Host-plant selection, diet diversity, and optimal foraging in a tropical leafcutting ant

Abstract

A month-long study was conducted on the comparative foraging behavior of 20 colonies of the leafcutting ant, Atta cephalotes L. in Santa Rosa National Park, Guanacaste Province, Costa Rica. The study was conducted during the middle of the wet season, when trees had mature foliage and the ants were maximally selective among species of potential host plants. The colonies always gathered leaves from more than a single tree species but on average one species constituted almost half the diet with the remaining species being of geometrically decreasing importance. Colonies exhibited greater diversity in their choice of leaves and lower constancy of foraging when the average quality of resource trees was lower, as predicted by elementary optimal foraging theory. Furthermore, the ants were more selective of the species they attacked at greater distances from the nest. However, the ants sometimes did not attack apparently palatable species, and often did not attack nearby individuals of species they were exploiting at greater distances.A classical explanation for why leafcutting ants exploit distant host trees when apparently equally good trees are nearer, is that the ants are pursuing a strategy of conserving resources to avoid long-term overgrazing pressure on nearby trees. We prefer a simpler hypothesis: (1) Trees of exploited species exhibit individual variation in the acceptability of their leaves to the ants. (2) The abundance of a species will generally increase with area and radial distance from the nest, so the probability that at least one tree of the species will be acceptable to the ants also increases with distance. (3) The ants forage using a system of trunk-trails cleared of leaf litter, which significantly reduces their travel time to previously discovered, high-quality resource trees (by a factor of 4- to 10-fold). (4) Foragers are unware of the total pool of resources available to the colony. Therefore once scouts have chanced upon a tree which is acceptable, the colony will concentrate on harvesting from that tree rather than searching for additional sources of leaves distant from the established trail.

Harvesting of Protium heptaphyllum (Aubl.) March. seeds (Burseraceae) by the leaf-cutting ant Atta sexdens L. promotes seed aggregation and seedling mortality

“…Atta species usually (1) forage on seeds via aboveground trails (Farji-Brener & Silva 1996, Dalling & Wirth 1998; (2) carry seeds to nests far away from parental plants and lose significant numbers of them during the transport along the extended foraging trails (Lugo et al 1973, Leal & Oliveira 1998, Wirth et al 2003; and (3) discard seeds in subterraneous or external refuse dumps (Peternelli et al 2003, Farji-Brener & Medina 2000. Our findings represent new aspects of seed foraging in LCAs and extend our understanding about plasticity and opportunistic foraging of LCAs as highlighted by Rockwood and Hubbell (1987), and Kost et al (2005). Removal of foliar vegetation in the vicinity and above the nest has been recorded for many LCAs species (Farji-Brener & Illes 2000, Hull-Sanders & Howard 2003.…”

Costs of trail construction and maintenance in the leaf-cutting ant Atta columbica

“…In most cases, recruitment trails are temporary, and are abandoned when the specific food resource to which recruitment is taking place is exhausted. However, in a few genera such as Pogonomyrmex (Hölldobler 1976), Formica (Rosengren 1971), and Atta (Weber 1972), trails may last for months or years, and may be partially or completely cleared to facilitate locomotion (Rockwood and Hubbell 1987). In some cases these "trunk" trails are long-lasting because they serve high-quality, long-lasting resources such as aphid colonies (Rosengren and Sundström 1987), but for other species the reasons for trail longevity remain unclear.…”

“…Single trunk trails may exceed 200 m in length (Lewis et al 1974) and appear to represent a significant investment in time and energy by the colony (Lugo et al 1973;Shepherd 1982). This investment is repaid with a four-to tenfold reduction in travel costs for ants using the trail system versus those traveling over uncleared ground (Rockwood and Hubbell 1987). In addition, the application of trail pheromone to cleared and relatively smooth substrates may greatly increase the strength and persistence of major trails, allowing ants to rapidly relocate and exploit favorable resources after periods of inactivity.…”

“…Wilson's (1980) detailed analyses of caste optimization took place in the laboratory and so excluded the tasks involved in trail construction and maintenance under natural conditions. Much work has focused on the performance of workers utilizing trails (Lutz 1929;Hubbell et al 1980;Rudolph and Loudon 1986;Lighton et al 1987;Waller 1989;Shutler and Mullie 1991;Wetterer 1994;Burd 1995Burd , 1996 and the use of trail systems by colonies in exploiting resources (Fowler and Robinson 1979;Fowler and Stiles 1980;Shepherd 1982;Rockwood and Hubbell 1987). However, our understanding of foraging economics in leaf-cutting ant colonies will remain incomplete until the costs of trail construction and maintenance are included in colony time and energy budgets.…”

Animal transportation networks

“…Howard [58] estimated that the total annual cost of trail maintenance in leafcutter ants was 11 000 ant hours or the energy equivalent of carrying 8000 leaf burdens. However, while the absolute cost of trail maintenance may seem high, the colonies investment was relatively low considering the number of available workers [58], and the potential benefit of clearing, which can reduce travel times by up to 10-fold (Johnson and Hubbell, unpublished, cited in [69]). …”