The single male, called the dominant male, may be accompanied by another young male, called a "follower" male. Females that more closely associate with the dominant male are called "central females," while females who associate less frequently with the dominant male are called "peripheral females." Juvenile male offspring leave the harem and live either solitarily, or, with other young males in groups known as bachelor herds. Sexually mature female offspring may stay within their natal harem, or may join another harem. The females in a harem may be, but are not exclusively, genetically related. For instance, the females in Hamadryas baboon harems are not usually genetically related because their harems are formed by "kidnapping" females from other harems and subsequent herding. In contrast, Gelada baboon harems are based on kinship ties to genetically related females. Multiple harems may assemble into larger groups known as "clans" or "teams".
Harem cohesiveness is mediated by the dominant male who fights off invading males to keep claim over the harem. In some harem-forming species, when a dominant male vacates his harem (due to death, defection to another harem, or usurpation) the incoming male sometimes commits infanticide of the offspring. Because time and resources are no longer being devoted to the offspring, infanticide often stimulates the female to return to sexual receptivity and fertility sooner than if the offspring were to survive. Furthermore, while lactating, females do not ovulate and consequently are not fertile. Infanticide therefore has the potential to increase the incoming male's reproductive success.
Harems are beneficial social structure for the dominant male, as it allows him access to several reproductively available females at a time.  Harems provide protection for the females within a particular harem, as dominant males will fiercely ward off potential invaders.  This level of protection may also, such in the case of the common pheasant, reduce the energy in female expended on remaining alert to, or fleeing from, invading males.  Harems allow bonding and socialization among the female members, which can result in greater control over access to females as determined by the females' preferences. Harems also facilitate socialized behavior such as grooming and cooperative defense of territory.  
Harems can prove energetically costly for both males and females. Males spend substantial amounts of energy engaging in battles to invade a harem, or to keep hold of a harem once dominance has been established. Such energy expenditure can result in reduced reproductive success such in the case of red deer.  This is especially true when there is high turnover rates of dominant males, as frequent intense fighting can result in great expenditure of energy.  High turnover rate of dominant males can also be energetically costly for the females as their offspring are frequently killed in harems where infanticide occurs. Harems can also negatively affect females if there is intra-harem competition among females for resources. 
List of animals that form harems
Species that form harems include, but are not limited to:
- Hamadryas baboon 
- Gelada baboon 
- Golden snub-nosed monkey 
- Guinea baboon 
- Gray langurs 
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