Large Ants Research Articles

Larger Seeds are Dispersed Farther: the Long-Distance Seed Disperser ant Aphaenogaster famelica Prefers Larger Seeds

Seed dispersal by ants (myrmecochory) is an important life phase in flowering herbs of temperate deciduous forests, and long dispersal is basically preferable in terms of plant fitness. Although it is known that larger ants transport seeds farther, previous studies conflicted whether larger seeds are preferred by large ants. We examine how seed size and ant body size influence removal rate of seeds and their dispersal distance by ants. The obligate myrmecochorous, large-seed bearing Viola hondoensis and ant-ballistic diplochorous, small-seed bearing Viola spp. and Corydalis pallida were comparatively investigated. 1) Ant preference: attraction rate and transportation rate of large V. hondoensis seeds by ants were consistently high regardless of the disperser ant size whilst those rates of small diplochorous seeds were conspicuously lower for large Aphaenogaster ants than for small Pheidole ants, suggesting small seeds are not adapted to be carried by large ants. 2) Dispersal distance: large V. hondoensis seeds were transported for a long distance because they were carried by both large and small ants while the small diplochorous seeds were transported for a short distance because they were carried only by small ants. These results suggest that large seeds are advantageous to attract large ants and to be farther dispersed as far as ant-mediated dispersal is concerned. On the other hand, the diplochorous small seeds do not attract large ants, and so are transported short by ants although they disperse their seeds by ballistic dispersal.

Thermal adaptation generates a diversity of thermal limits in a rainforest ant community.

The Thermal Adaptation Hypothesis posits that the warmer, aseasonal tropics generates populations with higher and narrower thermal limits. It has largely been tested among populations across latitudes. However, considerable thermal heterogeneity exists within ecosystems: across 31 trees in a Panama rainforest, surfaces exposed to sun were 8°C warmer and varied more in temperature than surfaces in the litter below. Tiny ectotherms are confined to surfaces and are variously submerged in these superheated boundary layer environments. We quantified the surface CTmin and CTmax s (surface temperatures at which individuals grew torpid and lost motor control, respectively) of 88 ant species from this forest; they ranged in average mass from 0.01 to 57mg. Larger ants had broader thermal tolerances. Then, for 26 of these species we again tested body CTmax s using a thermal dry bath to eliminate boundary layer effects: body size correlations observed previously disappeared. In both experiments, consistent with Thermal Adaptation, CTmax s of canopy ants averaged 3.5-5°C higher than populations that nested in the shade of the understory. We impaled thermocouples in taxidermy mounts to further quantify the factors shaping operative temperatures for four ant species representing the top third (1-30mg) of the size distribution. Extrapolations suggest the smallest 2/3rds of species reach thermal equilibrium in <10s. Moreover, the large ants that walk above the convective superheated surface air also showed more net heating by solar radiation, with operative temperatures up to 4°C higher than surrounding air. The thermal environments of this Panama rainforest generate a range of CTmax subsuming 74% of those previously recorded for ant populations worldwide. The Thermal Adaptation Hypothesis can be a powerful tool in predicting diversity of thermal limits within communities. Boundary layer temperatures are likely key to predicting the future of Earth's tiny terrestrial ectotherm populations.

Load Capacity of Workers of Atta robusta During Foraging (Hymenoptera: Formicidae)

Worker ants are highly polymorphic in the genus Atta and they are usually classified into castes according to the specific functions they perform in a colony. Minor workers (head width ≤ 2.0 mm) help to maintain and grow the symbiotic fungus whereas larger workers (head width &gt; 2.0 mm) cut and transport plant fragments. This study investigated the roles in the cutting and transporting of different plant resources of different Atta robusta worker classes that were classified based on the size of their head capsule. Experiments were conducted in the restinga of Grussaí/Iquipari, São João da Barra, Rio de Janeiro State, Brazil. In each month between October 2009 and September 2010, we collected 100 ants and their respective loads from the trails of four nests of Atta robusta. The samples were individually transported to the laboratory, where the ants and their loads were weighed and the head capsules of the ants were measured. Large ants transported heavier loads. These ants usually transported more fruit and seeds than smaller ants.

