Do ants make direct comparisons?

Many individual decisions are informed by direct comparison of the alternatives. In collective decisions, however, only certain group members may have the opportunity to compare options. Emigrating ant colonies (Temnothorax albipennis) show sophisticated nest-site choice, selecting superior sites even when they are nine times further away than the alternative. How do they do this? We used radio-frequency identification-tagged ants to monitor individual behaviour. Here we show for the first time that switching between nests during the decision process can influence nest choice without requiring direct comparison of nests. Ants finding the poor nest were likely to switch and find the good nest, whereas ants finding the good nest were more likely to stay committed to that nest. When ants switched quickly between the two nests, colonies chose the good nest. Switching by ants that had the opportunity to compare nests had little effect on nest choice. We suggest a new mechanism of collective nest choice: individuals respond to nest quality by the decision either to commit or to seek alternatives. Previously proposed mechanisms, recruitment latency and nest comparison, can be explained as side effects of this simple rule. Colony-level comparison and choice can emerge, without direct comparison by individuals.     

Central place foraging by an ant (Cataglyphis bicolor Fab.): a model of searching

A computer model of the searching activity of Cataglyphis bicolor (Fab.), based on quantitative data obtained in the field in Greece, was investigated. This ant searches for individual items of food (mainly dead arthropods) in the open on flat ground, on which its paths can be followed and mapped. The model was based on an analogy with molecular diffusion from a centre, suggested by the fact that the average distance of ants from the nest increased in proportion to the square root of time, up to a distance more than half the radius of a circle enclosing 90% of the search area. Actual behaviour was well described by a model in which individual ants went out for the first time to search in a direction determined at random. Subsequently they went out in a straight line in a direction and to a distance related to that of the previous find before commencing a semi-random search, on a path consisting of straight portions branching at random angles, with restriction on turns in a backward direction. Individual real ants search in restricted areas, commonly sectors centred on the nest. This behaviour was reproduced in the model as were quantitative aspects of the nest behaviour. Communication between individuals was not required in the model.     

Weaving resistance: silk and disease resistance in the weaver ant <Emphasis Type="Italic">Polyrhachis dives</Emphasis>

Social insects are at risk from a diverse range of parasites. The antibiotic-producing metapleural gland is an ancestral trait in ants which is thought to be one of their primary mechanisms of resistance. However, the metapleural gland has been lost secondarily in three ant genera, which include weaver ants that are characterised by the remarkable construction of their nests using larval silk. Silken nests may have allowed reduced investment in costly disease resistance mechanisms like the metapleural gland if the silk has antimicrobial properties, as in other insects, or is a hygienic substrate. Here we examine this hypothesis in the weaver ant Polyrhachis dives. We found no evidence of a beneficial effect of silk. The presence of silk did not improve the already high resistance of ants to the entomopathogenic fungus Metarhizium, the ants only rarely interacted with the silk regardless of whether they were exposed to Metarhizium or not, and silk also did not inhibit the in vitro germination or growth of Metarhizium. Furthermore, silk was found in vitro to be heavily contaminated with the facultative entomopathogenic fungus Aspergillus flavus, and many more ants sporulated with this fungus when kept with silk in vivo than when they were kept without silk. Further work is needed to examine the effects of silk on other parasites and of silk from other weaver ants. However, the results in combination suggest that silk in P. dives is unlikely to provide protection against parasites and that it is also not a hygienic substrate. Alternative explanations may therefore be needed for the loss of the metapleural gland in weaver ants.     

The ‘truck‐driver’ effect in leaf‐cutting ants: how individual load influences the walking speed of nest‐mates

The foraging behaviour of social insects is highly flexible because it depends on the interplay between individual and collective decisions. In ants that use foraging trails, high ant flow may entail traffic problems if different workers vary widely in their walking speed. Slow ants carrying extra‐large loads in the leaf‐cutting ant Atta cephalotes L. (Hymenoptera: Formicidae) are characterized as ‘highly‐laden’ ants, and their effect on delaying other laden ants is analyzed. Highly‐laden ants carry loads that are 100% larger and show a 50% greater load‐carrying capacity (i.e. load size/body size) than ‘ordinary‐laden’ ants. Field manipulations reveal that these slow ants carrying extra‐large loads can reduce the walking speed of the laden ants behind them by up to 50%. Moreover, the percentage of highly‐laden ants decreases at high ant flow. Because the delaying effect of highly‐laden ants on nest‐mates is enhanced at high traffic levels, these results suggest that load size might be adjusted to reduce the negative effect on the rate of foraging input to the colony. Several causes have been proposed to explain why leaf‐cutting ants cut and carry leaf fragments of sizes below their individual capacities. The avoidance of delay in laden nest‐mates is suggested as another novel factor related to traffic flow that also might affect load size selection The results of the presennt study illustrate how leaf‐cutting ants are able to reduce their individual carrying performance to maximize the overall colony performance.     

