How does colony growth influence communication in ants?

Summary. As Lasius niger societies grow from incipient nests to mature colonies, their foraging strategies shift from the individual exploitation of food sources to mass recruitment. Colony size instead of age is the key factor that shapes the exploratory and foraging responses of Lasius niger: a drastic reduction (or increase) of the population elicits an activity profile similar to that observed in younger (or older) societies of the same size. As a colony grows, the proportion of patrollers significantly decreases while the proportion of conveyors remains rather constant. As regards the energetic return, it increases with incipient nest size due to the replacement of minims by ordinary workers of larger crop capacity. We also demonstrate that minims of incipient nests modulate their trail-laying behaviour according to the social context, in this case the colony size. During their ontogenesis, L. niger colonies exhibit a progressive integration of individual foragers into a network of communication, the adaptive significance of which is discussed.     

Persistent Intercolonial Trunkroute-marking in the African Weaver Ant Oecophylla longinoda Latreille (Hymenoptera,Formicidae): Tom Thumb's versus Ariadne's Orienting Strategies

The territorial African weaver ant Oecophylla longinoda forages in a 3-dimensional system when attending honeydew secreted by coccid colonies. The orienting strategy allowing workers to reach a static food resource was studied in laboratory conditions. Contrary to other studies devoted to territoriality in O. longinoda, our experiments show that, when moving off the nest, territorial faecal marking by the major workers is not randomly placed all over the home-range area. The chemical trail leading from the nest to a given food site is reinforced by faecal materials both on the horizontal and the vertical planes. In addition to visual cues and chemical trails laid by the major workers (Holldobler & Wilson 1978), foragers use the territorial marking both to localize the food site and to come back to the nest. Thus, anal-drop deposition in O. longinoda also has a clear dual-purpose function: territorial and orientationai marking. The ecological value of such a discrete Tom Thumb's orienting strategy enables workers to quickly reach a food location even after more than 5 months of inactivity on this given site. The persistent marked trail also has an intercolonial effect. This could allow the keeping of exploitation of a definite food site by alien colonies of the same species by reducing time costs for new exploring phases or for learning new spatial cues when foraging on an unknown environment.     

How does food distance influence foraging in the ant Lasius niger: the importance of home-range marking

We study the influence of food distance on the individual foraging behaviour of Lasius niger scouts and we investigate which cue they use to assess their distance from the nest and accordingly tune their recruiting behaviour. Globally, the number of U-turns made by scouts increases with distance resulting in longer travel times and duration of the foraging cycle. However, over familiar areas, home-range marking reduces the frequency and thereby the impact of U-turns on foraging times leading to a quicker exploitation of food sources than over unmarked set-ups. Regarding information transfer, the intensity of the recruitment trail reaching the nest decreases with increasing food distance for all set-ups and is even more reduced in the absence of home-range marking. Hence, the probability of a scout continuing to lay a trail changes along the homeward journey but in a different way according to home-range marking. Over unexplored setups, at a given distance from the food source, the percentage of returning trail-laying ants remains unchanged for all tested nest-feeder distances. Hence, the tuning of the trail recruiting signal by scouts was not influenced by an odometric estimate of the distance already travelled by the ants during their outward journey to the food. By contrast, over previously explored set-ups, a distance-related factor – that is the intensity of home-range marking – strongly influences their recruiting behaviour. In fact, over a home-range marked bridge, the probability of returning ants maintaining their trail-laying behaviour increases with decreasing food distance while the gradient of home-range marks even induces ants which have stopped laying a trail to resume this behaviour in the nest vicinity. We suggest that home-range marking laid passively by walking ants is a relevant cue for scouts to indirectly assess distance from the nest but also local activity level or foraging risks in order to adaptively tune trail recruitment and colony foraging dynamics. Received 13 July 2004; revised 26 January and 20 May 2005; accepted 2 July 2005.     