How fire ant architects connect to build balls

For red fire ants (Solenopsis invicta), rain gently drumming on the ground is the trigger for a mass exodus. Streaming from their nest as the water levels rise, the ants rapidly assemble and grip onto their nearest neighbours, forming a raft to carry them to safety. What is more miraculous is that each individual ant is denser than water and in danger of sinking if submerged. However, the ants don't just draw the line at constructing rafts: they routinely form bivouacs, assemble towers and even coalesce into droplets when swished in a cup. ‘You can consider them as both a fluid and a solid’, explains David Hu from the Georgia Institute of Technology, USA, who is most interested in the ants because they are large enough for him to begin to find out how they interact to pull off these remarkable engineering feats. Hu teamed up with Paul Foster and Nathan Mlot to investigate how balls of living fire ants self-assemble (p. 2089).Gently swirling 110 ants in a beaker to form a sphere, the team then swiftly froze the structure in liquid nitrogen and coated it in Super Glue™ vapour to preserve the minute contacts within, ready for Angela Lin to visualise the structures in a CT scanner. ‘With the CT scan we can focus on individual ants and see how they are connected to their neighbours’, explains Hu, who adds that processing the images could only be partially automated because it is hard to tell where one ant ends and another begins.However, after months of painstaking scrutiny, Foster and Hu discovered that on average, each ant participated in 14 contacts – reaching out with all six legs to grip neighbours and receiving eight contacts back to its body – although large ants participated in as many as 20 contacts and the smallest ants participating in only eight. ‘It turns out that 99% of the legs are connected to another ant and there are no free loaders’, says Hu, who admits that he was impressed by the high degree of connectivity.Next Foster digitally removed all of the limb connections so that he could take a closer look at the ways that the ants' bodies packed together, and he was amazed to see that instead of clustering together in parallel, like grains of rice in a jar, the ants had actively oriented their bodies perpendicular to each other. ‘They have to be alive to do that,’ says Hu, adding, ‘It requires some intelligence, and suggests that somehow they sense their relative orientation.’ The duo also analysed how closely the ants' bodies packed together and realized that the smaller ants were packing in to fill the gaps between the larger ants to increase the number of contacts. They also noticed that the ants were actively pushing on each other, using their legs like tiny jacks to increase the distance between neighbours and reduce the density of the ball. Hu explains that by introducing air pockets between their bodies, the ants increase their water repellency and buoyancy, which is why their rafts are so effective.Finally, Hu and Foster took a closer look at the contacts made by individual ants with a scanning electron microscope and saw that the insects rarely used their mandibles to grip on to other ants. Instead they mainly used their legs, holding on with hooks on their feet and the sticky pads that allow them to walk on vertical surfaces.So, having discovered how fire ants self-assemble to form light but stable structures, Hu is keen to know how they react to reinforce weak points in structures where ant architecture could fail.

Foraging Ecology of the Tropical Giant Hunting Ant Dinoponera australis (Hymenoptera: Formicidae)—Evaluating Mechanisms for High Abundance

AbstractA central goal of ecology is to understand the mechanisms behind variation in the abundance of species. Food web theory predicts higher biomass for animals at lower trophic levels. However, some high trophic level species may reach great abundance via highly efficient foraging behaviors. We evaluated ecological and behavioral traits of the giant tropical ant Dinoponera australis related to these mechanisms. We determined its distribution and abundance, documented its foraging behavior, and measured its trophic position in a population at P.N. Iguazú, Argentina. We report that D. australis colonies are overdispersed, and the species reaches a wet biomass of more than 2.5 kg/ha at this site. Dinoponera australis foraging behavior is characterized by route fidelity of individual workers, with different individuals specializing on different areas around the nest. Finally, stable isotopic evidence and direct observation suggest these ants are among the top predators in this terrestrial invertebrate community. We interpret our findings in the context of how the behavior of an abundant top predator creates an exception to the usual trade‐off between biomass and trophic level. Together these data provide insight into the biology of one of the world's largest ants and why they may be able to attain such high densities where they occur.

