A harem is an animal group consisting of one or two males, a number of females, and their offspring. The dominant male drives off other males and maintains the unity of the group. If present, the second male is subservient to the dominant male. As juvenile males grow, they leave the group and roam as solitary individuals or join bachelor herds. Females in the group may be inter-related. The dominant male mates with the females as they become sexually active and drives off competitors, until he is displaced by another male. In some species, incoming males that achieve dominant status may commit infanticide.
For the male, the main benefits of the harem system is obtaining exclusive access to a group of mature females. The females benefit from being in a stable social group and the associated benefits of grooming, predator avoidance and cooperative defense of territory. The disadvantages for the male are the energetic costs of gaining or defending a harem which may leave him with reduced reproductive success. The females are disadvantaged if their offspring are killed during dominance battles or by incoming males.
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 
- 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. Print.
- Kummer, Hans. Social Organization of Hamadryas Baboons. A Field Study. Basel: Karger, (1968.) Print.
- David, J. H. M. "The Behaviour of the Bontebok, Damaliscis Dorcas Dorcas, (Pallas 1766), with Special Reference to Territorial Behaviour." Zeitschrift für Tierpsychologie 33 (1973): 38-107. Print.
- Qi, Xiao-Guang; Li, Bau-Guo; Garber, Paul A.; Ji, Weihong; and Wanatabe, Kunio. "[http://www.academia.edu/download/45243064/Social_dynamics_of_Golden_snub-nosed_mon20160501-5010-1pfo0mh.pdf Social Dynamics of the Golden Snub-Nosed Monkey (Rhinopithecus Roxellana): Female Transfer and One-Male Unit Succession[." American Journal of Primatology 71 (2009): 670-79. Print.
- Ortega, Jorge; Maldonado, Jesus E.; Wilkinson, Gerald S.; Arita, Hector T.; and Fleischer, Robert C. "Male Dominance, Paternity, and Relatedness in the Jamaican Fruit-eating Bat (Artibeus Jamaicensis)." Molecular Ecology 12.9 (2003): 2409-415. Print.
- Greenwood, Paul J. "Mating Systems, Philopatry and Dispersal in Birds and Mammals." Animal Behaviour 28.4 (1980): 1140-162. Print.
- Mori, Akio; Iwamoto, Toshitaka; Mori, Umeyo; and Bekele, Afework. "Sociological and Demographic Characteristics of a Recently Found Arsi Gelada Population in Ethiopia." Primates 40.2 (1999): 365-81. Print.
- Schreier, Amy L.; and Swedell, Larissa. "The Fourth Level of Social Structure in a Multi-level Society: Ecological and Social Functions of Clans in Hamadryas Baboons." American Journal of Primatology 71.11 (2009): 948-55. Print.
- Bonenfant, Christophe; Gaillard, Jean-Michel; Klein, François; and Maillard, Daniel. "Variation in Harem Size of Red Deer (Cervus Elaphus L.): The Effects of Adult Sex Ratio and Age-structure." Journal of Zoology 264.1 (2004): 77-85. Print.
- McCann, T. S. "Aggression and Sexual Activity of Male Southern Elephant Seals, Mirounga Leonina." Journal of Zoology 195 (1981): 295-310. Web.
- Ridley, M. W.; and Hill, D. A. "Social Organization in the Pheasant (Phasianus Colchicus): Harem Formation, Mate Selection and the Role of Mate Guarding." Journal of Zoology 211 (1987): 619-30. Print.
- Swedell, Larissa; and Tesfaye, Teklu . "Infant Mortality after Takeovers in Wild Ethiopian Hamadryas Baboons." American Journal of Primatology 60.3 (2003): 113-18. Print.
- Horev, Aviad; Yosef, Reuven; Tryjanowski, Piotr; and Ovidia; Ofer. "Consequences of Variation in Male Harem Size to Population Persistence: Modeling Poaching and Extinction Risk of Bengal Tigers (Panthera Tigris)." Biological Conservation 147.1 (2012): 22-31. Print.
- Searcy, William A.; and Yasukawa, Ken. "Alternative Models of Territorial Polygyny in Birds." The American Naturalist 134.3 (1989): 323-43. Print.
- Latty, Tanya M.; Magrath, Michael J. L.; and Symonds, Matthew R. E. "Harem Size and Oviposition Behaviour in a Polygynous Bark Beetle." Ecological Entomology 34.5 (2009): 562-68. Print.
- 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.
- Agoramoorthy, Govindasamy. "Adult Male Replacement and Social Change in Two Troops of Hanuman Langurs (Presbytis entellus) at Jodhpur, India." International Journal of Primatology 15.2 (1994): 225-38. Print.
- Codenotti, Thaïs L.; and Alvarez, Fernando. "Mating Behavior Of The Male Greater Rhea." The Wilson Bulletin 113.1 (2001): 85-89. Print.
- Kelly, Clint D.; and Jennions, Michael D. "Sexually Dimorphic Immune Response in the Harem Polygynous Wellington Tree Weta Hemideina Crassidens." Physiological Entomology 34.2 (2009): 174-79. Print.
- Colin, P. L. "Spawning and larval development of the hogfish, Lachnolaimus maximus (Pisces: Labridae)." Fish. Bull. 80 (1982): 853–862.
- Coleman, Ron. "Something Old Doing Something New". Cichlid News Magazine (1998): 30-31