Large Ants Research Articles

Effects of abiotic restoration through rock addition on invertebrate functional diversity in native temperate grasslands

Invertebrates account for over 90% of all described species and provide crucial ecosystem services. Land clearing, including the removal of abiotic habitat features, threatens invertebrate biodiversity, making environmental restoration crucial for conservation. However, little is known about the effects of abiotic restoration compared to the restoration of biotic features. To determine if rock addition increased invertebrate biodiversity, we introduced 120 t of rocks to 10 sites across five grassland reserves in the Australian Capital Territory in a before‐after‐control‐impact design. We sampled invertebrates using pitfall traps before treatment in 2016 and after rock addition in 2019 (during record‐breaking drought). We recorded 29,164 individual invertebrates from 19 orders and undertook a functional trait‐based analysis of rock effects on the invertebrate community. No effects of rock addition were observed on the probability of occurrence, richness, diversity, community similarity, or abundance of any taxonomic or functional group. The abundance of snare‐building spiders and large ants was higher in the control plots at the end of the experiment. These abundance responses in 2019 did not differ from the control or treatment plots in 2016. Thus, some combination of drought and rock addition appears to have neutralized a positive temporal change that was evident without rocks. Despite a regionwide, replicated restoration effort and an extensive functional analysis of invertebrate communities, very little positive response to rock addition was recorded. Rock addition did not have a negative effect on grassland invertebrate biodiversity and may still be beneficial to the long‐term conservation and restoration of vertebrate and invertebrate communities.

Cicada feast and food links: Grey-headed flying fox preys on black prince in south-East Australia

Cicada mass emergence offers a plentiful and energy-rich food source for a variety of predators, mainly birds and spiders. Megabats (Pteropodidae) mostly feed on nectar and fruits, but occasionally prey on insects such as sap-sucking cicadas and fruit-eating beetles. We observed and documented with photographs the behavioural repertoire of the Grey-headed Flying Fox Pteropus poliocephalus while hunting and consuming the Black Prince cicada Psaltoda plaga in Sydney, New South Wales, Australia. The bats used three main tactics while hunting the cicadas: a) flushing and/or catching cicadas on tree foliage while clambering over branches or landing on the foliage, b) lunging from a perch after spotting a resting cicada, and c) pursuing cicadas on the wing. Upon securing a cicada the bats flew off and landed on a feeding perch, where they chewed the prey and spat the remains as a pellet below this perch. On the ground, the pellets were usually sought by large ants that tore off pieces from these remains, the ants transporting them to the nest. Small ants aggregated on the pellets probably to seek the remaining sap, after which they left the pellets. The above described events showed three main food links: a) cicadas feeding on plant sap, b) bats preying on cicadas and spitting the remains as pellets, and c) ants feeding on bat pellets. The links illustrate a food chain that include plants, cicadas, bats, and ants. Additional food links we observed were: 1) cicadas feeding on different plants, 2) cicadas preyed on by diurnal animals, 3) dead cicadas consumed by ants, 4) dead bats consumed by birds, and 5) nocturnal raptorial bird that may prey on bats. This report seems the first to mention food links within a megabat-insect trophic interaction.

Wind disrupts trail pheromone communication in the leaf-cutting ant Acromyrmexlobicornis

Although wind is a ubiquitous component of ecological systems and might affect pheromone communication, its effects have not been studied in depth. To test whether wind impacts trail pheromone behaviour, we studied the behaviour of the leaf-cutting ant Acromyrmex lobicornis in natural nests. We conducted observations under (1) unaltered scenarios with different wind intensities and (2) manipulative experiments aimed at altering trail-marking pheromone volatilization rates. In our manipulative experiments, we placed filter paper on ant trails, then removed the pheromone-marked paper 24 h later and either exposed or did not expose the marked paper to artificial wind (2 km/h) for 1 or 3 h. We then put the marked paper back on the trail and filmed ants' responses for 1 min. As controls, we filmed ants’ responses to filter paper immediately before it was removed as well as their responses to new unmarked filter paper. We measured ant flux, speed and walking sinuosity of ants over trails and filter paper, the number of ants tapping their gaster against the substrate (an indication of pheromone-marking frequency), the number of head-on encounters among workers (as alternative modes of communication) and ant body size. Wind negatively affected ant traffic by decreasing ant flux and speed. Colonies increased the number of U-turns and head-on encounters among workers, with a higher response as wind exposure time increased, but trail pheromone deposition was constant among treatments. The size frequency distribution of foragers over the treatment area was skewed towards larger ants on trails with unmarked paper and on trails with paper that had been exposed to wind for 1 or 3 h or not exposed to wind but removed for 3 h. This could be the result of small ants focusing on pheromone trail maintenance, while larger ones focused on foraging on the filter paper irrespective of whether it had been marked. Our results suggest that division of labour and behavioural plasticity might allow leaf-cutting ants to mitigate the effects of wind on pheromone communication and continue foraging. • Wind negatively affected ant traffic on trails by decreasing ant flux and speed. • Ants performed more U-turns and head-on encounters on trails exposed to wind. • Wind exposure did not affect trail pheromone deposition. • Wind exposure influenced the size frequency distribution of forager ants.