A revision of the ant-like sac spider genus Apochinomma Pavesi 1881 (Araneae: Corinnidae) in the Afrotropical Region

The genus Apochinomma Pavesi, 1881 is revised in the Afrotropical Region. The male and female of the type species, A. formicaeforme Pavesi, 1881, are redescribed. Five new species are described: A. malkini sp. nov. (♂ ♀) from Nigeria, A. parva sp. nov. (♂) from Guineé, and A. tuberculata sp. nov. (♀) from the Ivory Coast in the A. formicaeforme species group, and A. decepta sp. nov. (♂ ♀) from South Africa and Mozambique, and A. elongata sp. nov. (♂) from Botswana, Malawi and Tanzania in the A. decepta species group. Another undescribed species, known only from juveniles, belongs to the latter species group. Members of the A. formicaeforme species group mimic Polyrhachis ants and are thought to be primarily arboreal, whereas the A. decepta species group has ground-dwelling or grass-dwelling representatives that mimic large ground-dwelling ponerine ants. A single case of A. formicaeforme feeding on its model, Polyrhachis gagates, is noted.http://www.zoobank.org/urn:lsid:zoobank.org:pub:04116235-B195-48B0-B27E-076DAF4F51FD

Dealing with water deficit in Atta ant colonies: large ants scout for water while small ants transport it

SummaryLeafcutter ants (Atta sexdens rubropilosa) (Forel 1908) have an elaborate social organization, complete with caste divisions. Activities carried out by specialist groups contribute to the overall success and survival of the colony when it is confronted with environmental challenges such as dehydration. Ants detect variations in humidity inside the nest and react by activating several types of behavior that enhance water uptake and decrease water loss, but it is not clear whether or not a single caste collects water regardless of the cost of bringing this resource back to the colony. Accordingly, we investigated water collection activities in three colonies of Atta sexdens rubropilosa experimentally exposed to water stress. Specifically, we analyzed whether or not the same ant caste foraged for water, regardless of the absolute energetic cost (distance) of transporting this resource back to the colony. Our experimental design offered water sources at 0 m, 1 m and 10 m from the nest. We studied the body size of ants near the water sources from the initial offer of water (time = 0) to 120 min, and tested for specialization. We observed a reduction in the average size and variance of ants that corroborated the specialization hypothesis. Although the temporal course of specialization changed with distance, the final outcome was similar among distances. Thus, we conclude that, for this species, a specialist (our use of the word “specialist” does not mean exclusive) task force is responsible for collecting water, regardless of the cost of transporting water back to the colony.

A Carnivorous Plant Fed by Its Ant Symbiont: A Unique Multi-Faceted Nutritional Mutualism

Scarcity of essential nutrients has led plants to evolve alternative nutritional strategies, such as myrmecotrophy (ant-waste-derived nutrition) and carnivory (invertebrate predation). The carnivorous plant Nepenthes bicalcarata grows in the Bornean peatswamp forests and is believed to have a mutualistic relationship with its symbiotic ant Camponotus schmitzi. However, the benefits provided by the ant have not been quantified. We tested the hypothesis of a nutritional mutualism, using foliar isotopic and reflectance analyses and by comparing fitness-related traits between ant-inhabited and uninhabited plants. Plants inhabited by C. schmitzi produced more leaves of greater area and nitrogen content than unoccupied plants. The ants were estimated to provide a 200% increase in foliar nitrogen to adult plants. Inhabited plants also produced more and larger pitchers containing higher prey biomass. C. schmitzi-occupied pitchers differed qualitatively in containing C. schmitzi wastes and captured large ants and flying insects. Pitcher abortion rates were lower in inhabited plants partly because of herbivore deterrence as herbivory-aborted buds decreased with ant occupation rate. Lower abortion was also attributed to ant nutritional service. The ants had higher δ15N values than any tested prey, and foliar δ15N increased with ant occupation rate, confirming their predatory behaviour and demonstrating their direct contribution to the plant-recycled N. We estimated that N. bicalcarata derives on average 42% of its foliar N from C. schmitzi wastes, (76% in highly-occupied plants). According to the Structure Independent Pigment Index, plants without C. schmitzi were nutrient stressed compared to both occupied plants, and pitcher-lacking plants. This attests to the physiological cost of pitcher production and poor nutrient assimilation in the absence of the symbiont. Hence C. schmitzi contributes crucially to the nutrition of N. bicalcarata, via protection of assimilatory organs, enhancement of prey capture, and myrmecotrophy. This combination of carnivory and myrmecotrophy represents an outstanding strategy of nutrient sequestration.

