Optimal foraging behavior in desert harvester ants (Veromessor pergandei)

Carlee Bowen1, George Eskander2, Brian Lai3, Mariam Samy2

University of California, Santa Cruz1, University of California, Riverside2, University of California, Merced3

Optimal foraging behavior is exhibited when individuals in a species maximize their nutritional intake, while minimizing their energy expenditure during the foraging process. In this study, we explored the foraging behavior of Black Harvester Ants (Veromessor pergandei) to assess if their foraging habits coincided with optimal foraging theory. We observed 35 Veromessor colonies in Anza Borrego State Park in a controlled experiment where piles of large and small pumpkin and oat seeds were placed on a foraging path both near and far from the nest. We found that Veromessor preferred small seeds over large, near seeds over far, and pumpkin seeds over oat seeds. These results supported our predictions that Veromessor would collect higher value and lower cost foods, aligning with patterns predicted by optimal foraging theory. Investigating these foraging behaviors allows us to better estimate future patterns of not only Veromessor, but other foragers as well.

Keywords: O.F.T (optimal foraging theory), Veromessor pergandei, foraging cost, food preference, fitness

INTRODUCTION individuals that maximize foraging efficiency (Pyke et al. 1977). Optimal Optimal foraging theory predicts that foraging behavior may also be seen in ants animals are adapted to minimize their as they forage for food to bring back to the energy expenditure while simultaneously colony. This evolutionary behavior increases maximizing their nutrient intake (Orians and the overall fitness of the colony, allowing Pearson 1979). For example, the bee foragers to retrieve a wide range of foods species Apis mellifera will select flowers varying from plant seeds to other animals with higher sugar concentration rather than when resources are limited (Cerda 1997). lower sugar concentration in order to Although ants are small in size, they have maximize honey production efficiency a relatively large ecological impact on their (Roubik and Buchmann 1984). Optimal surrounding plant communities. They can foraging patterns can also be seen in shape the structure and composition of soil Lepomis macrochirus, a species of fish that and plant communities through aeration, aims to optimize foraging time by selecting seed dispersal, and increased nutrient prey that are of higher abundance and content in the soil leading to greater fertility visibility (Werner and Hall 1974). Optimal (MacMahon et al. 2000). Ants are capable foraging has been naturally selected for of performing these ecological functions because of the increased fitness of through their unique foraging behaviors.

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Many species of ants communicate while METHODS foraging by using pheromones notifying other ants of a food source, enabling them 2.1 Sampling Design. to move copious amounts of food from surrounding areas to their nest. This study was performed in the Sonoran Black harvester ants (Veromessor Desert in San Diego County at the pergandei, hereafter referred to as Steele/Burnand Anza-Borrego Reserve o o Veromessor), commonly found throughout (33.2559 N, 116.375 W) located in Borrego the Sonoran Desert, is a eusocial Springs, California. This reserve is polymorphic ant species. In other words, dominated by creosote bush. We observed they live in colonies and have individuals of 35 ant colonies in total. Veromessor are drastically different morph sizes. However, highly abundant throughout the area. The despite this advantageous morphology, study was conducted from November 2-5, there are still questions regarding how 2019 from 9:30-11:30 and 14:30-16:30, the these foraging ants are strategically working most active times of day for Veromessor to obtain their food in order to provide for colonies. their colony (Traniello 1989). The forager’s Food type treatments included small and purpose in a Veromessor colony is to large oat and pumpkin seed particles provide food to the queen, larvae, and (hereafter referred to as seeds). Oats other ants, making foraging efficiency contain 1.4% fat and 2.4% protein whereas crucial. Food size, distance from the nest, pumpkin seeds contain 19% fat and 19% and nutritional content is important protein. For food size manipulation, a soil regarding optimal foraging theory because sieve was used to separate small and large there may be a preference of smaller and seeds of both food treatments after closer foods due to lower foraging costs and manually crushing them into various sizes. higher nutrient foods due to greater Large oat seeds were greater than 2 mm metabolic rewards. and small oat seeds were between 1 mm - 2 In this study, we conducted an mm. Large pumpkin seeds were greater experiment to test the effects of foraging than 1 mm while small pumpkin seeds were cost and food quality on foraging behavior smaller than .5 mm. Treatments were of Veromessor. We manipulated food size measured this way in order to control for and distance from the ant nest as a proxy foraging cost due to pumpkin seeds having for foraging cost and food type as a proxy a higher density than oat seeds. for nutritional quality most favorable 2.2 Experimental Design nutritional content. In this case, foods containing higher fat and protein content To manipulate distance, seeds were are referred to as higher nutrient foods. We placed 1 meter (near) and 2 meters (far) hypothesized that Veromessor would prefer away from the ant nest. At each colony, half smaller food sizes, food closer to the ant a teaspoon of each treatment was colony, and food with the highest measured and placed in a randomized order nutritional content. at both distances, perpendicular to the foraging path (Figure 1). We placed the