Colony spatial structure in polydomous ants: complimentary approaches reveal different patterns

Eusocial insects often live in colonies comprised of an extensive network of interconnected nests and estimating colony spatial structure and colony boundaries may be difficult, especially in cryptic, subterranean species. A combination of aggression assays and protein marking was used to estimate nest spatial distribution in field populations of the highly polydomous cornfield ant, Lasius neoniger. The estimates were first obtained via 1-on-1 aggression tests for workers collected from different nests within the research plots. The aggression tests were followed by mark-recapture field studies which utilized rabbit IgG protein. The ants were allowed to self-mark by feeding on sucrose solution spiked with the IgG protein. Colony spatial structure was detected by sampling ants from different nests and analyzing them for the presence of the marker using an ELISA test. Estimates based on aggression tests were substantially higher relative to those based on protein marking. The average colony size based on aggression tests was 2.0 ± 0.2 m² and was significantly higher than the 1.1 ± 0.4 m² estimate based on protein marking. The estimate based on protein marking was even lower, 0.2 ± 0.1 m², when a Fluon-coated ring restricted ant feeding to the focal nest and prevented ants from other nests from feeding on the protein-marked sucrose. No significant correlation was detected between internest aggression and internest distance. Likewise, no correlation was detected between distance from the focal nest and the percentage of workers testing positive for the protein marker. The results show that both approaches have their own limitations, but their simultaneous use allows for a more accurate assessment of colony spatial structure. The advantages and limitations of each technique are discussed.     

Does lack of intraspecific aggression or absence of nymphs determine acceptance of foreign reproductives in <Emphasis Type="Italic">Macrotermes</Emphasis>?

The rejection or acceptance of a foreign reproductive by an alien colony may not always be as straightforward as cue recognition between worker termites. This paper aims to determine whether adoption of foreign reproductives is caused simply by lack of intraspecific aggression or is contingent on the reproductive status of the host colony. In the fungus-culturing termites, Macrotermes gilvus (Hagen) and Macrotermes carbonarius (Hagen), major workers showed low intraspecific aggression towards non-nestmates irrespective of geographic distance between source colonies. Our results indicated that workers were hardly aggressive towards non-nestmates. In royal cell-swapping experiments, both species responded in a similar way: (1) in host colonies with nymphs present, the foreign reproductives were rejected; while (2) in host colonies without nymphs the foreign reproductives were either accepted and breeding resumed or the host colonies died eventually. Workers from the host colonies preferentially maintained offspring nymphs from which adultoid replacement reproductives develop rather than accepting foreign reproductives. There is no fitness gain for the queenless workers in accepting foreign reproductives; however, there is overall benefit to the newly born population.     

Interactions between extrafloral nectaries, aphids and ants: are there competition effects between plant and homopteran sugar sources?

Broad bean (Vicia faba), an annual plant bearing extrafloral nectaries (EFN) at the base of the upper leaves, is regularly infested by two aphid species, Aphis fabae and Acyrthosiphon pisum. EFN and A. fabae are commonly attended by the ant, Lasius niger, while Ac. pisum usually remains uninfested. Sugar concentration and sugar composition of extrafloral nectar did not change significantly after aphid infestation. The sugar concentration was significantly higher in EFN (c. 271 µg µl^-1) than in the honeydew of A. fabae (37.5 µg µl^-1). The presence of small A. fabae colonies had no significant effect on ant attendance of EFN, which remained at the same level as that on plants without A. fabae. Obviously, there was no significant competitive effect between the two sugar sources. We suggest that the high sugar concentration in the extrafloral nectar may outweigh the higher quality (due to the presence of melezitose) and quantity of aphid honeydew. Ants and the presence of EFN influenced aphid colony growth. While A. fabae colonies generally grew better in the presence of ants, Ac. pisum colonies declined on plants with EFN or A. fabae colonies. We conclude that EFN may provide some degree of protection for V. faba against those sucking herbivores that are not able to attract ants.     