Evolution of nest-weaving behaviour in arboreal nesting ants of the genus Polyrhachis Fr. Smith (Hymenoptera: Formicidae)

Abstract  Polyrhachis ants represent one of the most taxonomically and ecologically diverse ant genera, with over 500 described species organised into 12 subgenera. Nesting habits range from subterranean localities to arboreal nests incorporating silk produced by the ants' own larvae (nest weaving). In this article, we combine scanning electron microscopy and gross observations of the nests of 35 species representing nine subgenera thought to contain individuals that nest above the ground (Cyrtomyrma, Hagiomyrma, Hedomyrma, Hemioptica, Myrma, Myrmatopa, Myrmhopla, Myrmothrinax, Polyrhachis) to revaluate the relationship between nest locale, the type of nest material used and the use of larval silk for nest construction. Nesting habits are highly diverse, ranging from truly arboreal nests on or between leaves and branches, to lignicolous nests inside hollow stems or bamboo internodes and lithocolous nests on the sides of rock walls. Flat sheets of larval silk are used only by arboreal nesting species within the subgenera Cyrtomyrma , Hemioptica , Myrma , Myrmatopa , Myrmothrinax and Polyrhachis . Lignicolous nesting habits were demonstrated predominantly by Hedomyrma spp., but these habits also occur in Myrma and Myrmhopla . Lithicolous nesting habits occur within Hagiomyrma and Hedomyrma , though the actual nesting material used can be either carton or dense masses of spider silk. Based on existing phylogenetic hypotheses, the use of larval silk for nest construction has evolved independently within the genus, and has evolved independently of the construction of silk nests per se . Further examination of the exact type of silk found in the colonies of 'nest-weaving' Polyrhachis is warranted.     

The carrying of ants (Cataglyphis bicolor Fab.) by others of the same nest

The carrying of ants from the nest by others of the same species ( Cataglyphis bicolor Fab.) is described. Two types of carrying are distinguished, systematic removal of many adults and larval stages to a new nest, and carrying of individuals, alive or dead, that are dropped at distances up to 35 metres from the nest. The rate of carrying of individual ants from a nest varied from 0–15 per day in a population of foragers estimated as about 200, making 1500–2000 exits per day. Live and dead ants appeared to be treated similarly. Live ants, in the cases examined, went back to the nest. Evidence was obtained by marking ants that carriers were distinct in habits from non-carriers, and also excavated earth from the nest.     

Near-infrared spectroscopy identifies the colony and nest of origin of weaver ants, <Emphasis Type="Italic">Oecophylla smaragdina</Emphasis>

The ability of social insects to differentiate between colony members and others is essential for the survival of the colony. It enables individuals to direct altruistic behavior towards colony mates, while protecting the colony from intruders. Colonies have a distinct chemical signature that facilitates colony-mate recognition. However, in large polydomous colonies, this signal is likely to be modified by factors unique to each nest. We demonstrate, using near-infrared spectroscopy (NIRS), that individual weaver ants, Oecophylla smaragdina, can be differentiated with respect to their colony and nest of origin. 76.5% of individuals from four colonies could be correctly assigned to their colony of origin; and 79.6% of individuals could be assigned to the correct nest (of two) within their colony. Despite the differences between nests within colonies, in most cases individuals from one nest were more similar to individuals from the other nest within the colony than they were to individuals from any nest outside the colony. Therefore, a distinctive colony identity is maintained despite differences between nests within colonies. We discuss the advantages of using NIRS as a faster and less expensive alternative to the analysis of cuticular hydrocarbons following extraction and identification with gas chromatography/mass spectroscopy. Received 26 November 2007; revised 22 January 2008; accepted 25 January 2008.     

