The temporal development and sensory mediation of social discrimination in domestic chicks

We conducted a series of experiments to investigate the temporal development and sensory basis of social discrimination by young chicks, Gallus gallus domesticus. In the initial experiment, 1-day-old chicks were housed in pairs for 6-9 or 11-14 h. After this treatment period, interactions between paired chicks that had been housed together for 6-9 h did not differ from those of unfamiliar chicks. However, pairs of chicks that had been together for 11-14 h pecked each other significantly less than did unfamiliar pairs. Reduced pecking by the familiar partners indicates that 11-14 h of direct contact is sufficient for the establishment of individual discrimination. In a second experiment we assessed the role of olfaction in the development of social discrimination. Chicks in the anosmic condition were housed in pairs after their external nares were blocked with acrylic cement. Control chicks had cement applied to the beak (the nares remained uncovered) before being housed in pairs. When tested after the pair-housing period, pecking frequencies were lower among familiar cagemates than among unfamiliar pairs of chicks in both the anosmic and control conditions. Anosmic chicks, unlike the controls, showed little reaction to mint odour, thereby verifying that the blocked-nares treatment disrupted olfactory perception. In a final experiment, there was no reliable difference between the pecking rates of paired chicks that had been housed together in the dark and those of unfamiliar dark-reared pairs. We conclude that access to social partners' visual phenotype, but not olfactory familiarization, is necessary for the rapid development of individual discrimination by chicks.