Donald H. Feener

Energetics of Locomotion and Load Carriage and a Model of the Energy Cost of Foraging in the Leaf-Cutting Ant Atta colombica Guer

Standard rates of O₂ consumption (V̇o2) and net, gross, and minimum costs of transport (NCOT, GCOT, and Mrun) were measured in the leaf-cutting ant Atta colombica. Both closed (running wheel respirometer) and flow-through (treadmill) systems were used. The relation between body mass (.004–.035 g) and standard V̇o2 in workers was V̇o2 = .074 M.62 where V̇o2 is ml h⁻¹ at 28 C and M is mass in grams. When combined with published data for 30 ant species, at 20 C this equation becomes V̇o2 = .137 M.838. Equations that allow calculation of NCOT and GCOT from body mass and running speeds are presented. NCOT in A. colombica at 28 C was 18.6 ml O₂ g⁻¹ km⁻¹ (mass .015 g, running speed 5.2 cm s⁻¹). Both NCOT and GCOT decreased with increasing body mass. Load carriage decreased running speed and increased NCOT proportionally to the increase of body mass + load mass. Cost of transporting a unit of load and a unit of body mass were therefore equivalent. Mrun, was 10.1 ml O₂ g⁻¹ km⁻¹ (n = 11, mean mass .0314 g), which does not differ significantly from values predicted on the basis of published equations relating Mrun to body mass in vertebrates and insects. A model is developed to predict the energy costs of foraging and maintenance of an Atta colony. The cost of maintaining a 100-m trail with a traffic rate of 60 loaded and 60 unloaded workers min⁻¹ at 28 C was approximately 2.2 W.

Competition between ant species: Outcome controlled by parasitic flies

Experimental evidence demonstrates that the parasitic phorid fly Apocephalus shifts the competitive balance between the ant species Pheidole dentata and Solenopsis texana by interfering with the defensive behavior of Pheidole dentata major workers (soldiers). This represents one of the first examples of a parasite affecting competitive interactions anmong terrestrial animals in natural communities. Similar complex interactions are probably common in many ant communities.

Defense against parasites by hitchhikers in leaf-cutting ants: a quantitative assessment

SummaryA curious behavior in leaf-cutting ants in the genus Atta is the “hitchhiking” of small minim workers on leaf fragments carried by larger workers. Two functions of these hitchhikers have been proposed: (1) defense of leaf carriers against parasitic flies in the family Phoridae (ant protection hypothesis; Eibl-Eibesfeldt and Eibl-Eibesfeldt 1967) and (2) reduction of transport costs of small minims that collect plant fluids (energy conservation hypothesis; Stradling 1978).We studied hitchhiking behavior in colonies of Atta colombica on Barro Colorado Island, Panama, and found strong evidence in favor of the ant protection hypothesis. Females of Apocephalus attophilus (Diptera: Phoridae) attack leaf carriers of A. colombica and deposit eggs in the head capsules of these ants. Our observations indicate that parasites require leaf fragments to stand on during oviposition, and, as a result, only leaf carriers are susceptible to parasitic attack. The presence of hitchhikers reduces significantly both the time parasites spend on leaf fragments and the probability that they will land in the first place. Results of experimental introductions of parasites and a year of biweekly censuses at ten colonies indicate that leaf-cutting ants adjust the level of hitchhiking to accommodate both daily and seasonal changes in the abundance of parasites.We found little evidence in support of the energy conservation hypothesis. If it is assumed that all minim workers hitchhike back to the nest, our calculations indicate that total transport costs along a foraging trail are reduced by 10% or less. However, our observations indicate that only 50% of returning minim workers hitchhike, and therefore energy savings are actually considerably less than 10%. Leaf-cutting ants in the genera Atta and Acromyrmex are attacked by over 20 species of parasitic phorids. In the discussion we review what is known about these associations and suggest that these parasites have influenced the ecology and evolution of polyethism in leaf-cutting ants.