Dispersed central place foraging in Australian meat ants

Summary Australian meat ants often inhabit colonies with widely dispersed nest holes, and this study examines how resource is harvested and distributed in a colony ofIridomyrmex sanguineus Smith (Formicidae: Dolichoderinae). The three principal types of foragers (tenders, honeydew transporters, scavengers) exhibited nest hole fidelity, where harvested resource was consistently delivered to the same nest hole by each foraging individual. Australian meat ants thus use a harvesting system based on dispersed central place foraging. Evidence of frequent larval transport among nest holes, age polyethism developing in the direction of foraging, and the tendency for nest-associated workers to accept new nest holes more readily than foragers, suggests that workers develop fidelity to the particular nest hole in which they eclose. Coupled with larval transport, nest hole fidelity may allow a colony with widely dispersed nest holes to adjust its structure to more efficiently harvest a resource distributed unevenly in space or time.     

Larval gregariousness and neonate establishment of the eucalypt-feeding beetle Chrysophtharta agricola (Coleoptera: Chrysomelidae: Paropsini)

The selective advantage offered to individuals living within groups may relate to natural enemy defence, but in leaf feeding insects may also relate to overcoming plant defences, especially with respect to feeding establishment. We conducted a series of experiments focusing on neonate larval survival, examining the effect of group size and leaf age on the survival of a eucalypt-feeding beetle, Chrysophtharta agricola , which formed groups of up to 43 larvae on the foliage of Eucalyptus nitens in the field. In the laboratory, in the absence of natural enemies, we found that initial density, leaf age and damage to the leaf margin significantly affected larval survival. Survival of solitary first-instar larvae on young foliage was around 80% whereas on older foliage it was around 11%. Prior damage to the leaf margin significantly increased survival on older leaves to around 61%. Initial larval density also affected survival, although mortality was always significantly higher on older leaves. On older leaves the larval group size above which mortality increased no further was over two-fold that on young leaves. Observations of group feeding behaviour at each instar showed that the majority of larvae (75.7%) were aligned facing away from the feeding site and that only around 7.5%, or just 1–2 larvae per group, fed at any one time. Feeding larvae chewed the leaf edge by straddling the leaf margin. Measurements of leaf margins showed that older leaves had significantly thicker leaf margins and 'thickness' ratios (leaf margin to leaf lamina proper). In the field, approximately 85% of all larvae occurred on the first two expanded leaf pairs, and larval mortality was highest between eclosion and establishment of the first instar. However, beetles apparently did not adjust clutch size according to leaf age.     

Tomb evaders: house-hunting hygiene in ants

House-hunting ants avoid otherwise excellent potential nest sites that contain dead ants which may pose risks from poor hygiene. This discovery adds another category to the exceptionally long list of nest site attributes that ants evaluate. It further indicates the importance of disease as a selection pressure on social insects.     

The locomotor activities on sites covered by silk produced by related and unrelated spider mites in Tetranychus urticae (Acari: Tetranychidae)

The two-spotted spider mite Tetranychus urticae is a silk producer known to live in groups with a common silk web that can cover entire plants and protect mites against predators, rain and wind. Silk also plays an important role during collective migration by aerial dispersal or by walking. In this context, we studied the locomotor activity i.e. time in movement, in resting, and in static exploration (probing behaviour) of an individual confronted to clean places and/or places covered by silk coming from its own population or from another. Two populations were tested, one of the red form (Tunisia) and another of the green form (Belgium). The experimental results showed that the presence or the absence of silk did not influence the relative proportion of each behavioural item either for the red or the green population. Individuals of the green form population spent more time moving and less time resting than individuals of the red form population and this, whatever the substrate (red/green silk, clean). Moreover, the silk from the red form population attracted individuals from both populations, whereas the silk produced by the green form population did not attract any individual either from the red or the green form. This surprising result might have been due to a difference in the quantity and/or quality of silk woven by the two forms. We discuss how the differences observed in the behaviour of these two populations may result from differences in their strategy to rapidly increase the population of the colony and reinforce the silk nest.     