The Influence of Relatedness, Neighbourhood and Overall Distance on Colony Mate Recognition in a Polydomous Ant

The ‘dear enemy phenomenon’ predicts that territorial animals respond less aggressively towards more familiar neighbours than towards unfamiliar conspecifics if potential losses to strangers are more costly than potential losses to neighbours. Conversely, territorial animals should respond more aggressively to neighbours, if potential losses to them are more costly than potential losses to strangers. In social insects the question of how colony members distinguish neighbours from strangers, however, is intertwined with the more general question of how colony members discriminate themselves from non‐colony members; both genetic and spatial distance can correlate with levels of inter‐colonial aggression. In this paper I disentangle the role of experience, genetic and spatial distance on inter‐colonial aggression in a polydomous population of Iridomyrmex purpureus. In I. purpureus, aggression is related to the spatial distance between colonies irrespective of genetic similarity. Spatial distance affected aggression in two different ways. First, workers were more likely to exhibit aggression towards alien conspecifics of adjoining rather than non‐adjoining territories, suggesting the opposite of the dear enemy phenomenon. Second, workers were more often aggressive towards conspecifics of more distant colonies, implying that environmental cues play a role in the recognition system of I. purpureus.     

Field experiments on colony foundation byLasius niger (L.) andL. flavus (F.) (Hym., Formicidae)

Summary Queens ofLasius flavus (F.) andL. niger (L.) were observed to choose sunlit bare areas for colony foundation and shading was found to reduce their success in founding colonies. Large colonies of these species killed queens of the opposite species first thus favouring the co-existence brought about by their habitat selection.     

Social wasp parasites affect the nestmate recognition abilities of their hosts (<Emphasis Type="Italic">Polistes atrimandibularis</Emphasis> and <Emphasis Type="Italic">P. biglumis</Emphasis>, Hymenoptera,Vespidae)

Summary. Queens of the parasitic social wasp, Polistes atrimandibularis, temporarily mimic the odor of their host species, Polistes biglumis, but their offspring have parasite-specific odors. As a consequence, in parasitized colonies individuals with different odors co-inhabit the colony and host workers, who are responsible for colony defense, accept wasps with different odors. In order to verify whether this particular condition causes a change in recognition abilities of hosts, we tested nestmate/non-nestmate discrimination in field colonies invaded by social parasites (and in non-parasitized colonies as controls). Results show that parasitized colonies distinguish between nestmates and non-nestmates, distinguish their parasite queen from those that usurped alien colonies, and accept their parasite's non-mimetic offspring but make more recognition errors than non-parasitized colonies. The optimal acceptance threshold model predicts that when the frequency of encountering non-kin increases, residents become less permissive towards intruders. However, my data show that parasitized colonies are more permissive towards non-nestmates with respect to non-parasitized colonies but they are also more aggressive towards nestmates, suggesting that host workers' learning abilities are impaired.     

Survival of transplanted nests of the red wood ant Formica aquilonia （Hymenoptera： Formicidae）： The effects of intraspecific competition and forest clear-cutting

The fitness and survival of ant colonies depend on the resources near their nests. These resources may be limited due to poor habitat quality or by intra- and interspecific competitions, which in extreme cases may cause the ant colony to perish. We tested the effect of intraspecific competition and habitat degradation （forest clear-cutting） on colony survival by transplanting 26 nests of the red wood ant （Formica aquilonia Yarrow, 1955） in 26 different forest areas that contained 0-11 conspecific alien nests per hectare. F. aquilonia is highly dependent on canopy-dwelling aphids, thus the removal of trees should cause food limitation. During the course of the 4-year experiment, 9 of the forests were partially clear- cut. We found that while forest clear-cutting significantly decreased the colonies＇ survival, intraspecific competition did not. As a highly polygynous and polydomous species, E aquilonia seems to tolerate the presence of alien conspecific colonies to a certain extent.     

