Polymorphism: a weak influence on worker aggregation level in ants

Abstract.  1. Aggregation of individuals, a basic behaviour in social species, plays an essential role in many aspects of animal life (reproduction, defence, and alimentation). Understanding how this phenomenon is modulated is important to comprehend the social organisation of the group.
2. In social insects, aggregation is influenced by environmental (e.g. the light level) and social (e.g. polyethism in monomorphic ants) factors. Ants display a great variation of biological characteristics (e.g. queen number, polymorphism, division of labour, etc.) that are likely to influence the level of inter-attraction and so the aggregation.
3. The present research focused on one biological characteristic: the morphological castes (minors, majors), testing the hypothesis that minors will aggregate more than majors due to their greater need to fight against the loss of heat and to increase their self-protection.
4. Aggregation experiments were conducted on two highly polymorphic species, Atta sexdens rubropilosa and Solenopsis interrupta , using the two extreme morphological castes (majors and minors).
5. All castes exhibited a low level of aggregation: 40–50% of workers assembled for both species, the biggest cluster involving 20% of the total population. The lack of difference between morphological castes in the aggregation shows the weak influence of polymorphism on the interactions between ants.
6. It is concluded that the main factor modulating the aggregation behaviour is polyethism, i.e. the division of labour associated with the presence of an outside-the-nest experience: workers that only take care of the brood, without outside world experience (brood-tenders) assembling more than foraging workers (foragers).     

Effect of social and environmental factors on ant aggregation: a general response?

In social species, the interactions that occur between individuals and also between individuals and their environment can lead to various spatial distributions. Our goal was to understand how both factors (social and environmental) can affect such spatial distribution of individuals. Aggregation experiments were conducted on the ant Crematogaster scutellaris: the level of inter-attraction between individuals was characterized and the influence of an environmental factor (light intensity) was studied. Two ethological castes (brood-tenders and foragers) were tested, showing a strong and similar aggregation pattern—although faster for foragers. This difference could be explained by a higher degree of inter-attraction for brood-tenders, resulting in longer interaction time and therefore a slower aggregation rate. We also showed the influence of light conditions by carrying out experiments in total darkness and under red light. C. scutellaris appeared highly sensitive to the red light, aggregating just underneath it. Nevertheless, the level of aggregation is not affected by the luminosity condition. These results are discussed in relation to the ecology of the species, and are compared with results obtained in another ant species, Lasius niger, which exhibits a similar response to aggregation. We propose, therefore, a general link between the level/pattern of aggregation and polyethism in ants.     

Task-related variation of postpharyngeal and cuticular hydrocarbon compositions in the ant Myrmicaria eumenoides

In the ant Myrmicaria eumenoides we investigated postpharyngeal and cuticular hydrocarbons. At eclosion the glands contained almost no hydrocarbons and there were no lipid inclusions in the glandular epithelium. During the first 3 weeks of adult life the amount of hydrocarbons in the gland increased until day 5, and then remained constant while the lipid content in the epithelium increased steadily. Intracolonial hydrocarbon compositions were not uniform. Compositions of post-pharyngeal and cuticular hydrocarbons in individual ants varied simultaneously, but in different manner depending on the tasks of the ant (brood-tenders, foragers, scouts). Variations on the cuticle were greater than in the gland, but they were strongly correlated. Independent of ants' age and task, cuticular hydrocarbon compositions were dominated by alkenes and alkadienes. Task-specific differences in cuticular compositions were mainly in the amount of alkenes (high in foragers) and alkadienes (high in brood-tenders). Variation of hydrocarbons was low in ants up to 10 weeks old. Thereafter, ants fell into two groups: (1) ants that did not change their hydrocarbons and remained in the nest, and (2) ants that changed their hydrocarbon compositions and became foragers. These results contribute to an ongoing discussion of the dynamic relationship between post-pharyngeal and cuticular hydrocarbons.     

The influence of red light on the aggregation of two castes of the ant, Lasius niger

Insects are generally described as essentially blind to red wavelengths but sensitive to ultraviolet. Therefore, ants are generally reared in nests where "obscurity" is obtained by the use of a red filter. Yet, no study has been done in order to confirm this idea and to verify whether ants behave differently with or without red light. We performed aggregation experiments with the ant Lasius niger under red light and in total darkness to control the sensitivity of workers. Aggregation is a basic behavior associated with the origin of sociality and with the spatial organization of the colony. We demonstrate that L. niger is sensitive to red wavelengths. Moreover, we show that workers behave differently depending on their ethological caste: foragers aggregate well in total darkness but show low assembly under red light, whereas brood-tenders aggregate well in both conditions. For the first time, a link between vision, social organization and spatial patterns is revealed. The results are discussed relative to their adaptive value and relative to the physiology of the workers. Hypotheses are formulated concerning the acquisition of this change of behavior between castes.     