Ant visitation of extrafloral nectaries of Passiflora : the effects of nectary attributes and ant behavior on patterns in facultative ant-plant mutualisms

Extrafloral nectary (EFN) plants are widespread and can be quite species-rich in some communities. Thus, ants that utilize extrafloral nectar may have the opportunity to discriminate among a wide variety of nectar sources, resulting in variation in the ant attention EFN plants receive. In this study, we compare ant visitation rates of three Passiflora species that coexist in an early successional neotropical forest. These three vine species (Passiflora auriculata, P. biflora, and P. oerstedii) differ in their extrafloral nectary structure and placement, and thus may attract different numbers or species of ants. Through censuses of ants tending extrafloral nectaries, we found that P. auriculata received significantly higher numbers of ant visitors than P. oerstedii, but did not differ significantly from P. biflora in its attractiveness to ants. We also found that termite worker baits (simulating herbivores) placed on P. auriculata and P. biflora were discovered by ants significantly more quickly than baits placed on P. oerstedii. In both ant visitation censuses and in termite bait trials, we found no significant associations between Passiflora species and the species of ant visitors. We also performed experimental manipulations of several characteristics of P. auriculata, which resulted in changes in levels of ant visitation. When petiolar nectaries of P. auriculata were experimentally blocked, visitation by the common ant Ectatomma ruidum declined, even though nectaries on the leaf surfaces were still functional. Connections with other vegetation also had an effect on ant visitation. Though experimental creation of connections between growing P. auriculata shoots and other vegetation did not enhance ant visitation, eliminating connections resulted in a significant decline in the number of ant visitors. The results of this study suggest factors that may contribute to variation in ant visitation of extrafloral nectary plants. In addition, this study demonstrates that extrafloral nectary plants co-occurring in a habitat and available to the same ants may differ in patterns of visitation by ants and perhaps in the quality of protection from herbivores that they receive.

Effects of parasites on foraging and defense behavior of a termitophagous ant, Pheidole titanis Wheeler (Hymenoptera: Formicidae)

SummaryPheidole titanis Wheeler, an ant that occurs in desert and deciduous thorn forest in the southwestern United States and western Mexico, is a predator on termites. In the dry season well-coordinated raids against termite foraging parties occur early in the morning or late in the afternoon, whereas in the wet season most raids occur at night. This seasonal shift in the timing of raids is due to the increased activity of a fly (Diptera: Phoridae) that is a specialist parasitoid on P. titanis workers and soldiers. When parasitic flies discover P. titanis nest entrances or raiding columns, workers stop foraging and defend themselves against oviposition attacks. Flies are only active during the day and never interfere with foraging at night. However, P. titanis does not increase the frequency of raids at night and, as a result, colonies collect less food in the wet season compared to the dry season. Presence of parasitic flies also interferes with normal defense behavior of P. titanis against conspecific and heterospecific enemy ants. Dissections of P. titanis workers and soldiers suggest that the parasitism rate by flies is less than 2% and observations indicate that parasitic flies are much rarer than their host workers and soldiers. Nonetheless, these parasites exert a strong ecological impact on their host.

Maintaining Diversity in an Ant Community: Modeling, Extending, and Testing the Dominance‐Discovery Trade‐Off

Ant communities often consist of many species with apparently similar niches. We present a mathematical model of the dominance‐discovery trade‐off, the trade‐off between the abilities to find and to control resources, showing that it can in principle facilitate the coexistence of large numbers of species. Baiting studies of dominance and discovery abilities in an ant community from the Chiricahua Mountains of Arizona indicate that real communities fail to fit the assumptions of the simple model in several ways: (1) dominance depends on the size of the food resource; (2) for some ants, dominance depends on the presence or absence of specialist parasitoids; (3) pairwise dominance is not an all‐or‐nothing trait; and (4) a consistent negative relationship between pairwise differences in per capita discovery rates and dominance can be detected for only one bait type. Extended models incorporating these factors successfully predict the coexistence of five of the six most abundant members of this community but fail to accurately predict their relative abundances. Sensitivity analysis indicates that each complicating factor enhances the extent of coexistence.