Out of sight but not out of mind: modulation of recruitment according to home range marking in ants

Animals can acquire a global knowledge about their environment that exceeds their individual capacities by estimating the local density and activity of nestmates in an area. In ants, home range marking can indicate the density and activity of nestmates, allowing scouts to assess the potential interest of the area as a foraging site. We investigated how home range marking through footprints influences the foraging behaviour of Lasius niger scouts at a sugary food source (1 M, 1.5 ml). Over a marked apparatus the discovery time of food sources decreased while the probability of scouts recruiting nestmates and of continuing to lay a trail increased. For ants making U turns on their return to the nest, home range marking helped them to resume laying a trail after the U turn and delayed the occurrence of the U turn. As a result, the trail intensity and the rate at which information about food was conveyed by scouts to nestmates depended on home range marking. Such modulation of information reduces the number of foragers mobilized to less frequented areas that are potentially dangerous and promotes recruitment and exploitation of food sources to better known sites.     

Social organization, reproductive behavior and ecology of Leptothorax acervorum (Hymenoptera, Formicidae) from the Sierra de Albarracin in central Spain

Summary: Leptothorax acervorum, an ant species with holarctic range, occurs in an isolated population in the Spanish Sierra de Albarracin. Dissection of dealate females and laboratory observations revealed that in contrast to other European populations, the colonies are monogynous, with one reproductive queen each and a variable number of virgin or mated dealate but not laying females. Most of the latter probably just hibernate in the mother nests, leaving them in the following spring, but a few remain there for longer time, without reproducing. Such colonies then are functionally monogynous. Alate females exhibit a stationary sexual calling, and mating behavior could be studied in the laboratory. Mated females return to the mother nest where they soon shed wings. When developing fertility before or after hibernation they are evicted from the nests; in nature they probably form daughter colonies. Patchy habitat and rough climatic conditions in the Sierra de Albarracin may be responsible for the particular reproductive behavior of L. acervorum in this area. The generally small size difference between queens and workers in the subgenus Leptothorax entails high costs of dispersal and colony foundation by single queens who have to forage for their first brood. Some kind of dependent colony foundation therefore is frequently met with in the subgenus. Notwithstanding the marked biological and a few slight morphological differences between central European L. acervorum and the Spanish population its taxonomic status as yet is unsettled. We refer to this population provisionally as "L. acervorum Albarracin".     

Aliens among us: nestmate recognition in the social huntsman spider, Delena cancerides

Unlike all other social spiders, the social huntsman spider, Delena cancerides, has been reported to rapidly respond to non-nestmates with lethal aggression, similar to the behavior of some eusocial insects. We tested for the presence of nestmate recognition in D. cancerides under laboratory conditions by introducing 105 unrelated alien conspecifics into foreign colonies and comparing their behavior to 60 control spiders removed and returned to their natal colony. Spiders demonstrated nestmate recognition by investigating alien spiders far more than nestmates and by resting closer to nestmates than to aliens. Serious attacks or deaths occurred in 23% of all trials; however, aggression was not directed significantly more toward aliens than to nestmates. Most notably, aggression was largely mediated by the adult females (resident or alien), who were most likely to attack or kill other subadult or mature individuals. Young individuals (resident or alien) were largely immune from serious aggression. Spiders recently collected from the field tended to be more aggressive than spiders born and raised in the laboratory, possibly due to blurring of recognition cues related to laboratory husbandry. Our findings support the prediction that nestmate recognition should evolve when there is a benefit to discriminating against non-kin, as in this social spider system where foraging individuals may enter a foreign colony and the colony retreat is a limited resource.     