Dry season herbivore utilization of open grasslands in Lower Zambezi National Park, Zambia

Utilisation of open grassland plains by large herbivores (≥100 kg) and harvester ants (Messor capensis) in Lower Zambezi National Park, Zambia was assessed every September from 1997 to 2007. A point intercept method was used to estimate percent cover for grass, shrub, bare, litter, herbivore droppings and presence of harvester ants in the Jeki open grassland vegetation community as indicators of range condition. Results showed a reduction in grass cover from 35% in 1997 to 10% in 2007 while litter remained stable. Incidence of herbivore droppings and harvester ants showed a decline while shrub cover and species composition of shrubs increased significantly. As grass cover and grass species composition declined, herbivore droppings and incidence of harvester ants also decreased. Unpalatable species such as Vernonia spp. were assumed to be signs of poor quality range. Increase in bush encroachment indicated heavy utilization by herbivores in the dry season when quality of range deteriorated as a consequence of over utilization of grass species. Further research is required to determine population estimates and grazing capacity of key herbivores such as buffalo (Syncerus caffer). Key words: Range utilization, overgrazing, flood plains, valley floor, shrub encroachment.

Ants as Bioindicators of Natural Succession in Savanna and Riparian Vegetation Impacted by Dredging in the Jequitinhonha River Basin, Brazil

We investigated the response of ant species to landscape and geomorphologic parameters of a long‐term (7–11 years) restoration project in the Jequitinhonha River (Northern State of Minas Gerais, Brazil) margins, previously dredged by a diamond mining company. Geomorphological changes from the dredging were severe and the area is unlikely to be adequately restored, mainly due to the negative effects of flooding. Our hypothesis is that ant species assemblages bioindicate successional stages and soil characteristics. We studied the association of effects from the river's flooding zone, the native vegetation, and sedimentary grain size with that of ant species diversity, abundance, and composition. An ant sampling program was conducted in April 2005, using three methods: baits, pitfall traps, and direct collection. Grain size was measured by sieving. In total, 10,784 ants were sampled, belonging to 7 subfamilies, 24 genera and 45 morphospecies. Ant species richness was greater in the undisturbed savanna area than in the restored habitats, and equivalently greater in the ecotone and intermediate zone habitats than on the river bank, the poorest habitat.Atta sexdens rubropilosaindicated a condition related to small forest remnants having well‐structured soil. On the other hand, ants with a body length of under 0.5 cm (Dorymyrmex pyramicusandPheidole fallax) predominated in sandy areas, where the majority of the granules were the finest. The lack of organic matter and soil structure for constructing suitable nests may prevent large ants from colonizing such areas, and thus inhibit the advance of natural succession.

Birds and ants provide complementary seed dispersal in a neotropical savanna

Summary 1. A substantial portion of the crop of fruiting trees falls beneath parent plants as a result of dispersal failure. Such diaspores are considered as waste because the likelihood of plant recruitment is usually very small close to parent trees. However, many animals may rescue fallen diaspores and provide them with another chance of dispersal and establishment. 2. We investigated the effectiveness of two broad types of seed dispersal vectors for the regeneration of Xylopia aromatica in the Brazilian cerrado savanna: birds that remove diaspores from plant canopies and ants that harvest diaspores on the ground under the parent plant (as rescuers) or from bird feces (as secondary dispersers). 3. Birds removed a mean of 32% of the crop from plant canopies, but removal was independent of crop size. A large part of the crop (mean of 25%) landed beneath parent plants or was dropped after manipulation by vertebrate frugivores as viable diaspores. Ants from at least five genera removed most fallen diaspores (up to 83%) within 24 h. Ants influenced the fate of a large amount of the crop, and for some trees ants removed as many diaspores as birds. 4. Large ants rescued some diaspores to distances beyond the parent plant crown, but birds may remove diaspores 40‐fold farther. However, seedlings of X. aromatica were only found close to nests of large ants, probably due to diaspore rescuing and/or directed secondary dispersal by certain ant groups following primary dispersal by birds. Although an unknown percentage of seeds was lost to granivorous ants, diaspore removal by ants potentially enhances the likelihood of plant recruitment due to distance‐related benefits and directed dispersal to ant nests, while birds play a premier role in long‐distance seed dispersal and metapopulation dynamics. 5. Synthesis. Birds and ants provide complementary seed dispersal at different spatial scales to X. aromatica. Since ants remove most fallen diaspores beneath parent plants, the use of diaspore removal rates from plant canopy as a surrogate of plant fitness may be misleading. By acting as secondary dispersers, ants may also provide a fine‐tuned dispersal following long‐distance dispersal by birds (i.e. diplochory).