Marking as a decision factor in the choice of a new resting site in <Emphasis Type="Italic">Lasius niger</Emphasis>

Summary In ants, aggregation is mainly based on the attraction between individuals. We confirmed the existence of ground marking in Lasius niger and demonstrated its influence on the aggregative behaviour of ants: the place of the gathering is leading by the existence of a ground marking which favours clustering by increasing the rate of the process but does not change the aggregation level. This factor could play a role in the choice of a new nest site and in the spatial distribution of ants inside the nest. This marking, presuming to have a long duration, could represent an external memory which could play an important role in various situations such as the choice of new nest or the reorganisation of ants after a disturbance.Received 23 July 2003; revised 16 December 2003; accepted 19 December 2003.     

A Basis for Spatial and Social Patterns in Ant Species: Dynamics and Mechanisms of Aggregation

Aggregation is usually studied in functional terms, forgetting mechanisms. In this paper, experimental results on the ant Lasius niger, complemented by a model, allow us to understand the mechanisms responsible for aggregation and to study the influence of the population density on this phenomenon. The results show (1) a high level of aggregation and the emergence of a large cluster; (2) that aggregation results from an amplification mechanism—the greater the number of ants inside a cluster, the greater the time spent by one ant in this cluster; and (3) that population density has only a weak influence on the aggregation process. This method of analysis and these results can certainly be extended not only in social insects but also in other species, like subsocial arthropods.     

Spatial organization in ants’ nests: does starvation modify the aggregative behaviour of Lasius niger species?

Ant colonies that undergo long starvation periods have to tune their exploratory and foraging responses to face their food needs. Although the number of foragers is known to increase with food deprivation in the ant Lasius niger, such enhanced food exploitation is not related to a more intense recruitment by successful scouts. We thus suggest that the colony’s response to a food shortage could result from changes at the level of the ant recruits, in particular from changes in their spatial organization inside the nest. Since aggregation plays a key role in the social organization of ants, we assume that the colony’s response to starvation could be due to changes in the aggregative behaviour of L. niger nestmates.We thus compared the aggregation dynamics of inner-nest workers and foragers having undergone either a short or a long-lasting starvation period. Whatever the ethological group (foragers or inner-nest workers), there was no significant influence of starvation on the aggregation dynamics nor on any feature of the observed clusters. This result shows that an increased foraging response to food shortage cannot be explained by changes in the tendency of nestmates to aggregate within the nest. Finally, we discuss other behavioural mechanisms, in particular changes in behavioural thresholds that could underlie the adaptive changes seen in colony foraging after long starvation periods. Received 25 June 2007; revised 21 January 2008; accepted 24 January 2008.     

Task-specific expression of the foraging gene in harvester ants

In social insects, groups of workers perform various tasks such as brood care and foraging. Transitions in workers from one task to another are important in the organization and ecological success of colonies. Regulation of genetic pathways can lead to plasticity in social insect task behaviour. The colony organization of advanced eusocial insects evolved independently in ants, bees, and wasps and it is not known whether the genetic mechanisms that influence behavioural plasticity are conserved across species. Here we show that a gene associated with foraging behaviour is conserved across social insect species, but the expression patterns of this gene are not. We cloned the red harvester ant (Pogonomyrmex barbatus) ortholog (Pbfor) to foraging, one of few genes implicated in social organization, and found that foraging behaviour in harvester ants is associated with the expression of this gene; young (callow) worker brains have significantly higher levels of Pbfor mRNA than foragers. Levels of Pbfor mRNA in other worker task groups vary among harvester ant colonies. However, foragers always have the lowest expression levels compared to other task groups. The association between foraging behaviour and the foraging gene is conserved across social insects but ants and bees have an inverse relationship between foraging expression and behaviour.     

Division of labour and specification of castes in the red imported fire ant Solenopsis invicta buren

Division of labour in Solenopsis invicta follows a familiar pattern: younger, smaller ants tend toward brood care while older, larger ants tend toward foraging. However, long-term observations of marked individuals reveal that length of nursing and foraging ‘careers’ and the age of transition between these activities vary considerably between and within size groups, and are related to length of life. Experiments with entire colonies show that larger ants are more likely than smaller ants to forage for insect prey. There are two main worker castes, ‘nurses’ and ‘foragers’, whose members span a wide age-size range, and a large ‘reserve’ subcaste, heterogeneous in age, size, and behaviour: reserves may nurse, forage, store liquid food, or relay food from nurses to foragers. The proportion of ants engaged in foraging decreases with colony size because many ants in large colonies are not exposed to recruitment signals.     

