature and test its impact on colony occupancy. In addition, black-painted and white-painted rocks of similar size were placed in the field to test for a temperature effect on nest site colonization. Likelihood of nest persistence and colonization decreased under cooler, white-painted rocks and depended strongly on rock warming capacity. Experimental results showed that nest site colonization and occupancy are favored in microsites maintaining mid-day temperature ≥22°C. Across the altitudinal gradient, 66-90% of nest microsites occupied by F. neorufibarbis met this temperature threshold. Conversely, none of the exposed microsites and only 15% of those microsites under randomly selected rocks warmed to 22°C at the high altitude site (3,900 m). Results support the conclusion that nest site preference contributes to the broad altitudinal distribution of F. neorufibarbis.     

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.     

Worker reproduction in the ant Formica fusca

We studied the kin conflict over male parentage in the ant Formica fusca. The conflict arises because each worker and queen is most related to her own sons and is thus predicted to lay eggs. Microsatellite analysis of eggs revealed that workers laid eggs in more than half the queenright experimental nests. Nevertheless, almost exclusively diploid offspring were reared in the presence of a queen. This also occurred when worker-laid haploid male eggs were experimentally introduced in to the nests. Because our experimental setup allowed us to exclude the possibility of queen policing, we conclude that worker laid eggs are removed by other workers, either as a response to their parentage or gender. Our results suggest that worker reproduction in F. fusca is ultimately an interplay of conflicts over male parentage and sex allocation and that both worker and self policing have roles as proximate mechanisms of resolution.     

Restricted gene flow between two social forms in the ant Formica truncorum

We studied genetic differentiation between two social forms (M-type: single queen, independent nest founding; P-type: multiple queens, dependent nest founding, building of colonial networks) of the ant Formica truncorum in a locality where the social types characterize two sympatric populations. The genetic results indicate restricted gene flow between the social forms. Female gene flow between the forms appears to be absent as they did not share mitochondrial haplotypes. Significant nuclear differentiation and the distribution of private alleles suggest that male gene flow between the forms is weak. However, the assignment analysis indicates some gene flow with P males mating with M females. The results have potentially important implications concerning social evolution within the forms but they need to be confirmed in other localities before they can be generalized. The colonies in the M-type population have earlier been shown to produce split sex ratios, depending on the mating frequency of the queens. The inferred gene flow from the P to the M type means that the split sex ratio is partly suboptimal, possibly because the P populations are not long-lived enough to influence the behavioural decisions in the M colonies.     

Nestmate recognition in the unicolonial ant Formica paralugubris

In unicolonial populations of ants, individuals can mix freelywithin large networks of nests that contain many queens. Ithas been proposed that the absence of aggression in unicolonialpopulations stems from a loss of nest mate recognition, butfew studies have tested this hypothesis. We investigated patternsof aggression and nest mate recognition in the unicolonial woodant, Formica paralugubris. Little aggression occurred, evenbetween workers from nests separated by up to 5 km. However,when aggression took place, it was directed toward non–nestmates rather than nest mates. Trophallaxis (exchange of liquidfood) occurred very frequently, and surprisingly, workers performedsignificantly more trophallaxis with non–nest mates thanwith nest mates (bias 2.4:1). Hence, workers are able to discriminatenest mates from non–nest mates. Higher rates of trophallaxisbetween non–nest mates may serve to homogenize the colonyodor or may be an appeasement mechanism. Trophallaxis rate andaggression level were not correlated with geographical distanceand did not differ within and between two populations separatedby several kilometers. Hence, these populations do not representdifferentiated supercolonies with clear-cut behavioral boundaries.Overall, the data demonstrate that unicoloniality can evolvedespite well-developed nest mate recognition. Reduced levelsof aggression might have been favored by the low rate of interactionswith foreign workers, high cost of erroneously rejecting nestmates, and low cost of accepting foreign workers.     

The moderating effects of production feedback and standards on the relationship between worker job attitudes and productivity

An experimental study was performed to investigate the impact of production feedback and standards, singly as well as jointly, as moderators of the worker job attitudes-productivity relationship in a repetitive production task. There was no substantial or consistent correlation between worker job attitudes and productivity and the correlation coefficient values in general were not statistically significant. The provision of production feedback and standards affected worker job attitudes and productivity differently and this in turn had an effect on the job attitudes-productivity relationship.     

