Life in larger social units such as horse herds requires hierarchic structures, with communication within the group and expressive behaviour that is clearly understood by all. This is the only way of avoiding constant conflicts over rank, which waste energy and often end in injury. This expressive social behaviour or body language is inborn (Hassenberg 1971; Schäfer 1993). Young horses are socialised within the herd while growing up, an important requirement for dealing with their own kind later on in life, or for dealing with the humans who take their place.