A harem (in zoological terms) is (typically) a group of females that mate with and are protected by a singular dominant male and perhaps a sub-male.
Dynamics of the Harem
Defending a monkey harem - Clever Monkeys - BBC Earth
[]
Harems can be seen in a wide variety of organisms including animals such as elephant seals, baboons (Maestripieri et al., 2007), hogfish, deer, and even fruit bats (Storz et al., 2000). Males, frequently relying on both sexual ornamentation and physical force, defend the harem from other males that may try to breed with any of the females within the group.
However, this means that the offspring of the harem are all genetically related and any new males, once they reach sexual maturity may be chased from the group. Sometimes these outcast males form bachelor groups which may challenge dominant males for control of harems (Emlen & Oring, 1977). These confrontations may prove physically draining on dominant males which could lead to subsequent inferior fertility pay off.
Additionally, if a dominant male is overthrown by rival males then the rival male may kill off any of the previously dominant males offspring (that have not reached sexual maturity). New dominant males may even harass pregnant females into miscarrying the offspring of the previous male (Emlen &Oring, 1977).
Bibliography
Emlen, S. T., & Oring, L. W. (1977). Ecology, sexual selection, and the evolution of mating systems. Science, 197(4300), 215–223.
Maestripieri, Dario; Mayhew, Jessica; Carlson, Cindy L.; Hoffman, Christy L.; and Radtke, Jennifer M. "One-Male Harems and Female Social Dynamics in Guinea Baboons." Folia Primatologica 78.1 (2007): 56-68.
Storz, Jay F.; Bhat, Hari R.; and Kunz, Thomas H. "Social Structure of a Polygynous Tent-making Bat, Cynopterus Sphinx (Megachiroptera)." Journal of Zoology 251.2 (2000): 151-65. Print.
