Bachelor herd

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.

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.

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. In the Serengeti, immature or older males will usually form their own bachelor herds, while males of reproductive age are more often in mixed groups with females.

Being actively territorial in the Serengeti is physically demanding for male impala, so males occupy this role for about 3 months. Males will then join a bachelor herd, though this results in them occupying a social dominance status at the bottom of the linear rank hierarchy until their physical condition returns to pre-territorial levels. Bachelor herds may coexist with territorial males in the same area, but these individual males are always dominant above bachelor males.

Within the herds, bachelor males are less territorial toward each other than males in mixed herds. These males maintain, on average, a relatively large distance of approximately 2.5–3 metres (8.2–9.8 ft) between them. However, bachelor males exhibit reciprocal grooming despite occasional aggressive interactions between bachelors.

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. These bachelor herds are large in size, ranging from 15,000 to more than 20,000 seals living in one area, referred to as a rookery.