A bachelor herd is a herd of (usually) juvenile male animals who are still sexually immature or 'harem'-forming animals who have been thrown out of their parent groups but not yet formed a new family group. It may also refer to a group of males who are not currently territorial or mating with females.
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. Some animals, for example New Zealand fur seals, live in a bachelor herd all year except for the mating season, when there is a substantial increase in aggression and competition.
In many species, males and females move in separate groups, often coming together at mating time. 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.
Types
The social structure, aggression level, population size, and duration of presence of these herds across species varies greatly. Bachelor herds are most often found in mammals and are especially common in the grasslands.
Impala
Male impala form small bachelor herds during both the wet and dry seasons. These bachelor herds are generally smaller than herds of females, numbering around 4 members, compared to upwards of 10. Juvenile males begin to join bachelor herds at 8 months of age. Bachelor herds may coexist with territorial males in the same area, but these individual males are always dominant above bachelor males. However, bachelor males exhibit reciprocal grooming despite occasional aggressive interactions between bachelors.
Fur seals
thumb|Fur seal rookery during the non-mating season at [[Saint Paul Island (Alaska)|St. Paul Island, Alaska]]
Male fur seals, as a family, commonly live in bachelor herds during the non-breeding season. During the breeding season (April–September in the Northern Hemisphere, September–January in the Southern Hemisphere), the size of herds greatly diminishes. In fact, it has been observed that stallions often try to prevent foals from leaving the herd.
Within Cape mountain zebra bachelor herds, there is usually no social hierarchy. Dominance is given to the more senior members of the herd and when the oldest males leave to form a breeding herd, the next oldest bachelors take on the leadership role. There is minimal intragroup aggression and no observed fighting between members for a higher social position. They then join bachelor herds, in which they spend most of the year. This hierarchy is determined by both body size and the size of the stag's antlers, with older stags having on average larger antlers.