Trail laying behaviour during food recruitment in the antLasius niger (L.)

Summary The trail-laying behaviour of foragers of the antLasius niger was observed in the laboratory on a 20 cm bridge between the nest and the food source. We measured both the frequency of trail laying, as defined by the proportion of trips during which trail laying occurred, and its intensity, as defined by the number of marks laid during one bridge crossing.Foragers do not exhibit trail-laying behaviour until a food source is discovered. Trail laying then occurs more or less equally both to and from the nest, and both its frequency and intensity decrease as the recruitment proceeds. Foragers from very small colonies less than a year old appear to have quantitatively the same trail laying behaviour as those from older and much larger colonies.Groups of recruiters and recruits were individually marked. Their trail laying intensity was similar, both for trips to and from the nest, and for an ant's first, second, third and fourth trip. The frequency diminished rapidly with the number of trips made by each individual, and was 2–3 times higher for recruiters than for recruits, for trips both to and from the nest. Even though foragers stop marking after a variable number of passages, they continue to move between the nest and the food source, and other ants start marking. Different foragers appear to have widely different levels of trail laying, although we cannot say whether these differences are stable between different recruitments.Trail laying is strongly affected by the foragers' position on the bridge, especially for ants returning to the nest which lay up to five times more on the segment closest to the source than that closest to the nest. Foragers on a weakly marked trail appear to mark more than those on a well-marked trail. However, this effect is weak and could partly be attributed to their lower speed.Finally, a model using the experimental data gathered on the individuals' trail-laying behaviour reproduced satisfactorily the colony's overall trail laying.     

Spartan defense in the Thermopylae pass: Strategic defense by aggregations of <Emphasis Type="Italic">Pseudomyrmex spinicola</Emphasis> (Hymenoptera,Formicidae) on the trunk of <Emphasis Type="Italic">Acacia collinsii</Emphasis> (Mimosaceae)

Acacia ants (Pseudomyrmex spinicola) usually prune all of the vegetation growing in the vicinity of their host plant (Acacia collinsii). This lack of branches in contact with the host tree makes the main stem of the acacia the only point of access for ants invading the resident colony. The present study assessed whether the intrusion site affects the probability of the intruder being detected. It also investigated whether ants that remain inactive on the main trunk base (a) are involved in defensive duties and (b) are morphologically different from other colony members (e.g. foraging ants). In the field, intruder ants were placed on the trunk and leaves of acacias inhabited by P. spinicola; colony members on the trunk base and the leaves were marked and monitored; and head length, head width, and thorax length of ants that were either foraging on the leaves or immobile on the trunk were measured. P. spinicola ants found intruder ants entering their host acacia via the trunk more rapidly than those entering by means of the leaves. Ants on the base of the trunk do not exchange tasks with foragers: between 5 and 47% of the 1063 ants originally marked on the trunk were re-observed there, whereas only two of the 804 ants marked on the leaves were seen on the trunk. However, ants on the trunk were similar to foragers in morphology, suggesting that defensive ants may be part of a behavioral caste. Received 30 November 2007; revised 29 February 2008; accepted 17 March 2008.     

Behavior of red wood ants (Hymenoptera,Formicidae) during interaction with different symbiont partners

A comparative analysis of the behavior of Formica polyctena Först during interaction with different symbionts (free-living aphids Aphis grossulariae Kalt. and hidden larvae of the sawfly Blasticotoma filiceti Klug) was carried out. Red wood ants demonstrate different levels of functional differentiation in relatively constant groups of foragers collecting honeydew. A deep “professional” specialization with clear division of a number of tasks among foragers was studied in groups of ants tending aphids. Four professional groups of foragers with different tasks were revealed: “shepherds,” “guards,” “transporters,” and “scouts” (or “coordinators”). The groups of foragers caring for sawfly larvae mainly consist of unspecialized ants. Only few ants (about 5%) remain on duty on the fern plant near B. filiceti larvae and protect the food resource from competitors, especially from other ants. In addition, the ants demonstrate simpler behavior while collecting the larval excretion, resembling that at the sugar feeders. On the whole, the behavior of red wood ants is rather flexible. The level of functional differentiation in groups of foragers collecting honeydew is determined not only by the colony size and requirements but by the nature of their interaction with trophobionts, particularly, by the possibility of direct contact.     

