Aposematic colouration enhances memory formation in domestic chicks trained in a weak passive avoidance learning paradigm.

The one-trial passive avoidance learning task is commonly used in avian research to explore anatomical, cellular and molecular parameters of learning and memory. Many factors are known to influence the effectiveness and/or duration of such learning events. Combinations of novel odours, such as pyrazine, and aposematic colours, such as brig ht yellow or red, have been shown to induce a long-lasting aversion to food crumbs in 'visual' predators, including birds such as the domestic chick (1). The aim of this study was to (a) examine whether visual complexity played a role in the generation of an aversive response to a novel visual stimulus and (b) to establish whether the duration of memory of an aversive experience could be modified by altering the visual properties of the stimulus. In the first experiment, naive domestic chicks were trained on a weakly aversive one-trial passive avoidance bead task, in which chicks were allowed to peck at a bead coated with a 10% solution of the bitter-tasting and odorous substance methylanthranilate (MeA). The chicks were trained with (allowed to peck) one of four differently coloured beads dipped in 10% MeA. Chrome, black, yellow or black-and-yellow striped beads were used. 'Recall' of the aversive bead was examined by presenting the (clean) training bead 24h after training and monitoring avoidance to it compared to a 'neutral' white bead. A high proportion (63%) of chicks trained with the black and yellow striped bead avoided it 24h after training, whereas little or no avoidance was seen in response to chrome, yellow or black beads. In a second experiment naive domestic chicks were all trained once only with a black and yellow striped bead coated in a 10% MeA solution, but this time, were tested 24h later, once only, with either a black, a yellow or a black and yellow striped bead. Nearly 60% of chicks tested with a black and yellow striped bead showed avoidance of the bead, whereas only 23% of those tested with a black bead and 14% tested with a yellow bead showed avoidance. These results confirm the importance of complex warning colouration, when paired with a novel olfactory cue and a bitter taste, in avoidance learning. We conclude that the chicks' response to monochromatic colours (e.g. yellow or black) is not affected by their previous experience with a conspicuously patterned stimulus (yellow and black stripes). Moreover, it suggests a predisposition for chicks to attend to aversive cues associated with 'naturalistic' high contrast colour cue combinations such as black and yellow.