Does resilience to fire confer resilience to grazing in savanna ant communities of Northern Australia?

IntroductionEvolutionary traits acquired in response to one type of disturbance will potentially confer resilience to other disturbances that have similar environmental impacts, even if the biota has no evolutionary history of such disturbances. In grassy ecosystems the environmental impacts of grazing have important similarities to those of fire through the removal of grass biomass, and we hypothesise that high resilience to frequent fire confers high resilience to grazing.AimsWe test this hypothesis by investigating the resilience of highly fire-resilient ant communities to grazing in a mesic Australian savanna, which has not historically experienced such high levels of mammalian grazing.MethodsWe sampled ants using pitfall traps at Annaburroo Station in the Australian seasonal tropics using ten plot triplets, with each triplet representing no, low and heavy grazing. Grazing has had a major impact on the basal area of perennial grasses and the cover of bare ground. We considered large (> 4 mm) ants only, which tend to be particularly sensitive to disturbance.ResultsWe recorded 28 species of ‘large’ ants from 14 genera. Neither ant species richness nor overall composition varied significantly with grazing, and only one of the eight most common species responded to grazing.DiscussionAnt communities at Annaburoo Station are highly resilient to livestock grazing. The limited number of relevant studies suggest that ant communities in Australian savannas more generally have higher resilience than those in southern rangelands where fire frequency is relatively low. This supports our hypothesis that an evolutionary history of frequent fire confers resilience to grazing. Many more studies are required, but we suggest that resilience to grazing might be related more to evolutionary history in relation to fire, a more pervasive remover of plant biomass globally, than to grazing.Implications for insect conservationGrazing by cattle is the dominant land-use in Australian savannas and the Australian savanna ant fauna is one of the richest on Earth; our findings of high resilience is therefore good news for insect conservation. Such good news is likely to have wide applicability to other fire-prone grassy ecosystems subject to livestock grazing.

How does the feeding behaviour of the common forest toad Rhinella henseli (Anura: Bufonidae) vary in space and time? Trophic ecology, chemical and antimicrobial activity

Studies in trophic and chemical ecology, in particular in amphibians, have gained increasing attention in recent years, given that this is the vertebrate group that has suffered the greatest decline in recent years, caused by the degradation of natural ecosystems and emerging diseases. The assessment of food preferences and prey availability between areas and seasons provides important parameters for the understanding of the population dynamics of leaf-litter toads. The study of the secretions of the parotoid macroglands of these toads also provides insights into the role of these secretions in fighting frog pathogens and their potential applications to combat pathogens that are harmful to humans. In the present study, we describe the trophic ecology of Rhinella henseli (Lutz, 1934), and the variation in its diet between seasons and areas. We also attempt to identify the chemical composition of the secretions of the parotoid macrogland found in the parotoid glands and test their potential antimicrobial activity. We sampled two toad populations in the Atlantic Forest of southern Brazil. The composition of the diet was analysed by season (warm vs. cool) and study area, with the prey items being identified to genus, whenever possible, and classified using the Index of Relative Importance. The parotoid secretions were removed manually from the parotoid glands and analysed via HPLC-MS/MS. We ran microdilution and agar plug diffusion tests to assess antimicrobial activity. The principal prey of these toads are large ants, primarily Pachycondyla sp., which vary in abundance between seasons and, to a lesser extent, between areas. We identified 21 chemical compounds, primarily steroidal bufadienolides. One of the populations presented a subset of 14 of these 21 compounds, reflecting the variation in their spatial distribution. These compounds presented anti-pathogenic properties against Candida albicans and, to a lesser extent, Staphylococcus saprophyticus. Our results indicate that the diet of R. henseli varies significantly between areas and seasons, as do the secretions of their parotoid macroglands between areas. The toxins exhibit antimicrobial activity, although the compounds must be tested in isolation to confirm this.