Phylogeny, scaling, and the generation of extreme forces in trap-jaw ants

Trap-jaw ants of the genus Odontomachus produce remarkably fast predatory strikes. The closing mandibles of Odontomachus bauri, for example, can reach speeds of over 60 m s(-1). They use these jaw strikes for both prey capture and locomotion - by striking hard surfaces, they can launch themselves into the air. We tested the hypothesis that morphological variation across the genus is correlated with differences in jaw speeds and accelerations. We video-recorded jaw-strikes at 70 000-100 000 frames s(-1) to measure these parameters and to model force production. Differences in mean speeds ranged from 35.9+/-7.7 m s(-1) for O. chelifer, to 48.8+/-8.9 m s(-1) for O. clarus desertorum. Differences in species' accelerations and jaw sizes resulted in maximum strike forces in the largest ants (O. chelifer) that were four times those generated by the smallest ants (O. ruginodis). To evaluate phylogenetic effects and make statistically valid comparisons, we developed a phylogeny of all sampled Odontomachus species and seven outgroup species (19 species total) using four genetic loci. Jaw acceleration and jaw-scaling factors showed significant phylogenetic non-independence, whereas jaw speed and force did not. Independent contrast (IC) values were used to calculate scaling relationships for jaw length, jaw mass and body mass, which did not deviate significantly from isometry. IC regression of angular acceleration and body size show an inverse relationship, but combined with the isometric increase in jaw length and mass results in greater maximum strike forces for the largest Odontomachus species. Relatively small differences (3%) between IC and species-mean based models suggest that any deviation from isometry in species' force production may be the result of recent selective evolution, rather than deep phylogenetic signal.

Consumption of introduced prey by native predators: Argentine ants and pit-building ant lions

When populations of native predators are subsidized by numerically dominant introduced species, the structure of food webs can be greatly altered. Surprisingly little is known, however, about the general factors that influence whether or not native predators consume introduced species. To learn more about this issue, we examined how native pit-building ant lions (Myrmeleon) are affected by Argentine ant (Linepithema humile) invasions in coastal southern California. Compared to areas without L. humile, invaded areas contained few native ant species and were deficient in medium-sized and large bodied native ants. Based on these differences, we predicted that Argentine ants would negatively affect ant lion larvae. Contrary to this expectation, observational surveys and laboratory growth rate experiments revealed that Myrmeleon were heavier, had longer mandibles, and grew more quickly when their main ant prey were Argentine ants rather than native ants. Moreover, a field transplant experiment indicated that growth rates and pupal weights were not statistically different for larval ant lions reared in invaded areas compared to those reared in uninvaded areas. Argentine ants were also highly susceptible to capture by larval Myrmeleon. The species-level traits that presumably make Argentine ant workers susceptible to capture by larval ant lions—small size and high activity levels—appear to be the same characteristics that make them unsuitable prey for vertebrate predators, such as horned lizards. These results underscore the difficulties in predicting whether or not numerically dominant introduced species serve as prey for native predators.