Visual Navigation during Colony Emigration by the Ant Temnothorax rugatulus

Many ants rely on both visual cues and self-generated chemical signals for navigation, but their relative importance varies across species and context. We evaluated the roles of both modalities during colony emigration by Temnothorax rugatulus. Colonies were induced to move from an old nest in the center of an arena to a new nest at the arena edge. In the midst of the emigration the arena floor was rotated 60°around the old nest entrance, thus displacing any substrate-bound odor cues while leaving visual cues unchanged. This manipulation had no effect on orientation, suggesting little influence of substrate cues on navigation. When this rotation was accompanied by the blocking of most visual cues, the ants became highly disoriented, suggesting that they did not fall back on substrate cues even when deprived of visual information. Finally, when the substrate was left in place but the visual surround was rotated, the ants'' subsequent headings were strongly rotated in the same direction, showing a clear role for visual navigation. Combined with earlier studies, these results suggest that chemical signals deposited by Temnothorax ants serve more for marking of familiar territory than for orientation. The ants instead navigate visually, showing the importance of this modality even for species with small eyes and coarse visual acuity.     

The attraction of silk: base of group cohesion and collective behavior in social spiders

Social spiders differ from social insects by the production and the use of silk to build irregular webs. This silk prevents dispersion of the individuals and ensures the group cohesion during swarming and collective displacements, playing a part similar to tracks of pheromones in ants. A social spider Anelosimus eximius is attracted by conspecific silk and does not show any discrimination relative to its origin, excluding any group closure. The quantity of silk and the state of satiety of the individual modulate this attraction, and might explain how the spider society adapts the size of the web to its nutritional needs.     

鼎突多刺蚁的营巢习性

1985—1986年在浙江永康县城东南10公里的山坡上观察鼎突多刺蚁(Polyrhachis vicina Roger)的营巢习性;见到蚁巢大多分布于山脚附近、温暖而阳光充足的南坡,一般生活于地面,以刺芒野古草(Arundinella setosaTrin.)为主的禾本科草丛中.蚁巢由工蚁使幼虫吐丝把植物残体、昆虫尸体、泥沙和碎石等物缀织而成.地下部分的深度常受气温的影响:5—10月,气温较高,巢深2—4cm、11—4月,气候寒冷,深度可达10—20cm.每个蚁巢内的所有个体为一相对独立的群体,相邻蚁巢的群体间常相互联系、交换幼期个体和食物等.群体常因季节的变化而有迁移现象:秋季至春季,蚁巢大多位于阳光充足的草地上,夏天,由于气候炎热,蚁巢自草地移向遮荫度较大的树林.8—9月,部分上树筑巢.1—8月,蚁巢数不变,8月以后,蚁巢数开始增加,到10月达到最大,以后又下降,到12月降至通常水平.由此可见,9—10月,蚂蚁分巢,11—12月,部分蚁巢又合并.平均每巢工蚁数夏季最多,其次是春季和冬季,秋季由于分巢而最少.     

Simple rules based on pile slope are used in the self organization of sand pile formation by <Emphasis Type="Italic">Pheidole oxyops</Emphasis> ants

We tested the hypothesis that slope influences where worker ants deposit excavated soil on piles near the nest entrance. We predicted that ants will deposit their load near the top of a pile where the slope changes from upward to downward, to prevent material rolling back towards the entrance. We tested this hypothesis by studying five natural colonies of Pheidole oxyops ants at a field site at S?o Sim?o, Brazil. At this site, each colony was dumping sandy soil excavated from its underground nest in a crescent-shaped pile c. 13 cm from and perpendicular to the nest entrance. Each nest was given an experimental sand pile of symmetrical curved cross section on a plywood platform that could be tilted 15 degrees up or down. From videos, the locations where individual ants dumped their soil loads were measured in relation to the inner (position = 0) and outer (position = 1) edges of the pile. When the platform was tilted down the ants deposited their loads significantly closer to the inner edge (0.458 ± 0.007) than when not tilted (0.530 ± 0.006). When the platform was tilted up the ants deposited their loads significantly further from the inner edge (0.626 ± 0.006) than when not tilted (0.522 ± 0.006). These results support the hypothesis that ants use pile slope in deciding where to dump their load. A similar rule is probably used in other ant species that place excavated soil in steep piles near the nest entrance. Received 5 February 2007; revised 10 June 2007; accepted 9 October 2007.     

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.     

