Male stickleback fish develop red colour on their ventral side in the breeding season and maintain territories. When a conspecific male intruder enters their territory, resident males perform an aggressive display. The ethologist Niko Tinbergen presented models of different shapes to territorial male stickleback fish. He found that models of any shape elicited aggressive displays, provided the ventral part of the models was coloured red. This observation led to the development of which one of the following concepts?