The sequence of territorial courtship and mating behaviours was described in detail by Niko Tinbergen in a landmark early study in ethology. Tinbergen showed that the red colour on the throat of the territorial male acts as a simple sign stimulus, releasing aggression in other males and attracting females. The red colouration may also be used by females as a way to assess male quality. Red colouration is produced from carotenoids found in the diet of the fish. As carotenoids cannot be synthesised "de novo", the degree of colouration gives an indication of male quality (ability to find food), with higher-quality males showing more intense colouration. Also, males that bear fewer parasites tend to exhibit brighter red colours. Many studies have shown that females prefer males with brighter red colouration. However, the response to red is not universal across the entire species, with black throated populations often found in peat-stained waters.