Comparative analysis of experimental testing procedures for the elicitation of rescue actions in ants

Rescue behavior is observed when 1 individual provides help to another individual in danger. Most reports of rescue behavior concern ants (Formicidae), in which workers rescue each other from various types of entrapment. Many of these entrapment situations can be simulated in the laboratory using an entrapment bioassay, in which ants confront a single endangered nest mate entrapped on a sandy arena by means of an artificial snare. Here, we compared numerous characteristics of rescue actions (contact between individuals, digging around the entrapped individual, pulling at its body parts, transport of the sand covering it, and biting the snare entrapping it) in Formica cinerea ants. We performed entrapment tests in the field and in the laboratory, with the latter under varying conditions in terms of the number of ants potentially engaged in rescue actions and the arena substrate (marked or unmarked by ants’ pheromones). Rescue actions were more probable and pronounced in the field than in the laboratory, regardless of the type of test. Moreover, different test types in the laboratory yielded inconsistent results and showed noteworthy variability depending on the tested characteristic of rescue. Our results illustrate the specifics of ant rescue actions elicited in the natural setting, which is especially important considering the scarcity of field data. Furthermore, our results underline the challenges related to the comparison of results from different types of entrapment tests reported in the available literature. Additionally, our study shows how animal behavior differs in differing experimental setups used to answer the same questions.     

Not everything that counts can be counted: ants use multiple metrics for a single nest trait

There are claims in the literature that certain insects can count. We question the generality of these claims and suggest that summation rather than counting (sensu stricto) is a more likely explanation. We show that Temnothorax albipennis ant colonies can discriminate between potential nest sites with different numbers of entrances. However, our experiments suggest that the ants use ambient light levels within the nest cavity to assess the abundance of nest entrances rather than counting per se. Intriguingly, Weber's Law cannot explain the ants' inaccuracy. The ants also use a second metric, independent of light, to assess and discriminate against wide entrances. Thus, these ants use at least two metrics to evaluate one nest trait: the configuration of the portals to their potential homes.     

Leaf caching in Atta leafcutting ants: discrete cache formation through positive feedback

We examined the occurrence, mechanism and costs and benefits of leaf caching in laboratory colonies of two species of leafcutting ants, Atta cephalotes and A. colombica. If foragers returning to the nest are unable to enter because of a temporary bottleneck caused by leaves building up they may deposit their leaf pieces outside the nest entrance, forming a leaf cache. Similar leaf caches occur in the field at foraging trail junctions, obstacles on the trail and within nest entrance tunnels. Foraging ants carrying leaves were presented with different-sized leaf caches and the number dropping their leaves on the cache was recorded. The probability of a forager dropping her leaf was positively correlated with the size of the cache that she encountered. Therefore, positive feedback played a role in the formation of nest entrance caches. Cached pieces were more likely to be retrieved than noncached pieces but the time taken to retrieve leaf pieces from a cache was greater than from scattered groups of leaves. We suggest that the strategy of flexible nest entrance caching is an adaptive response to fluctuating food availability and collection. Copyright 2000 The Association for the Study of Animal Behaviour.     

How ants find each other; temporal and spatial patterns in nuptial flights

Reproduction is a key factor in understanding population ecology and therefore species occurrence. However, patterns in reproductive behaviour for distinct ant species remain insufficiently known. In this paper strategies in mate finding are studied for six ant species (Lasius niger, Lasius umbratus, Temnothorax nylanderi, Myrmica rubra, Myrmica ruginodis, Stenamma debile) in a forest – forest edge – agricultural field gradient. Using window traps, we studied whether these species had a restricted nuptial flight season, displayed swarming behaviour, and whether the alates aggregated at the forest edge. The flight season was limited to one month or less for L. niger, T. nylanderi, M. rubra, M. ruginodis and S. debile. Swarming behaviour occurred in all but one (L. umbratus) species. Although none of the six species seemed to have highest nest density at the forest edge, three of them, M. rubra, M. ruginodis and S. debile, showed male aggregations there, indicating this to be the main reproduction site. This last finding could be due to a more suitable micro-climate, but most likely, edges are conspicuous land marks which are used by ants to meet mates. The behavioural patterns of ant sexuals at the forest edge can influence dispersal possibilities in fragmented landscapes, reproductive success and nest densities. Received 27 November 2007; revised 27 March; accepted 5 April 2008.