DIRECT AND INDIRECT EFFECTS OF ANTS ON A FOREST-FLOOR FOOD WEB

Interactions among predators that prey on each other and are potential competitors for shared prey (intraguild [IG] predators) are widespread in terrestrial ecosystems and have the potential to strongly influence the dynamics of terrestrial food webs. Ants and spiders are abundant and ubiquitous terrestrial IG predators, yet the strength and consequences of interactions between them are largely unknown. In the leaf-litter food web of a deciduous forest in Kentucky (USA), we tested the direct and indirect effects of ants on spiders and a category of shared prey (Collembola) by experimentally subsidizing ants in open plots in two field experiments. In the first experiment, ant activity was increased, and the density of ants in the litter was doubled, by placing carbohydrate and protein baits in the center of each plot. Gnaphosa spiders were almost twice as abundant and Schizocosa spiders were half as abundant in baited plots relative to controls. There were more tomocerid Collembola in baited plots, suggesting possible indirect effects on Collembola caused by ant-spider interactions. The second experiment, in which screening of two mesh sizes selectively excluded large and small worker ants from a sugar bait, revealed that the large ants, primarily Camponotus, could alone induce similar effects on spiders. Gnaphosa biomass density was almost twice as high in the plots where large ants were more active, whereas Schizocosa biomass density was reduced by half in these plots. Although tomocerid densities did not differ between treatments, tomocerid numbers were negatively correlated with the activity of Formica, another large ant species. Path analysis failed to support the hypothesis that the ant Camponotus indirectly affected tomocerid Collembola through effects on densities of spiders. However, path analysis also revealed other indirect effects of Camponotus affecting tomocerids. These results illustrate the complexity of interactions between and within two major IG predator groups with disparate predatory behaviors, complexities that will have consequences for functioning of the forest-floor food web.

Nectar foraging behaviour is affected by ant body size in Camponotus mus

The nectivorous ant Camponotus mus shows a broad size variation within the worker caste. Large ants can ingest faster and larger loads than small ones. Differences in physiological abilities in fluid ingestion due to the insect size could be related to differences in decision-making according to ant size during nectar foraging. Sucrose solutions of different levels of sugar concentration (30% or 60%w/w), viscosity (high or low) or flow rate (ad libitum or 1 μl/min) were offered in combination to analyse the behavioural responses to each of these properties separately. Differences were found depending on ant body size and the property compared. A regulated flow produced smaller crop loads for medium and large ants compared to the same solution given ad libitum. All foragers remained longer times feeding at the regulated flow source but larger ants often made longer interruptions. When sugar concentration was constant but viscosity was high, only large ants increased feeding time. Constant viscosity with different sugar concentration determined longer feeding time and bigger loads for the most concentrated solution for small but not for large ants. Small ants reached similar crop loads in a variety of conditions while large ants did not. These differences could be evidence of a possible specialization for nectar foraging based on ant body size.

Development ofPseudacteon obtusus(Diptera: Phoridae) onSolenopsis invictaandSolenopsis richteriFire Ants (Hymenoptera: Formicidae)

We studied the developmental performance of the large biotype of Pseudacteon obtusus Borgmeier, a promising biological control agent of imported fire ants. We measured selected life history traits of this parasitoid as a function of (1) host species (Solenopsis invicta Buren versus Solenopsis richteri Forel), (2) temperature (22 versus 28°C), and (3) size distributions of available host ants (big versus mixed-size classes of workers). Survivorship on S. invicta was much greater than that on S. richteri under most conditions. Developmental time was strongly influenced by temperature with total developmental periods shortened by 21–34% at the 6°C higher temperature treatment. However, developmental periods were weakly influenced by hosts across temperatures. We found that larval, pupal, and total developmental periods of this phorid fly were up to 6.2% longer on S. richteri than on S. invicta, although these periods depended on temperature. Total developmental time was slightly shorter (by 4.8%) on S. invicta than S. richteri at 22°C but longer (by 5.7%) on S. invicta at 28°C. The relationship between host size and sex of emerging flies contrasts with that of previously documented Pseudacteon species, in that males were produced from all host size classes while females only came from larger ants. Sex ratios favor females when a mixture of ant size classes were offered in comparison to when only big ants could be chosen. Pseudacteon obtusus seems to be a promising candidate for biological control releases in that in addition to its high host specificity, it develops better on red fire ants, attacks ants on trails and mounds, and chooses a worker size range that complements the two other Pseudacteon species already released.

Can solitary spiders (Araneae) cooperate in prey capture?