Myrmecomorphy in early nymphs of Acontista concinna (Perty) (Mantodea: Acanthopidae: Acontistini), with notes on a new chromatic form

Summary Ant resemblance is common among praying mantis nymphs but switching the model of resemblance across the post-embryonic instars has been previously recorded only for Mantoida maya Saussure & Zehntner and Acontista aurantiaca (Burmeister). This work describes in detail the myrmecomorphy of early nymphs of Acontista concinna (Perty), based on ample material reared from oothecae collected in the field. Mimic models are proposed, and the possible occurrence of transformational mimicry is discussed. The 1st instar nymphs are defended as mimics of Pseudomyrmex gibbinotus (Forel) (Formicidae), but sudden and intense changes in colour and size during the transition to the 2nd instar make them similar to larger ants, namely Camponotus Mayr. This is maintained throughout the 3rd instar, but disappears on the 4th instar, giving place to typical mantid nymphs. A dark brown chromatic form for females of A. concinna is recorded for the first time, and its existence casts further doubts about the value of overall body colour in separating Acontista Saussure & Zehntner from Raptrix Terra.

Relações alométricas entre os tamanhos de sementes artificiais removidas e de formigas em um fragmento florestal na Amazônia Central

As formigas são os principais dispersores invertebrados de sementes encontradas no solo. No entanto, ainda conhecemos pouco sobre a história natural das espécies de formigas e como elas poderiam atuar na dispersão de sementes. Em busca de padrões alométricos entre formigas e sementes, que poderiam ser extrapolados para outros locais e outras espécies, comparamos modelos de sementes artificiais com tamanhos diferentes e os relacionamos com algumas medidas de tamanho das formigas. As sementes artificiais foram oferecidas em seis transectos com dez pontos de observação instalados no campus da Universidade Federal do Amazonas, em Manaus. Foram registradas 20 espécies de formigas interagindo com as sementes artificiais. Formigas do gênero Ectatomma removeram uma quantidade maior de sementes, seguidas por Pheidole, Odontomachus e Pachycondyla. A maior distância percorrida foi de 5,10 m, em um evento de dispersão por E. brunneum Smith, 1858. Formigas menores de 2 mm não removeram nenhum dos modelos de sementes artificiais, mas consumiram o arilo artificial no local onde a semente foi oferecida. Nossos resultados reforçam que a qualidade da dispersão de sementes é dependente da identidade do parceiro (espécie de formiga), mas que formigas maiores removem mais sementes e a distâncias maiores

Ant Morphology Mediates Diet Preference in a Neotropical Toad (Rhinella alata)

Despite the widespread occurrence of myrmecophagy in anurans, it is unclear whether ant-specialists feed on ants opportunistically or whether they prefer species with specific morphological, ecological, or nutritional traits. We flushed 105 stomachs of a lowland neotropical toad, Rhinella alata, and identified each consumed ant to species level. We calculated linear selectivity to determine predator preference for ants by comparing the abundances of consumed species to their relative abundances in the leaf litter community on Barro Colorado Island, Panama. We conducted linear regression models to test whether linear selectivity or general predator preference related to seven morphological characteristics and two measurements of nutritional content. Rhinella alata preferentially ate 24 ant species. Other species were either avoided (n = 34) or were eaten opportunistically (n = 26). Preferred ant species were large and textured with hair and/or rugosity. We found that prey preference did not relate to prey nutritional content, that small ants were avoided even if they were superabundant in the environment, and that chemically defended and aggressive ants were preferred if they were large enough. We propose that R. alata prefers large ants because they represent a more efficient prey item in terms of prey handling time and because they are easier to see than are smaller ants. Furthermore, we hypothesize that predation attempts are more successful when prey are textured because microstructures on the tongue and prey surface may increase prey adhesion. The ant specialist R. alata is not specializing on any particular ant species but rather maximizing prey quantity over quality by only eating the largest ants, despite their scarcity in the environment.