Marking of Nest Entrances and Vicinity in Two Related Tetramorium Ant Species (Hymenoptera: Formicidae)

Workers of the related ants Tetramorium impurum and T. caespitum mark the vicinity of their nest entrances in a species-specific manner, as seen by similarities between the behavior of nestmates and that of alien conspecifics (e.g., concerning aggregation, locomotion, orientation, tendency to move, and agonistic behavior). Additionally, they mark the inside of their nest entrances in a colony-specific manner, as seen by the following differences in behavior. Nestmates aggregate on these areas, walk rather slowly, but freely and essentially in the middle of the areas, come toward and very near such areas, are not inclined to escape, and are ready to attack possible intruders. Alien conspecifics do not aggregate, walk quickly, and are reluctant to stay on the areas, come neither toward nor very near the areas, are inclined to escape, and often open their mandibles, mainly when in front of a resident. The marking of the nest entrances is performed by T. impurum in 30 min and by T. caespitum in 15 min. If not reinforced, the marking by both species vanishes in 60 and 50 min, respectively. Extracts of hindlegs, metathorax, or metapleural glands produce in unmarked areas the ethological effect of marked entrances. It may be hypothesized that the marking factor is produced by the workers' metapleural glands and deposited onto the ground, via the hindlegs of ants leaving the nest. A worker's head has a species- but not a colony-specific ethological effect. An isolated alien conspecific's head is never attacked, whereas a thorax with abdomen is. This explains why, by opening its mandibles (and then presumably emitting a mandibular gland pheromone), a conspecific ant momentarily inhibits the attack of a nonnestmate. According to Hölldobler and Wilson (1990), the marking of the inside of T. impurum and T. caespitum nest entrances is a territorial and nest-entrance marking, whereas the marking of the close vicinity of the entrances is a home-range marking, as is the marking of the foraging area. These markings are in accordance with the fact that T. impurum foragers deposit their trail pheromone as far as the opening of the nest entrance.     

Ants distinguish neighbors from strangers

Summary Ants are known to distinguish their own nests and nestmates from all others, using colony-specific odors. Here I show that harvester ants (Pogonomyrmex barbatus) can further distinguish between two kinds of non-nestmates of the same species: neighbors and strangers. Interactions between colonies were thought to depend on the numbers of alien ants that each colony encounters on its territory. The results described here show that such interactions also depend on information about colony identity. Encounters on foraging trails with ants from neighboring colonies, deter foraging more than encounters with ants from distant ones. The history of interactions between particular pairs of colonies may have important effects on intraspecific competition for food.     

Intraspecific aggression and the colony structure of the invasive ant Myrmica rubra

1. Patterns of aggression between ants from different nests influence colony and population structure. Several species of invasive ants lack colony boundaries over large expanses, forming ‘supercolonies’ with many nests among which workers can move without encountering aggression. 2. Bioassays of aggression were used to determine the colony structure of the invasive ant Myrmica rubra (L.) at eight sites in Massachusetts, the state where the species was first discovered in North America. To improve the ability to distinguish systematic patterns from background variability in aggressiveness, a repeated‐measures design was used and replicate assays for each pair of nests were conducted. 3. Aggressive responses showed that populations at all sites consisted of multiple distinct colonies. Patterns of aggression were repeatable and transitive, with few exceptions. Colonies were identified as clusters of nests whose workers showed little to no aggression towards one another but were aggressive towards conspecifics from more distant nests. 4. The degree of aggression varied considerably among different colony pairs but did not depend in any consistent way on the distance of separation or on whether colonies were neighbours. 5. Territories of neighbouring colonies abutted, indicating that they were restricted by intraspecific competition. Mapped territories ranged in size from 0.03 to 1.2 ha, but colonies at the study sites have not undergone the enormous expansions seen in introduced populations of some other species of invasive ants, and neighbouring colonies compete locally.     