1 Cooperation in prey capture has never been reported for a solitary spider. Here, we conducted a laboratory experiment to determine if group hunting occurs in the ant-eating Zodarion spiders and if it yields higher individual benefit than solitary prey capture. 2 Spiderlings of the second instar were kept in groups of one, three and six individuals. Their foraging behaviour was studied with two ant species as prey: a large Messor and a small Tapinoma. 3 Groups of three and six spiders had better capture success than a single individual. The paralysis time of Messor was reduced as the number of spiders in a group increased, while that of Tapinoma ants was similar for all group sizes. The feeding time on Messor increased with the number of spiders in a group; all six individuals could feed simultaneously. For Tapinoma ants, six spiders fed for a significantly shorter time than the group of three spiders due to more intraspecific interactions. Only one to two spiders fed on Tapinoma at the same time. For spiders feeding on Messor the individual gain was on average 1·5 times greater than that when feeding on Tapinoma. The per capita gain decreased with group size but was not shared equally between the group members: the catchers gained about twice as much as the intruders. The catchers, however, experienced higher capture risk than intruders. 4 Our results show that solitary foraging is more beneficial when Zodarion spiders hunt small ants, while the strategy of group foraging is more beneficial when hunting large ants. The observed foraging behaviour of immature Zodarion spiders seems to be an example of mutual parasitism in which catchers are parasitized by intruders. Such interaction has a character of cooperation on a long-term scale.

Environmental rugosity, body size and access to food: a test of the size‐grain hypothesis in tropical litter ants

In terrestrial walking organisms, long legs help to decrease the cost of running, allowing animals to step over environmental interstices rather than walking through them. However, long legs can complicate the infiltration of these interstices, which may contain food sources and refugia. Since the number of environmental interstices perceived by an organism (rugosity) increases as it body size decreases (size‐grain hypothesis, SGH), natural selection should favor proportionally smaller legs with decreasing body size. Recent work demonstrated that ants fit this hypothesis. We experimentally tested the assumption of the SGH that small ants, which have proportionally smaller legs than larger ants, are more successful in exploring environmental interstices because they can easily penetrate them. We examined the ability of tropical litter ant species with different body sizes to access food baits in ‘landscapes’ (=plots) with different levels of rugosity and food exposure. In the first experiment, three levels of landscape rugosity were defined by manipulating the density of leaf litter placed on the ground plots: a) plain landscape: no litter fall, b) intermediate rugosity (∼0.5 kg of litter fall) and c) high rugosity (∼1 kg). In a second experiment, food baits were in plain landscapes, exposed or covered by leaf litter. The body lengths of ants that first accessed food baits ranged from 1.5 to 12 mm. Ants that first reached food baits in the most rugose landscapes were ∼40% smaller than ants that first found baits in plain landscapes. Smaller ants were also the first to access covered food. The application of a phylogenetic comparative method suggested the same patterns. We conclude that these results support the size grain hypothesis. Environmental rugosity might have operated as a selective force to shape the morphological characteristics of litter ant species.

Fragment-size determination and size-matching in the grass-cutting ant Atta vollenweideri depend on the distance from the nest

Fragment-size determination by workers of the grass-cutting ant Atta vollenweideri and the extent of size-matching between ant size and fragment size were investigated as a function of the distance from the nest. Foragers and their loads were sampled in four different sectors along a 30-m foraging trail in the field: directly on the harvested source, in the patch, on the trail and close to the nest. Ants sampled on the source immediately after cutting were significantly larger than carrying ants on any trail sector, whereas body mass did not differ among other samples. Thus, foraging was partitioned into at least two stages, with large ants cutting the source and smaller ants carrying the fragments to the nest. Fragments collected directly after cutting were significantly larger than those carried on the trail, indicating that the fragments were cut once again on their way to the nest. Size-matching depended on the trail sector considered. It was stronger in ants sampled closer to the nest, suggesting that carriers either collect dropped fragments according to their body mass, or cut fragments out of the dropped fragments in sizes corresponding to their body mass prior to transport.

Inventory and Bioindicator Sampling: Testing Pitfall and Winkler Methods with Ants in a South African Savanna

The sampling efficiency and consistency of pitfall traps and Winkler samples for inventory, bioindicator and ecological studies in savanna habitats was compared using ants. Pitfall traps are often used for ant collecting while Winkler litter sampling has until now had rather limited use. We test Winkler sampling for the first time in a South African savanna. Pitfall traps were more efficient and productive than Winkler sampling for epigaeic ants, with a greater total species richness and higher abundance of ants recorded. Winkler samples contributed few additional species. The relative abundance of different sized ants was different with the two collection methods. Winkler sampling was found to catch greater numbers of smaller ants than pitfall trapping, whereas pitfall trapping caught more larger ants. The standard collecting Winkler quadrat size of 1 m2 did not perform as well as 2×\( - \frac{1}{2}\)m2 quadrats combined for one sample.