Shear adhesive performance of leaf‐cutting ant workers (Atta cephalotes)

AbstractWingless arboreal ants must resist the force of gravity while traversing substrates in their environment. For leaf‐cutting ants like Atta cephalotes, foraging may also include a ca. 30 m vertical descent while carrying a load 1–6 times their body mass. We hypothesized that heavier and larger ants would carry heavier and larger loads and that adhesive performance would positively correlate with load mass. We found no relationship between ant mass, body length, head width, or adhesive performance, and the load size an ant carried. In addition to workers carrying vegetative loads (most often leaves), workers in an active foraging trail also include smaller workers riding on the leaves carried by larger workers, and large major workers, providing protection from aerial and ground attacks (Soldiers), respectively. Despite varying functional roles, all foraging ants require secure attachment to the substrate. We measured shear adhesive performance of each foraging role and found that Soldiers produced the highest shear adhesive forces. However, when controlling for tarsal pad area, we found that ants carrying loads have higher shear adhesive performance per unit area than those riding on leaves, and that Soldiers have the lowest shear adhesive performance per unit area. This suggests that while leaf choice does not appear to be dictated by size, mass, or shear adhesive performance of individual ants, overall, ants who carry leaves adhere more strongly given their pad size than those who do not.Abstract in Spanish is available with online material.

Effects of park size, peri-urban forest spillover, and environmental filtering on diversity, structure, and morphology of ant assemblages in urban park

Urban parks are likely the last green areas to preserve fauna diversity in urban ecosystems. We predicted that 1) species richness and ant diversity would increase with increasing park size, and species richness and ant diversity would be inversely related to distance from a peri-urban forest. 2) Larger ants would be predictably prevalent in less-complex habitats but the reverse should be true for small ants. The study was conducted in 47 parks of varying size located in Taichung City, Taiwan. In total, 14,324 ant individuals from 50 morphospecies and 22 genera were collected. No relationship between ant diversity and distance from a peri-urban forest. No significant clustering in functional composition with increasing park size and distance from a peri-urban forest. However, urban ant diversity marginally increased with increasing park size. Larger parks, characterize by heterogeneous fine-scale habitats, had the highest ant species and abundance of ants. The prevalences of opportunist, generalised myrmicinae (GM), and cryptic ant species were linked to increased soil moisture, number of tree species, and leaf litter depth. The positive association between GM and the number of trash bins. Our finding disproved the size-grain hypothesis, but hotter ground surface favored the presence of long-legged ants. The significant association between head width, inter-eye distance, eye width, and environmental variables such as understory vegetation cover, leaf litter depth, and soil temperature suggest that predaceous ants might be prevalent in heterogeneous fine-scale local microhabitats. In conclusion, our study evidenced the importance of heterogeneous fine-scale habitats in urban park to biodiversity.

Large ants don't carry their fair share: Maximal load carrying performance of leaf-cutter ants (Atta cephalotes)

Although ants are lauded for their strength, little is known about the limits of their load-carrying abilities. We determined the maximal load-carrying capacity of leaf-cutter ants by incrementally adding mass to the leaves they carried. Maximal load-carrying ability scaled isometrically with body size, indicating that larger ants had the capacity to lift the same proportion of their body mass as smaller ants (8.78times body mass). However, larger ants were captured carrying leaf fragments that represented a lower proportion of their body mass compared with their smaller counterparts. Therefore, when selecting leaves, larger ants retained a higher proportion of their load-carrying capacity in reserve. This suggests that either larger ants require greater power reserves to overcome challenges they encounter along the trail or leaf-cutter ants do not select loads that maximize the overall leaf transport rate of the colony.