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food treatments approximately three RESULTS centimeters away from each other to avoid any cross-contamination between ants Overall Veromessor ants preferred smaller gathering food. Each colony was observed seeds over larger seeds for both food types for 10 minutes immediately after all the (N = 35, t = 6.26, p < 0.0001, Figure 2). Ants seeds were placed. A hit was recorded each collected more seeds that were near the time an ant picked up a seed to bring back colonies than from far (N = 35, t = 2.92, p = to the nest. 0.003, Figure 3). Ants preferred pumpkin seeds over oat seeds (N = 35, t = 7.93, p < 0.0001, Figure 4). There were no interactions between the effects of food size, distance, or type.

Figure 1. Placement of food treatments. We conducted this experiment in the Sonoran Desert in San Diego County at the Steele/Burnand Anza- Borrego Reserve from November 2–5. Four different treatments of food were placed at one meter and two meters away from the colony perpendicular to the Veromessor foraging path. Bolded circles indicate the larger sized seeds. Shaded circles indicate pumpkin seeds. Non-shaded circles indicate oat seeds.

Statistical analyses were conducted in Figure 2. Number of seeds collected vs. seed size. Bars represent the average number of small and JMP Statistical Software v14. A factorial large seeds collected. For both food types, ants ANOVA model was used to determine collected a greater number of small seeds than large which of the three factors (food size, seeds. Error bars represent + 1 SEM. (N = 35, t = distance and type) was the most important 6.26, p < 0.0001). Numbers of hits were log- on Veromessor foraging behaviors and transformed to normalize data. whether or not there were any interactions between these factors. We log-transformed our number of hits to normalize our data.

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DISCUSSION

Our study supported the phenomena that ants exhibit optimal foraging behavior. All variables measured portrayed independent effects on ant foraging behavior. Veromessor selected more small seeds overall compared to large seeds, indicating their preference for lower foraging cost. Carrying a smaller seed requires less energy expenditure than bringing a larger seed back to the colony (Crist and MacMahon 1992, Tomer 2012). Our seed size results were consistent with past studies in which a different species of Figure 3. Number of seeds collected vs. distance harvester ants, Pogonomyrmex, generally from colony. Bars represent the average number of favored smaller seeds over larger seeds also near and far seeds collected. Ants took more seeds due to their lower foraging cost (Pirk and from near treatments than from far treatments. Error bars represent + 1 SEM. (N = 35, t = 2.92, p = Casenave 2009). 0.0038). Numbers of hits were log-transformed to Veromessor ants also exemplified foraging normalize data. behavior through the distance and food type manipulations. There were steady patterns seen with the selection of seeds collected closer to their nest as well a greater selection of pumpkin seeds. Pumpkin seeds have almost twenty times more protein and fat content than oat seeds, making them higher in nutrients ultimately leading to the higher preference shown by Veromessor. A related study was conducted on Veromessor and found that they also preferred seeds with higher protein and fat content (Gordon 1980). This pattern was also seen in previous patterns observed in Pogonomyrmex sp. where there were greater preferences for seeds with Figure 4. Number of seeds collected vs. seed type. higher nutritional content (Crist and Bars represent the average number of pumpkin and oat seeds collected. Ants took more pumpkin seeds MacMahon, 1992). than oat seeds. Error bars represent + 1 SEM. (N = Throughout all trials, Veromessor 35, t = 7.93, p < 0.0001). Numbers of hits were log- consistently exhibited strategies aligning transformed to normalize data. with the optimal foraging theory. Veromessor optimized their nutrient intake while also minimizing foraging costs by

