SHORT COMMUNICATION Queen regulates biogenic amine level and nestmate recognition in workers of the fire ant, Solenopsis invicta Robert K. Vander Meer & Catherine A. Preston & Abraham Hefetz Received: 27 March 2008 / Revised: 15 July 2008 / Accepted: 19 July 2008 / Published online: 13 August 2008 # Springer-Verlag 2008 Abstract Nestmate recognition is a critical element in social insect organization, providing a means to maintain territoriality and close the colony to parasites and predators. Ants detect the colony chemical label via their antennae and respond to the label mismatch of an intruder with aggressive behavior. In the fire ant, Solenopsis invicta, worker ability to recognize conspecific nonnestmates decreases if the colony queen is removed, such that they do not recognize conspecific nonnestmates as different. Here, we tested the hypothesis that the presence of the colony queen influences the concentration of octopamine, a neuromodulator, in worker ants, which in turn has an effect on nestmate recognition acuity in workers. We demonstrate that queenless workers exhibit reduced brain octopamine levels and reduced discriminatory acuteness; however, feeding queenless workers octopamine restored both. Dopamine levels are influenced by honeybee queen pheromones; however, levels of this biogenic amine were unchanged in our experiments. This is the first demonstra- tion of a link between the presence of the colony queen, a worker biogenic amine, and conspecific nestmate recogni- tion, a powerful expression of colony cohesion and territoriality. Keywords Formicidae . Octopamine . Dopamine . Modulation . Aggression Introduction The ability of colony members to discriminate between nestmates and nonnestmates is a critical element in social insect organization, providing a means to exclude harmful individuals such as predators, parasites, and conspecific competitors (Wilson 1971). Ants detect the colony chemical label via their antennae (Ozaki et al. 2005) and, through grooming and trophallaxis, they contribute to and experi- ence odors associated with their nest (Boulay et al. 2004). Upon contact with an intruder, the resident compares the cuticular chemistry of the intruder with the experience- based neural template of its colony odor (Vander Meer and Morel 1998) and in case of mismatch aggression occurs. Fire ants show an almost stereotypic response of high aggressiveness towards allospecific ants, presumably be- cause such intruders are easily recognized by qualitatively different cues. The intraspecific response is not stereotypic. Between individuals, aggressiveness varies even in re- sponse to identical stimuli. This is generally supported by the graded recognition–aggressive behaviors used in the quantification of nestmate recognition bioassays (Vander Meer and Morel 1998). The plasticity of individual recognition acuity is clearly illustrated by the powerful influence of the Solenopsis invicta queen on the conspecific recognition capabilities of workers. Removing the queen or if a new queen is adopted degenerates or restores, respectively, worker intraspecific recognition. We attributed this powerful queen effect to a putative recognition primer pheromone that increases the sensitivity of workers to subtle quantitative differences in Naturwissenschaften (2008) 95:1155–1158 DOI 10.1007/s00114-008-0432-6 R. K. Vander Meer (*) : C. A. Preston CMAVE, ARS, USDA, 1600 SW 23Rd Drive, Gainesville, FL 32608, USA e-mail: bob.vandermeer@ars.usda.gov A. Hefetz Department of Zoology, George S. Wise Faculty of Life Sciences, Tel Aviv University, Ramat Aviv, Tel Aviv 69978, Israel