Individual size as determinant of sugar responsiveness in ants

Social insects commonly exhibit division of labor in non-reproductive tasks. Task allocation may be related to size, form, and ergonomic differences when workers are anatomically variable. Carpenter ants Camponotus mus collecting nectar exhibit a wide forager size variation, thus raising the question of whether large and minor workers differ in their gustatory responsiveness and specialize, therefore, on different nectar sources. To answer this question, we first established the sucrose concentration at which small and large ants in the laboratory respond appetitively to a sugar solution (sucrose acceptance threshold, SAT) after experiencing a high or a low starvation regime (4- or 1-day carbohydrate deprivation, respectively). Under high starvation, no differences in SATs were found between larger and smaller ants. Under low starvation, both sizes increased their SATs but larger ants had a higher SAT, thus preferring more concentrated solutions while smaller ants responded mostly to more diluted sucrose solutions. In a field assay in which the distribution of larger and smaller ants on sugary food sources was analyzed, small and medium ants were found—in different proportions—at all food sources while larger ants were only found at nectar sources with a higher sugar flow rate, i.e., providing more sugar per unit time. Both field and laboratory assays supported that sugar-related parameters act as determinants of the size distribution of ants among food sources. In addition, interindividual differences in alternative non-sugar-related variables may contribute to this distribution, leading thereby to a potential nectar foraging specialization. Task specialization is crucial for the ecological success of social insects. Carpenter ants allocate individuals of variable size to foraging, thus raising the question of whether they differ in their food preferences. We determined the sugar concentration at which an appetitive response occurs in small and large carpenter ants, and analyzed their distribution on natural and artificial nectar sources in a field assay. Under low starvation, larger ants responded more than smaller ants to higher sucrose concentrations. Coincidently, in the field assay, they were mainly present at sources with higher sucrose delivery. This kind of specialization may reflect the fact that larger ants have larger feeding apparatuses, which may confer a better capacity to deal with the higher viscosity of more concentrated nectars and allow collecting more food at nectaries with higher sugar flow rates. Size specialization may thus increase colony success in the exploitation of variable food sources.

Colony size, habitat structure, and prey size shape the predation ecology of a social pseudoscorpion from a tropical savanna

Predation strategies are driven by habitat structure, prey’s nutritional value, and/or by predator characteristics such as developmental stage. Here, we evaluated the feeding habits of the social pseudoscorpion Paratemnoides nidificator in two areas of a Brazilian Cerrado savanna. These pseudoscorpions live under tree bark trunks of varying sizes; habitat structure could interfere with pseudoscorpion ambushing behavior and prey accessibility. We therefore assessed the hypotheses that (i) small and large colonies of P. nidificator will capture prey of distinct amounts and sizes; (ii) habitat structure will limit the captured prey size; (iii) there will be an age-dependent prey choice in P. nidificator. We evaluated the prey items, colony composition, and habitat structure of pseudoscorpions and determined whether P. nidificator presents age-dependent feeding preferences by offering prey items of different sizes. Colonies with more individuals captured more prey items and those prey presented a wider size variety. P. nidificator can capture a high variety of prey sizes by using openings in tree bark as a trap; however, only tree barks of intermediate size amplitude may be used for trapping most prey. Nymphs showed no preference for prey size, while adults mainly fed on larger ants. Tree bark may play a role as a phenotype extension by easing the process of large prey capture, which is considered a crucial factor for social species. Small prey might be a complementary food resource for nymphs, reducing intraspecific competition and their exposure to larger, dangerous prey. Habitat structure and prey’s traits such as size affect the predation strategies of several animals. How these features interfere in the feeding habits and prey accessibility of social arachnids is a matter of question. We showed that habitat structure and colony size drive the prey size preference of a social pseudoscorpion. Paratemnoides nidificator lives under tree trunk barks that vary in size, depth, and shape. The tree bark openings may play a crucial role by easing the capture process of different prey sizes, including large prey, which is considered a crucial factor for social species. According to the prey size hypothesis, social species require, collectively, higher amounts of food energy. Thus, we propose that the bark openings are related to the evolution of P. nidificator’s social behavior as they potentially allow the capture of larger and more nutritious prey.

Soil surface complexity has a larger effect on food exploitation by ants than a change from grassland to shrubland