Use of terrestrial chemical trails for nest orientation in an open nesting wasp, <Emphasis Type="Italic">Dolichovespula saxonica</Emphasis> F.

Summary The long-cheeked wasp Dolichovespula saxonica typically constructs exposed nests which can be reached by flying. Usually foragers do not walk on substrates in the close vicinity of the nests as cavity breeding wasps do (Steinmetz et al., 2002). Unexpectedly, when forced to walk outside the nest instead of flying in an artificial tunnel system, D. saxonica foragers lay a terrestrial trail and use it for orientation in the nest area in our experiments. 41% of the foragers followed the trail in a direction they were not accustomed to. We suggest that the foragers have employed the same orientation cues normally used for orientation in the close vicinity of the nest when approaching a free-hanging nest by flying, for example nest odour. Nest odour substances may have been transferred to the substrate as a trail as a consequence of foragers walking through the tunnels.Received 23 August 2002; revised 27 January 2003; accepted 30 April 2003.     

Patterns in foraging and nesting ecology in the neotropical ant, <Emphasis Type="Italic">Gnamptogenys moelleri</Emphasis> (Formicidae,Ponerinae)

Summary This study provides quantitative field data on the natural history and foraging behaviour of the Neotropical bromeliad-nesting ant Gnamptogenys moelleri (Ponerinae) in a sandy plain forest in Southeast Brazil. The ant nested on different bromeliad species and the nests were more frequently found in bigger bromeliads. The species used a wide array of invertebrates in its diet, hunting for live prey and scavenging the majority of the items from dead animals. The food items varied greatly in size (1 to 26 mm). Hunting was always performed by solitary workers. Retrieving was performed by solitary workers (small items), or by a group of 3 to 12 workers recruited to the food source (large items). Almost all G. moelleri foraging activity was restricted to the nest bromeliad. In the warm period more ants left the nest to forage, and foraging trips achieved greater distances compared to the cool season. Trap data revealed that overall availability of arthropod prey is higher in the summer than in the winter. The opportunism in nest site use and in foraging behaviour, the small foraging area, as well as the seasonal differences in foraging activity are discussed and compared with other tropical ants.Received 30 May 2003; revised 22 September 2003; accepted 3 October 2003.     

Context-dependent navigation in a collectively foraging species of ant, Messor cephalotes

More than 100 years of scientific research has provided evidence for sophisticated navigational mechanisms in social insects. One key role for navigation in ants is the orientation of workers between food sources and the nest. The focus of recent work has been restricted to navigation in individually foraging ant species, yet many species do not forage entirely independently, instead relying on collectively maintained information such as persistent trail networks and/or pheromones. Harvester ants use such networks, but additionally, foragers often search individually for food either side of trails. In the absence of a trail, these ‘off-trail’ foragers must navigate independently to relocate the trail and return to the nest. To investigate the strategies used by ants on and off the main trails, we conducted field experiments with a harvester ant species, Messor cephalotes, by transferring on-trail and off-trail foragers to an experimental arena. We employed custom-built software to track and analyse ant trajectories in the arena and to quantitatively compare behaviour. Our results indicate that foragers navigate using different cues depending on whether they are travelling on or off the main trails. We argue that navigation in collectively foraging ants deserves more attention due to the potential for behavioural flexibility arising from the relative complexity of journeys between food and the nest.     

Altruistic self‐removal of health‐compromised honey bee workers from their hive

Social insect colonies represent distinct units of selection. Most individuals evolve by kin selection and forgo individual reproduction. Instead, they display altruistic food sharing, nest maintenance and self‐sacrificial colony defence. Recently, altruistic self‐removal of diseased worker ants from their colony was described as another important kin‐selected behaviour. Here, we report corroborating experimental evidence from honey bee foragers and theoretical analyses. We challenged honey bee foragers with prolonged CO₂ narcosis or by feeding with the cytostatic drug hydroxyurea. Both treatments resulted in increased mortality but also caused the surviving foragers to abandon their social function and remove themselves from their colony, resulting in altruistic suicide. A simple model suggests that altruistic self‐removal by sick social insect workers to prevent disease transmission is expected under most biologically plausible conditions. The combined theoretical and empirical support for altruistic self‐removal suggests that it may be another important kin‐selected behaviour and a potentially widespread mechanism of social immunity.     