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collecting smaller sized pumpkin seeds Grüter, C., Czaczkes, T.J. & Ratnieks, F.L.W. 2011. closer to their colony compared to any Decision making in ant foragers (Lasius niger) facing conflicting private and social information. other seed treatment. These optimal Behavior Ecology Sociobiology 65:141. foraging patterns of Veromessor may be an indication of their level of fitness. Heinrich, B. 1983. Do bumblebees forage optimally, Maintaining an abundance of substantial and does it matter? Integrative and Comparative nutrients, such as protein or fats, provides a Biology 23(2):273–281. longer future for the colony. To further Holder, K. & Polis, G.A. 1987. Optimal and central- understand how adaptations of Veromessor place foraging theory applied to a desert harvester might evolve over time, a future study may ant, Pogonomyrmex californicus. Oecologia be conducted using obstacles in their 72:440–448. foraging path. This manipulation will assess Kretzer, J. & Cully, J. 2001. Prairie dog effects on their learning curve according to the harvester ant species diversity and density. predictions of foragers consistently aiming Journal of Range Management 54(1):11–14. to optimize their efficiency. Investigating these foraging behaviors allows us to Morehead, S.A. & Jr., D.H.F. 1998. Foraging behavior improve our comprehension of Veromessor and morphology: seed selection in the harvester ant genus, Pogonomyrmex. Oecologia 114:548– fitness and strengthen predictions of the 555. optimal foraging theory. MacMahon, J., Mull, J. & Crist, T.O. 2000. Harvester ACKNOWLEDGMENTS ants (Pogonomyrmex spp.): their community and ecosystem influences. Annual Review of Ecology Special thanks to Tim Miller, Krikor and Systematics 31(1):265–291. Andonian, Reina Heinz, and Jim Dice. This Pirk, J. and Casenave, J.L.D. 2010. Influence of seed work was performed at the University of size on feeding preferences and diet composition California’s Steele/Burnand Anza Borrego of three sympatric harvester ants in the central Desert Research Center, doi: Monte Desert, Argentina. Ecol. Res. 25(2):439– 10.21973/N3Q94F. 445.

REFERENCES Roubik, D., Buchmann, S., 1984. Nectar selection by Melipona and Apis mellifera (Hymenoptera: Apidae) and the ecology of nectar intake by bee Cerda, X., Retana, J., & Cros, S. 1997. Thermal colonies in a tropical forest. Oecologia 61:1–10. disruption of transitive hierarchies in Mediterranean ant communities. Journal of Traniello, J.F.A. 1989. Foraging strategies of ants. Animal Ecology 66(3):363–374. Annu. Rev. Entomol. 34:191–210. Flanagan, T.P., Letendre, K., Burnside, W.R., Fricke, Vittori, K., Talbot, G., Gautrais, J., Fourcassié, V., G.M. & Moses, M.E. 2012. Quantifying the effect Araujo, A.F.R., & Theraulaz, G. 2006. Path of colony size and food distribution on harvester efficiency of ant foraging trails in an artificial ant foraging. PLoS ONE 7(7):e39427. network. Journal of Theoretical Biology 239:507– https://doi.org/10.1371/journal.pone.0039427 515. Gordon, S. 1980. Analysis of twelve Sonoran Desert seed species preferred by the desert harvester ant. Madroño 27:68–78.

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