1. Changes in vegetation community composition, such as a transition from grassland to shrubland (woody encroachment), are associated with reductions in plant cover and increases in bare ground. Encroachment‐driven changes in surface cover at small spatial scales can alter ant community assemblages by changing their foraging behaviour and their ability to locate and monopolise resources.2. Artificial arenas with three levels of complexity were used to examine changes in ant foraging efficiency, body size and ability to monopolise food. The three levels of complexity included a control (no substrate), low‐complexity treatment (woody debris) and high‐complexity treatment (leaf litter).3. No difference was found in ant species composition within the complexity arenas between grassland and shrubland, but ant functional groups ‘generalised Myrmicinae’ and ‘subordinate Camponotini’ were more abundant in grassland arenas, whereas ‘opportunists’ were more abundant in shrubland arenas. Ants took twice as long to find baits in high‐complexity treatments, and 1.5 times as long in low‐complexity treatments, than in control treatments, which were bare arenas with no substrate. Ant body size declined with increasing surface complexity, suggesting that larger ants are discouraged from foraging in complex habitats.4. There was also significantly greater monopolisation of the protein bait (tuna) in low‐ and high‐complexity treatments, but there were no differences between tuna and carbohydrate (honey) in the control treatment. Consistently, no differences were found in ant behaviour between grasslands and shrublands.5. The present study shows that ants are more responsive to small‐scale alterations in soil surface complexity than to changes in vegetation community composition. Changes in soil surface complexity select for ants based on body size, which in turn influences their foraging success. Changes in vegetation complexity at small spatial scales are therefore likely to influence ant behaviour and abundance of some functional groups, potentially having an effect on the many ecosystem functions carried out by ants.

The flight ecology of ants (Hymenoptera: Formicidae)

Most of the world’s ant species rely on flight for reproduction and dispersal, during a solitary phase in which colony fitness depends only on the survival of individual queens. Flight-related selection shapes ant physiology, such that queens and males fly for short durations but carry heavy loads due to the nutrient demands of mating and colony founding. Ants vary by four orders of magnitude in flight distance, with larger ants or those with lighter abdomens flying farther than smaller or heavier ones. Flight tradeoffs explain much variation in ant life history, including the temporal segregation of flight and egg production, the continuum of ant mating systems from male aggregation to female calling syndromes, and the evolution of alternate colony founding strategies. Flight performance also constrains range expansions or shifts in response to invasions or climate change. Flying queens and males act as dispersal vectors for pathogenic or symbiotic organisms, and are eaten in large numbers by aerial insectivores. By entering aerial food webs, flying ants help mediate the flow of energy and materials through ecosystems. They are also model systems for addressing several questions, including nutrient allocation tradeoffs and the evolution of reproductive polymorphisms.

Seed Dispersal Distances by Ants Increase in Response to Anthropogenic Disturbances in Australian Roadside Environments

Ants provide a common dispersal vector for a variety of plants in many environments through a process known as myrmecochory. The efficacy of this dispersal mechanism can largely determine the ability of species to track changes in habitat availability caused by ongoing land-use and associated disturbances, and can be critical for population gene flow and persistence. Field studies were conducted in a typical fragmented agricultural landscape in southern NSW, Australia, to investigate the extent to which dispersal services by ants are influenced by anthropogenic disturbances associated with roadwork activities (i.e. soil disturbance as the result of grading of roads). Observational experiments were performed in road segments that were divided into disturbed and non-disturbed zones, where Acacia pycnantha seeds were offered at multiple bait stations and monitored. For combined species, the mean dispersal distance recorded in the disturbed zone (12.2m) was almost double that recorded in the non-disturbed zone (5.4m) for all roadside sites. Our findings show that myrmecochory is an unevenly diffuse mutualism, where few ant species contributed to much of the dispersal of seeds. Iridomyrmex purpureus was responsible for all seed dispersal distances > 17m, where a maximum of 120m in disturbed, versus 69m in non-disturbed zones, was recorded. Rhytidoponera metallica and Melophorus bruneus were important seed dispersers in non-disturbed and disturbed zones, respectively. In general, large bodied ants tended to move more seeds to longer distances in disturbed zones, as opposed to non-disturbed zones, where smaller bodied species carried out a greater percentage of short distance dispersals (< 1m). We also recorded secondary dispersal events from nests by I. purpureus, a phenomenon previously not quantified. Infrequent, long distance dispersal to suitable sites may be highly important for seedling recruitment in disturbed or modified habitats in otherwise highly fragmented rural environments.