Biology and oviposition behavior of the phorid Apocephalus attophilus and the response of its host, the leaf-cutting ant Atta laevigata

Atta laevigata (Smith) (Hymenoptera: Formicidae) foragers collected in the field and parasitized by Apocephalus attophilus Borgmeier (Diptera: Phoridae) (1.2% of sampled ants) were larger and had lower survival rates than similar non-parasitized foragers. Moreover, the size of parasitized ants was significantly correlated with the number of A. attophilus puparia and percentage of adult emergence. These results suggest that host size is important for the reproductive success of A. attophilus. Different from most ant-parasitizing phorids, A. attophilus exhibits a pre-oviposition behavior that involves walking towards the host and inspecting it, and this careful approach may be responsible for a relatively low percentage of parasitoid detection by A. laevigata. When an exotic resource (Acalypha sp. leaves) was placed on ants' foraging trails, more foragers were recruited, which resulted in the attraction of A. attophilus. The number and recruiting rates of small workers (minima) were significantly higher on leaves visited by A. attophilus, but larger foragers showed no response to phorids. These results demonstrated that minima react to the presence of A. attophilus and suggest a defensive role of these ants against phorid parasitism.     

Visual scanning behaviours and their role in the navigation of the Australian desert ant Melophorus bagoti

Ants are excellent navigators, using a combination of innate strategies and learnt information to guide habitual routes. The mechanisms underlying this behaviour are little understood though one avenue of investigation is to explore how innate sensori-motor routines are used to accomplish route navigation. For instance, Australian desert ant foragers are occasionally observed to cease translation and rotate on the spot. Here, we investigate this behaviour using high-speed videography and computational analysis. We find that scanning behaviour is saccadic with pauses separated by fast rotations. Further, we have identified four situations where scanning is typically displayed: (1) by naïve ants on their first departure from the nest; (2) by experienced ants departing from the nest for their first foraging trip of the day; (3) by experienced ants when the familiar visual surround was experimentally modified, in which case frequency and duration of scans were proportional to the degree of modification; (4) when the information from visual cues is at odds with the direction indicated by the ant’s path integration system. Taken together, we see a general relationship between scanning behaviours and periods of uncertainty.     

Interactions Increase Forager Availability and Activity in Harvester Ants

Social insect colonies use interactions among workers to regulate collective behavior. Harvester ant foragers interact in a chamber just inside the nest entrance, here called the ''entrance chamber''. Previous studies of the activation of foragers in red harvester ants show that an outgoing forager inside the nest experiences an increase in brief antennal contacts before it leaves the nest to forage. Here we compare the interaction rate experienced by foragers that left the nest and ants that did not. We found that ants in the entrance chamber that leave the nest to forage experienced more interactions than ants that descend to the deeper nest without foraging. Additionally, we found that the availability of foragers in the entrance chamber is associated with the rate of forager return. An increase in the rate of forager return leads to an increase in the rate at which ants descend to the deeper nest, which then stimulates more ants to ascend into the entrance chamber. Thus a higher rate of forager return leads to more available foragers in the entrance chamber. The highest density of interactions occurs near the nest entrance and the entrances of the tunnels from the entrance chamber to the deeper nest. Local interactions with returning foragers regulate both the activation of waiting foragers and the number of foragers available to be activated.     

Quarantining behaviour of ants towards infected aphids as an antifungal mechanism in ant–aphid interactions

The ecological success of social insects, including ants, is tightly connected with their ability to protect themselves and their food resources. In exchange for energy‐rich honeydew, ants protect myrmecophilous aphids from various natural enemies. Fungal infection can have disastrous consequences for both mutualist partners, wherein aphids can be disease vectors. Behavioural responses towards fungus‐infected aphids of ant species in nature have scarcely been studied. Here, we studied the behaviour of honeydew foragers of four ant species – Formica polyctena Foerster, Formica rufa L., Formica pratensis Retzius (Hymenoptera: Formicidae, Formicini), and Lasius niger (L.) (Formicidae, Lasiini) – towards Symydobius oblongus (von Heyden) aphids contaminated with the generalist fungal pathogen Beauveria bassiana (Balsamo‐Crivelli) Vuillemin in the field. Aphid milkers from Formica spp. quickly detected and removed infected aphids from the host plant (Betula pendula Roth., Betulaceae). Neither ant species, the degree of aphid‐milker specialization (medium or high), nor the number of honeydew foragers had significant effects on the behaviour of Formica milkers towards infected aphids. Unlike Formica ants, L. niger usually displayed non‐aggressive behaviour (tolerance, antennation, honeydew collection, grooming). By the immediate removal of infected insects, Formica ants seem to minimize the probability of infection of symbionts as well as themselves. Quarantining behaviour may play an important role in ant–aphid interactions as a preventive antifungal mechanism formed under parasite pressure and thus contributing to the ecological success of ants.