Covariation between colony social structure and immune defences of workers in the ant Formica selysi

Several ant species vary in the number of queens per colony, yet the causes and consequences of this variation remain poorly understood. In previous experiments, we found that Formica selysi workers originating from multiple-queen (=polygyne) colonies had a lower resistance to a fungal pathogen than workers originating from single-queen (=monogyne) colonies. In contrast, group diversity improved disease resistance in experimental colonies. This discrepancy between field and experimental colonies suggested that variation in social structure in the field had antagonistic effects on worker resistance, possibly through a down-regulation of the immune system balancing the positive effect of genetic diversity. Here, we examined if workers originating from field colonies with alternative social structure differed in three major components of their immune system. We found that workers from polygyne colonies had a lower bacterial growth inhibitory activity than workers from monogyne colonies. In contrast, workers from the two types of colonies did not differ significantly in bacterial cell wall lytic activity and prophenoloxidase activity. Overall, the presence of multiple queens in a colony correlated with a slight reduction in one inducible component of the immune system of individual workers. This reduced level of immune defence might explain the lower resistance of workers originating from polygyne colonies despite the positive effect of genetic diversity. More generally, these results indicate that social changes at the group level can modulate individual immune defences.     

The relationship between soil variables and leafcutter ant (Atta sexdens) nest distribution in the Colombian Amazon

We determined the relationships between several soil variables and the distribution of leafcutter ant (Atta sexdens) nests on the Colombian shore of the Amazon River. Seven habitats were identified in which nests were consistently present or absent. Soil samples for physiochemical analysis were taken near nest sites in three habitats where nests were present and from randomly selected sites in four other habitats where nests were absent. Percent canopy cover and soil resistance were also measured for each site. Almost all of the 20 identified soil variables were significantly different between habitats with and without A. sexdens nests, and the values of all variables were heterogeneous between the seven habitats. The most important variables correlated with leafcutter ant nest presence were percent canopy cover (positively), percent silt, soil resistance at 0–20 cm, and pH (all negatively). We deduced a binomial and a multinomial logistic regression which showed how each of these variables was related to nest presence and habitat, respectively. We describe how each variable may affect leafcutter ant nest development, especially in its earliest stages, and therefore influences nest distribution. We propose a suitable-soil hypothesis which, complementary to the palatable forage hypothesis, aims to explain patterns in leafcutter ant nest demography based on soil conditions.     

Reproductive sharing among queens in the ant Formica fusca

Reproductive sharing among cobreeders, in which reproductiveshares may vary from equal contribution (low reproductive skew)to reproductive dominance by one individual (high reproductiveskew), is a fundamental feature of animal societies. Recenttheoretical work, the reproductive skew models, has focusedon factors affecting the degree to which reproduction is skewedwithin a society. We used the parameters provided by skew modelsas a guideline to study determinants of reproductive sharingin polygyne ants. As a model system we used two-queen laboratorycolonies of the ant Formica fusca in which the reproductiveshares of each queen was assessed from offspring by using allozymesand DNA microsatellites. We tested how the different variablesincluded in reproductive skew models (queen-queen relatedness,potential fighting ability, productivity, and worker relatednessreflected by queen number in the colony of origin) affect reproductivesharing among queens. The results showed that the relatednessamong queens explained 26% of the variation in reproductiveskew. The size difference between queens (reflecting potentialfighting ability), colony productivity, and worker relatednessdid not have an effect on reproductive partitioning among cobreeders.To our knowledge, this is the first study to test for the effectsof various determinants of skew in an experimental setting.     

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.     

Reproductive conflicts in polyandrous and polygynous ant Formica sanguinea

The occurrence of multiple reproductives within an ant colony changes the balance between indirect fitness benefits and reproductive competition. We test whether the number of matings by an ant queen (polyandry) correlates negatively with the number of reproductive queens in the colony (polygyny), whether the patrilines and matrilines differ in their contribution to the sexual and worker progeny and whether there is an overall reproductive skew. For these aims, we genotyped both worker and sexual offspring from colonies of the ant Formica sanguinea in three populations. Most colonies were monogynous, but eight (11%) were polygynous with closely related queens. Most queens in the monogynous colonies (86%) had mated with multiple males. The effective paternity was lower than the actual number of mates, and the paternity skew was significant. Furthermore, in some monogynous colonies, the patrilines were differently represented in the worker pupae and sexual pupae produced at the same time. Likewise, the matrilines in polygynous colonies were differently present in worker pupae and male offspring. The effective number of matings by a queen was significantly lower in polygynous colonies (mean m_e = 1.68) than in monogynous colonies (means 2.06–2.61). The results give support to the hypotheses that polyandry and polygyny are alternative breeding strategies and that reproductive competition can lead to different representation of patrilines and matrilines among the sexual and worker broods.     

