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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 of the harem will leave the harem, either in solitary, or with other young males in a group known as a bachelor herd.  Sexually mature female offspring may stay within the harem in which they were born, or may join another harem.  The females in a harem may be, but not exclusively, genetically related.    For instance, the females that comprise the harems of the Hamadryas baboon are not usually genetically related, as harems are formed via "kidnapping" of females from other harems and via herding.  Harems of the Gelada baboon, by comparison, are formed based on kinship ties to genetically related females.  Multiple harems may assemble into larger groupings known as clans or teams. 
Harem solidarity is mediated by the dominant male. Dominant males will fight off invading males in order to keep claim over the harem.    In some harem-forming species, when a dominant males vacates his harem, either due to death, defection to another harem, or usurpation, the incoming male will commit infanticide against the existing offspring.  This allows the female to return to sexual receptivity and fertility sooner than if the offspring were to survive, as time and resources are no longer being devoted to the offspring. Furthermore, while lactating, females do not ovulate and consequently are not fertile. Thus, infanticide 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:
- Red deer 
- Elephant seal 
- Bengal tiger 
- Greater short-nosed fruit bat 
- Jamaican fruit bat 
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