The monoamine oxidase A gene (MAOA gene) produces the enzyme monoamine oxidase A, which has an important function in the endocrinological metabolism in the brain of mammals. Mutations on this gene cause aggressive behaviour in humans (the Brunner syndrome) and in laboratory mice. These mutations lead to a deficiency of monoamine oxidase A production, causing an excess of serotonin, dopamine and noradrenalin in the brain, which has an impact on behaviour.
A recent study examined the MAOA gene in in cattle. The study tested the MAOA gene in Spanish fighting cattle (Lidia), which is selected for aggression (“fighting spirit”), and breeds that are not selected for aggression and do not display excessively aggressive behaviour (Asturiana de los Valles, Morenas Gallega, Retinta, Rubia Gallega, Avilena, Limousine and Charolaise). They found considerable variation in sequences of the promotor region of the gene between Lidia and the non-aggressive breeds, indicating a possible influence of the gene on the behaviour of the cattle .
The million-dollar question now is, what were the sequences of the MAOA gene of the aurochs? Was it more like that of Lidia, or even identical as in Lidia, or more like that of the non-aggressive breed or even identical? As the full genome of the aurochs is resolved, it would be very interesting if someone would examine the MAOA gene of the aurochs. It could tell us if this was one of the many key genes that mutated during the domestication of cattle, and what the behaviour of the aurochs might have been like in terms of aggression, although the MAOA gene is probably not the only gene that is involved in aggressive behaviour.
 Eusebi et al.: Aggressive behaviour in cattle is associated with a polymorphism in the MAOA gene promoter. 2019.