This is not just a herd of cows! Their DNA is distinct from that of other cattle and they are thought to be descended from the original cattle which ran wild over much of Northern England and Southern Scotland.
First the history which forms a necessary condition for the creation of this herd: in the 14th century the King allowed the area of the park to be enclosed and thus a number of cattle were corralled. It is assumed that the main purpose was food but obviously it safeguarded the hunting in the area as well. From that time on, except for some supplementary feed, the cattle have been left strictly alone; for instance no veterinary attention is given. They are completely insulated from any other cattle and thus have survived only by interbreeding. I gather that King Edward the Seventh was the last person to hunt and kill one.
History has combined with the genetic behaviour of the herd. As is the case with a number of other wild animals, all calves are fathered by the 'King Bull' at the time, thus ensuring that, on one side, only top quality genes are handed on. Apparentlyfrom the skull size of some remains that have been found, the size of the cattle has diminished somewhat over time but otherwise the interbreeding does not seem to have caused any adverse consequences.
Challenges to the King are usually fairly formalized bouts of fighting and, provided that a defeated bull withdraws decently from the herd for a while, there is rarely a fatality.
There is apparently a ceremonial entry to the herd for new calves once they are of age. Being wild they are, of course, potentially dangerous and this particularly applies to cows with calves and to defeated bulls. Nobody is allowed into the area without the warden, who takes you as close as safety permits. His talk is highly informative - memory of it provides the basis of this page!
Chillingham Castle nearby has been owned by the same family, the Greys, who fought on the Lancastrian side in the Wars of the Roses [and one of whom was the first husband of Elizabeth Woodville who later married Edward the Fourth.] In the church are alabaster figures of a crusader, Ralph Grey, and his wife.