A bachelor herd is a herd of (usually) juvenile male animals who are still sexually immature, or of 'harem'-forming animals who have been thrown out of their parent groups but not yet formed a new family group.
Examples include seals, dolphins, lions, and many herbivores such as deer, horses, and elephants. Bachelor herds are thought to provide useful protection for social animals against more established herd competition or aggressive dominant males. Males in bachelor herds are sometimes closely related to each other. Red deer live in a bachelor herd all year except for the mating season (Fall) when there is a substantial increase in aggression.
In many species, males and females move in separate groups, often coming together at mating time, or to fight for territory or mating partners. In many species it is common for males to leave or be driven from the group as they mature, and they may wander as lone animals or form a bachelor group for the time being. This arrangement may be long term and stable, or short term until they find a new group to join.