Inbreeding-related trade-offs in stress resistance in the ant Formica exsecta

Inbred individuals and populations are predicted to suffer from inbreeding depression, especially in times of stress. Under natural conditions, organisms are exposed to more than one stressor at any one time, highlighting the importance of stress resistance traits. We studied how inbreeding- and immunity-related traits are correlated under different dietary conditions in the ant Formica exsecta. Its natural diet varies in the amount and nature of plant secondary compounds and the level of free radicals, all of which require detoxification to maintain organismal homeostasis. We found that inbreeding decreased general antibacterial activity under dietary stress, suggesting inbreeding-related physiological trade-offs.     

Reproductive specialization in multiple-queen colonies of the ant Formica exsecta

In polygynous (multiple queens per nest) colonies of socialinsects, queens can increase their reproductive share by layingmore eggs or by increasing the proportion of eggs that developinto reproductive individuals instead of workers. We used polymorphicmicrosatellite loci to determine the genetically effective contributionof queens to the production of gynes (new queens), males, and2 different cohorts of workers in a polygynous population ofthe ant Formica exsecta. For this purpose, we developed a newmethod that can be used for diploid and haplodiploid organismsto quantify the degree of reproductive specialization amongbreeders in societies where there are too many breeders to ascertainparentage. Using this method, we found a high degree of reproductivespecialization among nest-mate queens in both female- and male-producingcolonies (sex ratio is bimodally distributed in the study population).For example, a high effective proportion of queens (25% and79%, respectively) were specialized in the production of malesin female- and male-producing colonies. Our analyses furtherrevealed that in female-producing colonies, significantly fewerqueens contributed to gyne production than to worker production.Finally, we found significant changes in the identity of queenscontributing to different cohorts of workers. Altogether, thesedata demonstrate that colonies of F. exsecta, and probably thoseof many other highly polygynous social insect species, are composedof reproductive individuals differing in their investment togynes, males, and workers. These findings demonstrate a newaspect of the highly dynamic social organization of complexanimal societies.     

Highly variable social organisation of colonies in the ant Formica cinerea

Social organisation of colonies was examined in the ant Formica cinerea by estimating the coefficient of genetic relatedness among worker nest mates. The estimates based on microsatellite genotypes at three loci ranged from values close to zero to 0.61 across the populations studied in Finland. These results showed that a fundamental feature of colonies, the number of reproductive queens, varied greatly among the populations. Colonies in some populations had a single queen, whereas the nests could have a high number number of queens in other populations. There was a weak but non-significant correlation between the genetic and metric distance of nests within two populations with intermediate level of relatedness. Differentiation among nearby populations (within the dispersal distance of individuals) in one locality indicated limited dispersal or founder effects. This could occur when females are philopatric and stay in the natal polygynous colony which expands by building a network of nest galleries within a single habitat patch.     

Chemical basis of nest-mate discrimination in the ant Formica exsecta.

Distinguishing nest-mates from non-nest-mates underlies key animal behaviours, such as territoriality, altruism and the evolution of sociality. Despite its importance, there is very little empirical support for such a mechanism in nature. Here we provide data that the nest-mate recognition mechanism in an ant is based on a colony-specific Z9-alkene signature, proving that surface chemicals are indeed used in ant nest-mate recognition as was suggested 100 years ago. We investigated the cuticular hydrocarbon profiles of 10 Formica exsecta colonies that are composed almost entirely of a Z9-alkene and alkane component. Then we showed that worker aggression is only elicited by the Z9-alkene part. This was confirmed using synthetic Z9-alkene and alkane blends matched to the individual colony profiles of the two most different chemical colonies. In both colonies, only glass beads with 'nest-mate' alkene profiles received reduced aggression. Finally, changing the abundance of a single Z9-alkene on live ants was shown to significantly increase the aggression they received from nest-mates in all five colonies tested. Our data suggest that nest-mate discrimination in the social insects has evolved to rely upon highly sensitive responses to relatively few compounds.