The Effect of Leaf Fragment Size on Optimal Foraging in Atta cephalotes

Austen Hilding, Zoe Gerber, Kat Kurtenbach, Theresa Thao Nguyen

Atta cephalotes more commonly known as leaf-cutting ants are widely distributed throughout the world but most often found in forest below 2,000 meters of elevation in Costa Rica. A. cephalotes build extensive underground nests that can be up to 7 meters deep with openings up to 50 meters apart. Colonies of A. cephalotes are eusocial organisms with subdivided groups of workers including minima, media and soldiers. Each subdivision is characterized by specific responsibilities in the collection of leaf material and production of fungi. Media A. cephalotes are responsible for locating and collecting leaf material then returning it to the nest to be cleaned and tended to by minima. A. cephalotes use the collected leaf material to grow several hundred fungi gardens that are connected through complex interconnecting tunnels. A. cephalotes are also characterized by following the same well-cleared trail between foraging sites and nest openings. The media are able to remember and utilize the same trail by following the pheromones of the ant in front of them (Janzen 1983).

Due to the weight each media must carry over a long distance it is imperative to find a way to maximize fitness and minimize energy exerted. This is referred to as the Optimal Foraging Theory formulated by R.H. MacArthur and E.R. Pianka in 1966. The Optimal Foraging Theory asserts that natural selection favors organisms whose behavior maximizes net energy intake per unit of time spent foraging (Begon et al., 1990). A. cephalotes were ideal for testing this theory due to the consistent range of leaf fragment size collected that presented little variability and the the leaf fragments collected could be easily manipulated in terms of size. In addition, there was an abundant population of A. cephalotes following a clear visible trail making the ants and leaf fragments easily accessible. The purpose of this study was to assess whether or not A. cephalotes were achieving an optimum foraging strategy by carrying the largest possible leaf while exerting the least amount of energy. In order to statistically analyze the foraging strategy of the A. cephalotes the leaf weight of traveling ants was manipulated to be heavier or lighter to assess the change in speed of the ant. The A. cephalotes will achieve an optimum foraging strategy as that will maximize their fitness and minimize their energy exerted.

After analyzing the data of our tests, we found significant results relating to the mass of the leaf fragment being carried by the ants in relation to the time and energy required to carry that load. In the first test, reducing the weight of the leaves being carried resulted in those ants moving significantly faster than they were with the initial leaf fragment (Refer to slidshow, Fig. 1). This was expected, however, it does not indicate whether the ants were foraging in the most efficient way. When dividing the weight of leaf by the time it took to travel the 20cm with both the initial leaf fragment and the reduced fragment, we significantly reduced the efficiency of foraging (Fig. 2).

Likewise, we affected the time and efficiency of carrying leaves by adding a small piece of foil to the leaf fragments being carried by the ants. Adding foil to the leaves resulted in significantly slower travel time (Fig. 3). However, after calculating the efficiency of foraging for the initial leaf fragment compared to with the added foil, there was actually a slight increased in the grams carried per second (Fig. 4).

According to the results, reducing the size of the leaf fragments resulted in the ants carrying fewer grams per second, and therefore foraging at a less optimal level. However, the additional weight of tin-foil to the leaf fragments resulted in a slightly more efficient rate of foraging, indicating that the ants may be foraging at a suboptimal level. This possibility of suboptimal foraging could be attributed to several causes, such as being physically limited by the dimensions of their mouths. The morphology of their mouth may not allow the ants to cut larger fragments that they physically are capable of carrying. The ants, when cutting the leaf, may take into account that they might also need to carry minima ants on their fragment. To ensure that they will be able to carry their leaf load plus the added weight of the minima, they might decide to cut a smaller section of leaf. Burd also provides hypotheses of why individual ants foraging at a suboptimal level may actually be a component of optimal group foraging. Smaller loads may allow the ants to return more quickly to the nest and recruit other ants to the patch, and therefore increasing the rate that the whole colony acquires a resource (Burd 2000). However, this is unlikely because most often the ants have a continuous flow of traffic to one area. Individual suboptimal foraging may also be optimal for the whole nest because as ants take time to cut sections of leaves, they inadvertently hinder their nest mates’ access to the leaf margin (Burd 2000). The smaller leaf size could allow more workers to obtain fragments and return to the colony at a faster foraging rate than if they cut as large of a fragment as was possible to carry. Carrying larger loads could cause more congestion on the trail, resulting in a slower group rate of foraging return (Burd 2000). The smaller leaf size could also be a result of energy. We only tested the ants on the return phase to the nest, however, the outgoing trip and the cutting of the leaf requires energy as well. The ants were able to carry the heavier load more efficiently for 20cm, but they would be unable to maintain that rate for a full journey back to the nest. Our results are similar to other tests of foraging in Atta ants. However, the results indicating suboptimal foraging could be attributed to the difference in weight of the foil piece and the leaf over the same area. The foil was denser per unit of area than the leaf, so although the ants could carry the added weight, the dimensions of that same weight added as leaf area could create a more cumbersome load and slow the ant down to a less efficient rate. Even if the ants are not foraging at the optimal level, our results indicate that their foraging rate is close to the optimal level.

Our findings demonstrate that individual ants do not necessarily optimize their leaf-carrying capacity. In fact, these results may help show striking differences between optimality for individual versus grouped foraging. Because we believe that successful species would follow the Optimal Foraging Theory, we think there must be a reason beyond the individual for the load size that the ant chooses. The complex lifestyle of A. cephalotes points to them being highly resourceful and adaptive as a group, hence, individual ants most likely have discovered the optimal leaf size for the entire nest. Many questions arise as to how and why this eusociality evolves. Future tests should look further into how grouping may change the foraging of Atta cephalotes and other species, and if optimal group foraging would change under different circumstances like size of nest and resource being exploited.

References

Begon, M. and Harper L.J and Townsend, R.C. 1990. Ecology Individuals, Populations, and Communities. Blackwell Scientific Publication Oxford, London, UK.

Burd, M., 2000. Body size effects on locomotion and load carriage in the highly polymorphic leaf-cutting ants Atta colobica and Atta cephalotes. Behavioral Ecology Vol. 11, No. 2, p. 125-131

Burd, M., 2000. Foraging behaviour of Atta cephalotes (leaf-cutting ants): an examination of two predictions for load selection. 60: 781-787.

Janzen, H.D. 1983. Costa Rican Natural History. The University of Chicago, Chicago, USA.