Flexible task allocation and raid organization in the termite-hunting ant Megaponera analis

Division of labor is one of the main reasons for the success of social insects. Worker polymorphism, age polyethism and work division in more primitive ants, such as the ponerines, remain mostly unexplored. The group hunting, termite-specialist Megaponera analis conducts raids in column formations of 200–500 ants. Since these ants specialize on a defensive prey, adaptations to reduce their foraging costs can be expected. We found that the work division, task allocation and column formation during the hunt were much more sophisticated than was previously thought. The column-formation was remarkably stable, with the same ants resuming similar positions in subsequent raids and front ants even returning to their positions if displaced in the same raid, suggesting yet unknown regulatory mechanisms for the formation of the column. We identified three previously undescribed tasks during the hunting process of M. analis: lingerers, runners and raid-guards. Most of these tasks were not executed by predetermined members of the raid but were filled out as need arose during the hunt, with a clear preference for larger ants to conduct most tasks. The plasticity of task allocation was particularly well exemplified by the termite carriers, with the number of small ants carrying termites only starting to rise when less large ants were available. We, therefore, propose that the continuous allometric size polymorphism in M. analis allows for greater flexibility in task allocation, necessary due to the unpredictability of task requirements in an irregular system such as hunting termites in groups.

Experimental Study of Ant Movement in a Straight Passageway under Stress Conditions

In the present study, we investigated whether single-file traffic of the ant Camponotus japonicus would exhibit a transition to traffic-jamming with increasing density under stress conditions. Previous work indicated that this transition did not occur in non-stressed ants, in contrast to data from pedestrians and vehicular traffic. Citronella oil was used to elicit ant movement from one end of a unidirectional passage to the other. The movements were recorded with a video camera, and the speed and distance travelled were extracted using image processing. We provided fundamental diagrams of ant traffic for small and large ants under stress conditions. Examination of the relationship between flow rate and density revealed no evidence of jamming, different from pedestrian and vehicular traffic. Concerning the speed-density relationship, surprisingly, in contrast with human fundamental diagrams (vehicular and pedestrian), we found that speed seemed to be constant with density within the experimental error. This study provided novel experimental data on ant traffic and might inform future studies of collective behavior of social insects and traffic systems.

PLASTICIDADE DO COMPORTAMENTO DE FORRAGEAMENTO EM FORMIGAS CORTADEIRAS

The foraging behavior in social insects is a complex activity that involves a self-organizing system and can be studied at the individual or at the colony level. In the ant society, not all individuals participate in the gathering of food. The workers who perform this task are often older and larger ants. In Attini ants, the foraging is a complex process that involves selection, cutting and transport of food resources to the nest. These ants use several sources of food to develop the symbiotic fungus, which provides food to the colony. The genera Atta and Acromyrmex , known as leaf-cutter ant, have a well-defined pattern of foraging determined by physical and chemical trails. In the basal genera the worker ants forage alone, hardly ever cut fresh vegetation, do not follow trails and there is no fidelity to the resource gathered. Besides the differences between groups, foraging behavior may be different in the same specie as consequence of environmental factors and the adaptability of workers. Other phenomena that make the Attines an interesting model to study foraging behavior are the polymorphism and polyethism. Several studies carried in Atta species demonstrated the existence behavioral plasticity of foraging behavior of ants. In this context, this review analyzes the adaptability of leaf-cutter ants to solve problems fitting their behavior, both individually and collectively to adopt the most convenient strategy to exploit as society several sources of food.

Competition can lead to unexpected patterns in tropical ant communities

Ecological communities are structured by competitive, predatory, mutualistic and parasitic interactions combined with chance events. Separating deterministic from stochastic processes is possible, but finding statistical evidence for specific biological interactions is challenging. We attempt to solve this problem for ant communities nesting in epiphytic bird’s nest ferns (Asplenium nidus) in Borneo’s lowland rainforest. By recording the frequencies with which each and every single ant species occurred together, we were able to test statistically for patterns associated with interspecific competition. We found evidence for competition, but the resulting co-occurrence pattern was the opposite of what we expected. Rather than detecting species segregation—the classical hallmark of competition—we found species aggregation. Moreover, our approach of testing individual pairwise interactions mostly revealed spatially positive rather than negative associations. Significant negative interactions were only detected among large ants, and among species of the subfamily Ponerinae. Remarkably, the results from this study, and from a corroborating analysis of ant communities known to be structured by competition, suggest that competition within the ants leads to species aggregation rather than segregation. We believe this unexpected result is linked with the displacement of species following asymmetric competition. We conclude that analysing co-occurrence frequencies across complete species assemblages, separately for each species, and for each unique pairwise combination of species, represents a subtle yet powerful way of detecting structure and compartmentalisation in ecological communities.