Quantitative cognition in carpenter ants

Abstract

Processing information about quantities allows animals to make optimal decisions during many natural contexts, such as foraging, territorial defense, offspring care, mate choice, and intra-sexual competition. Compared to the wealth of information available in vertebrates, much less is known in invertebrates, even though the processing of quantities is equally relevant for both taxa. Here, we used two separate ecologically relevant tasks (brood pile preference and landmark-guided foraging) to investigate two dimensions of quantitative cognition in carpenter ants: spontaneous quantitative judgments and trained use of sequential landmarks. Individual ants spontaneously discriminated between two piles of dummy cocoons both when the choice involved smaller (1 vs. 2, 3, 4) and larger numerical contrasts (2 vs. 4, 6, 8). Ants used both chemical and visual/tactile cues and their performance was dependent on the numerical ratio. In the second task, ants preferentially searched near the trained landmark (out of five identical ones) despite alterations in its position, suggesting that they used ordinal information about its location when searching for food. In this experiment, ants showed a limit at four since their performance drastically decreased when they were trained to the 5th landmark. We showed that carpenter ants use both relative quantity and relative position to make efficient decisions. Our study contributes to the scant body of knowledge available on quantitative cognition in invertebrate species. There is ample experimental evidence that non-human animals are able to process quantitative information, in particular mammals, some birds, and fish, and that this ability is useful in a variety of ecologically relevant situations. Yet, information about quantitative cognition in invertebrates is very scarce. We show that individual ants are able to use both relative quantity and relative position of items. Nurse ants spontaneously discriminated between two brood piles of different sizes and foragers were able to learn to identify a landmark associated with food on the basis of its position in a series of identical ones. These results suggest that ants might use quantitative information to make optimal choices in their natural environment although the precise mechanisms underlying this ability should be further investigated.