Outside the rut, they run in bachelor herds and females-with-young groups, apart from the old stags, which tend to be solitary or team up with one other male. Round about now, the stags start establishing territories, then as the females come into oestrus, the stags fight constantly between themselves to maintain those territories. They lose a lot of weight built up over the summer as they barely eat or sleep. By the end of the rut, they are very lean and exhausted. This applies to fallow deer too, but not roe, which don't herd but stay in family groups.
Apologies for offtopic but thought some of us might find it interesting.