Food Selection and Sequential Load Transport Behavior in Anoplolepis gracilipes (Smith) (Hymenoptera: Formicidae)

Foraging behavior is crucial for an ant species to adapt to its environment. Ants may adopt different foraging strategies to cope with environmental change, e.g., available food size and food-patch distance. Therefore the objective of this study was to explore the effect of food size and foraging distance on the retrieval behavior of yellow crazy ant, Anoplolepis gracilipes in the field and in laboratory. Besides, we also investigate an interesting retrieval strategy, sequential load transfer behavior, and explore its adaptive value with modeling.

To explore the retrieval behavior and food selection of A. gracilipes, three species of fruit flies that differed in size (Drosophila virilis > D. albomicans > D. melanogaster in weight) were given to workers for choosing in two experimental designs. Both experiments showed that the workers prefer the fly of medium-sized. Choosing medium-sized food could be a result of trade-off between the cost and benefit of carrying different sizes of food. We also provided crickets (Gryllus bimaculatus) to ants as a large-sized food supply and fruit fly as small-sized food supply. We compare the foraging behaviors observed from the two kinds of food patches. The results showed that when workers encountered a large-sized food, i.e., cricket, they quickly returned back to nest and laid trail to recruit nestmates. Under this condition, foraging workers could be divided into two types, the scouts and the recruits. On the contrary, when the workers encountered small-sized food, all the workers foraged solitarily and their foraging routes were completely different. Therefore, foraging strategy of the yellow crazy ant is very plastic. They can adjust their foraging behavior to different types and sizes of food quickly.
In the food-patch distance experiment in the field, the short-distance patch was discovered and utilized much more quickly than the far-distance patch. When ants were given the choice between two food patches with equal quality but with different foraging distance. The results are consistent with the prediction of the optimal foraging theory, i.e., the near patch was utilized much more quickly than the far patch in various distance-ratio treatments and this preference is enhanced by the large distance ratio. Ant flows and retrieval speed in the near patch were also significantly higher than those in the far patch.

Many social insects show sequential cooperation in the foraging process, in which workers form a transport chain and transfer a load from one to another, this is known as task partitioning. In this study, sequential load transport in the ant, A. gracilipes, was analyzed, and the assumptions and predictions of the bucket brigades (BBs) hypothesis were tested. Our results suggested that in the vicinity of the nest entrance, 38% of foods were retrieved with tug behavior. Worker size and the retrieval speed increased with each transfer, and the foraging efficiency improved along the transport chain to the nest. These results support the BBs hypothesis of sequential cooperation. However, the retrieval time for the transport chain was significantly longer than that of individual foraging. Moreover, distances of both the transport chain and individual foraging were significantly longer than that of the shortest linear route, and the distance covered by larger workers was shorter. Thus the loaded worker probably showed avoidance behavior in order to avert being robbed. Food transfer might not be a cooperative strategy in A. gracilipes. Nepotism hypothesis that states load robbing for close kin is first proposed to account for the maintenance of a non-cooperative sequential load transport in a colony.
Finally, based on the data of seed-harvesting ant, we run the simulation of load transfer and test the effects of worker size, load size and retrieval speed on load transfer. Results showed that speed decreases abruptly with increasing loading ratio of a worker. The optimal load for a worker increases with increasing body size and there is an optimal load for an ant of specific size. Compared the speed ratio for a worker in a transfer chain to that in a single foraging, results showed that longer distance and higher loading mass carried by the worker favor the evolution of transfer chain.