Effect of treefall gaps on the patchiness and species richness of Neotropical ant assemblages

Abstract Natural formation of treefall gaps plays an integral role in the ecological and evolutionary dynamics of many tropical forests, affecting the spatiotemporal distribution of plants and the animals that interact with them. This study examines the impact of treefall gaps on the spatial and temporal patchiness of ant assemblages in a moist lowland forest in Panama. Using pitfall traps and honey baits, we compared ant assemblages in five 1 to 2-year-old treefall gaps (ca 100 m2) and five adjacent plots (ca 100 m2) in undisturbed forest understory at three different times of year (late wet season, late dry season, and early wet season). We found little evidence that ant assemblages respond dramatically to the formation of treefall gaps and the differences in habitat qualities they produce. Ant abundance, species richness, species composition, and rates of resource discovery did not differ between gaps and forest understory. However, we did find significant differences in numerical abundance related to forest stratum (ground vs vegetation) and resource type in pitfall traps (oil-cockroach vs honey), and significant differences in ant species richness and rates of resource discovery across seasons. While habitat effects by themselves were never statistically significant, habitat and seasonal differences in species richness interacted significantly to produce complex, season-dependent differences among gap and forest habitats. These results suggest that the formation of natural treefall gaps has less of an effect on Neotropical ant assemblages compared to other groups of organisms (e.g., plants, birds) or other causes of patchiness (e.g., ant mosaics, moisture availability, army ant predation). The results of our study also have important implications for the underlying causes of habitat differences in the distribution of ant-defended plants.

Curvilinear allometry, energetics and foraging ecology: a comparison of leaf-cutting ants and army ants

Allometric scaling in ants with polymorphic castes often remains curvilinear after logarithmic transformation. A technique for comparing non-linear scaling is developed and used to compare caste polymorphism in the leaf-cutting ant Atta colombica Guerin and the army ant Eciton hamatum (Fab.). Of the five morphological characters compared, the scaling of leg length to the cube root of body mass (M1l3) differed the most: relative leg length increased with M1l3 in E. hamatum and decreased with M1l3 in A. colombica. This difference reflected species differences in the energetics and foraging behaviour. Key-words: Atta colombica, Eciton hamatum, leafcutters, army ants, curvilinear allometry, foraging ecology

Assembling an ant community: species functional traits reflect environmental filtering.

Species should only persist in local communities if they have functional traits that are compatible with habitat-specific environmental conditions. Consequently, pronounced regional environmental gradients should produce environmental filtering, or a trait-based spatial segregation of species. It is critical to quantify the links between species’ functional traits and their environment in order to reveal the relative importance of this process to community assembly and promote understanding of the impacts of ongoing environmental changes. We investigated this relationship using epigaeic ants in an environmentally heterogeneous region of Florida. We found evidence for environmental filtering as environmental conditions such as groundcover, surface temperature, vapor pressure deficit, and plant diversity were strongly correlated with assemblage composition. Certain species traits appeared particularly important to persistence: (1) ants in environments with less groundcover have relatively longer legs but do not differ in size, (2) ants in hotter environments exhibit greater thermal tolerances, and (3) ants in hotter and drier environments do not exhibit greater desiccation resistance. These findings show surface complexity and temperature may interact with morphology and physiology to impact the spatial distribution of ants and underscore the importance of climate change. Climate warming is predicted to alter assemblage composition, competitive dynamics, and consequently impact ecosystem processes. We suggest environmental filters acting at regional scales, as shown here, act in tandem with more frequently studied local-scale competitive interactions to delimit ant community assemblages.

Foraging in the seed-harvester ant genus Pogonomyrmex: are energy costs important?

Energy intake and expenditure on natural foraging trips were estimated for the seed-harvester ants, Pogonomyrmex maricopa and P. rugosus. During seed collection, P. maricopa foraged individually, whereas P. rugosus employed a trunk-trail foraging system. Energy gain per trip and per minute were not significantly different between species. There was also no interspecific difference in energy cost per trip, but energy cost per minute was lower for P. maricopa foragers because they spent on average 7 min longer searching for a load on each trip. Including both unsuccessful and successful foraging trips, average energy gain per trip was more than 100 times the energy cost per trip for both species. Based on this result, we suggest that time cost incurred during individual foraging trips is much more important than energy cost in terms of maximizing net resource intake over time. In addition, because energy costs are so small relative to gains, we propose that energy costs associated with foraging may be safely ignored in future tests of foraging theory with seed-harvesting ant species.