Colony size,nestmate density and social history shape behavioural variation in Formica fusca colonies

In ants, individuals live in tightly integrated units (colonies) and work collectively for its success. In such groups, stable intraspecific variation in behaviour within or across contexts (personality) can occur at two levels: individuals and colonies. This paper examines how colony size and nestmate density influence the collective exploratory behaviour of Formica fusca (Hymenoptera: Formicidae), in the laboratory. The housing conditions of the colonies were manipulated to vary the size of colonies and their densities under a fully factorial design. The results demonstrate the presence of colony behavioural repeatability in this species, and contrary to our expectations, colonies were more explorative on average when they were kept at lower nestmate densities. We also found that experimental colonies created from larger source colonies were more explorative, which conveys that a thorough understanding of the contemporary behaviour of a colony may require knowing its social history and how it was formed. Our results also convey that the colony size and nestmate density can have significant effects on the exploratory behaviour of ant colonies.     

Intraspecific competition influences the symmetry and intensity of aggression in the Argentine ant

Cooperative social groups rely on the ability to distinguishmembers from nonmembers. Accordingly, social insects have evolveda variety of systems that allow discrimination of nest matesfrom non–nest mates. In this study, we show that experiencecan modify patterns of intraspecific aggression in Argentineants (Linepithema humile). In laboratory experiments, we foundthat aggression between colonies was often asymmetrical, butin all five cases, this asymmetry shifted to symmetrical aggressionafter contact with a hostile colony. Moreover, in the field,aggression between workers collected from colony borders wassymmetrical, whereas polarized aggression occurred between workerscollected 500 m away from colony borders. Coinciding with thisshift in aggression symmetry, we also observed an increase inboth the overall level of aggression and the frequency of aggressionin both the field and laboratory bioassays. We found littleevidence for colony-level competitive asymmetries stemming frompolarities in aggression at the worker level, either in thelaboratory or in the field. These results illustrate that recognitionsystems in Argentine ants are surprisingly dynamic and provideexperimental evidence for how recognition can be adjusted inresponse to specific circumstances—in this case the presenceof intraspecific competitors.     

Nest site selection and longevity in the ponerine ant Rhytidoponera metallica (Hymenoptera, Formicidae)

Summary: The size of the nesting cavity and the nature of the nest-building material can constrain colony growth in eusocial insects. Rocks protect colony members against extreme temperatures as well as serve as a supplementary source of heat, thereby maintaining optimal conditions for brood development. The frequency distribution of available rocks and those occupied by colonies of Rhytidoponera metallica in the Avon River State Forest, Victoria, Australia, showed that colonies of R. metallica avoided nesting under small rocks (<200 cm²). A strong correlation between colony size and rock size indicates that nesting under a larger rock promotes colony growth. Laboratory choice experiments revealed that workers are capable of recognizing larger rocks from external physical characters (dimension). Field data indicates that colonies leave nests frequently, with nest abandonment peaking in the summer months.     

Biological constraints and colony founding in the polygynous invasive ant Lasius neglectus (Hymenoptera, Formicidae)

Summary. The polygynous invasive ant Lasius neglectus was described from Budapest, Hungary, as an unicolonial species, with no apparent colony barriers, and inferred intra-nidal mating without a nuptial flight. Here we analyze additional morphological characteristics of gynes, their physiological condition at emergence and at the time of mating and we describe the productivity of different types of colony founding in the laboratory. A low increase in dry weight and in fat content from emergence to mating indicates that gynes can only succeed in dependent colony founding. However, the queen-worker thorax volume ratio is typical of a species with independent colony founding and we were able to demonstrate independent colony founding in the laboratory (both haplo- and pleometrotic). Brood development in independent founding is rapid and the number of nanitics higher than in other Lasius species. Both colony budding and dependent colony founding could also be demonstrated in the laboratory. Worker oviposition was absent. The carbohydrate content of newly mated queens is consistent with the observed loss of mating flight of this species. However the relative wing area clearly indicates that L. neglectus queens should be able to fly. Therefore, both queen morphology and physiological state at maturity show that L. neglectus is intermediate between a monogynous, free-living, non pest ant and a multiqueened (polygynic) invasive tramp ant. This neglected ant thus offers interesting opportunities to study the origin of unicoloniality and the spread of this species in northeast Spain.