Narrow definition: how an animal responds or acts to a stimulus where the stimulus maybe another animal or some aspects of environment. Theodosius dobzhansky j nothing in bio makes sense except in the light of evolution j we can naming, describing and categorizing thing without evo, yet cannot understand things without it. Present of food j move in different distances according to the genotype, lack of food j move in same amount distance. Rovers move much greater distance than sitters when food is patchy, more likely to find new food. Environmental effects on trait values can be described as plasticity. The infulence of genes and environment can be visualized with plots of the reaction norm. Reaction norm j the response of any trait to different environments. Genetic j environment interaction j we can not distinct those two aspects, have to thought together. Genes and environment infulence the behaviour j cannot be distinct!