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Cognitive abilities evolve by natural selection to help an organism cope with problems encountered in the organism’s typical environment. In acacia ants, coevolution with the acacia tree led workers to forage exclusively on the host plant (Bin-nest<^> foraging), instead of the central-place foraging typical for most social insects. To test whether foraging ecology altered the orientation skills of acacia ants, we developed a novel field disorientation assay to evaluate the ability of foraging workers to quickly reorient after being disoriented (rotated) in an experimental arena. We compared 10 behaviors among disoriented and sham-treated workers of three in-nest foraging species (Pseudomyrmex nigrocinctus, P. flavicornis, and P. spinicola) and two central-place foraging species that regularly forage off the host tree (P. gracilis, P. nigropilosus). We predicted that experimental disorientation of workers should affect in-nest foraging species (acacia ants) more than central-place foraging species. Behavioral differences between control and disoriented ants were not consistently associated with foraging ecology, although the species least able to recover after disorientation was an acacia ant (P. nigrocinctus), and the species performing best after disorientation was a central-place forager (P. gracilis). Only one of the 10 behaviors studied consistently differed in experimentally disoriented workers compared to controls in all three species of acacia ants, whereas none of the experimentally disoriented central-place foragers differed from control workers for this specific behavior. Future studies could evaluate additional ant species living in obligate associations with plants, to further compare the cognitive abilities of in-nest versus central-place foraging organisms.

Significance statement

Foraging ecology influences the evolution of spatial orientation abilities. Acacia ants exclusively nest and forage on acacia trees; unlike most other ants that are central-place foragers, acacia ants therefore do not face the challenge of finding the way back home after collecting food. We compared the performance of three species of acacia ants to the performance of two species of central-place foragers on a disorientation assay in the field. We found that the ability to reorient was not consistently associated with foraging ecology, although the species least able to recover after disorientation was an acacia ant (P. nigrocinctus), and the species performing best after disorientation was a central-place forager (P. gracilis). Other behaviors related with the mutualism with the acacia tree-such as pruning nearby vegetation and falling off branches to attack potential herbivores-could also select for orientation abilities because workers reorient back to the host tree.