In cultures that had both harems and eunuchs, eunuchs were sometimes used as harem servants. Many Arabs took women slaves as concubines in their harems. Concubines were at times housed in harems which maintained their modesty and privilege. Some harems were guarded by eunuchs. Monk seals are polygynous, and group together in harems. They are generally social animals that live in small harems to large herds. Most baboons live in hierarchical troops containing harems. Stallions form and expand their harems by abducting young mares from their natal harems. Active mainly at dawn and dusk, springbok form harems (mixed-sex herds). Dominant bulls will prevent cows from leaving their harems. Northern fur seal males aggressively select and defend the specific females in their harems. In addition, girls were taken in order to serve as odalisques in harems. The largest males often control harems containing three to 15 females. Grouper are protogynous hermaphrodites, who school in "harems" of three to fifteen females. Two or more harems unite repeatedly to form clans.