It is well understood that the urge to reproduce is a crucial driving force for a wide array of animal behaviors. At the end of the day, passing on genes to the next generation is the goal. There are many different strategies that animals use in selecting mates, and many social structures that emerge from these strategies.
Polygyny is when one male mates with many females. It is a very common mating strategy in mammals and because of relative gametic and reproductive cost. Sperm cells are small and can be produced throughout the life of a male. Egg cells are much larger than sperm, have finite quantity, and the female likely will put a lot of energy into either laying/tending to eggs, carrying a fetus, and also raising young. It seems it would best serve a male to mate with many females because he has little to lose. Females very rarely use this strategy because of the fitness cost. Sometimes, polygyny is competitive. Males maintain and defend a territory and the resources contained within it. Females enter the territory and mate, have access to the resources, and male-provided protection. An extreme form of this is a harem system, in which a dominant male controls a territory full of females, and he will be the only one to mate with all of them. The male fiercely guards his harem.
Polyandry, when one female mates with many males, is very rare. The benefit seems to be in that the female has enough help raising her young so that it outweighs the egg-cost. She is also likely to find a genetically optimal mate by chance alone if she has many partners, increasing her fitness through creation the strongest progeny. A female can do this in one of two ways. She can competitively establish control over a territory, mate with all the males within those lines, and then they collectively rear her young there (as the males don’t know which young are theirs). The other method is to mate with a male and then leave him to care for the young while she finds another mate. This again, is exceptionally rare, and occurs in a select few species.
There are mating strategies that involve multiple matings of both males and females. Polygynandrous (also called “polygamous”) systems are the more social of the two strategies. Males and females both have a few partners. Males benefit from having more parents of their young, and females can benefit from help rearing young and lesser chances of infanticide due to the males not knowing whose progeny are whose. Individuals are still selective over whom they chose to mate with, and often times the species is rather social. Bonobos are a prime example of a polygynandrous species. Promiscuous mating is far more random. Often times, environmental pressures outweigh the benefit of carefully choosing a mate in species who use this strategy.
Monogamy is when one male and one female create a stable pair bond. Both the male and the female work hard to raise the young that they know is both theirs. This is the case in for most birds. Social monogamy is when the pair mates for life; coming together season after season. If the pair is monogamous for a single mating season and then they go their separate ways and find new mates the next time around, they’re referred to as being serially monogamous. Monogamy (both types) is a good strategy if the offspring require care from both parents to survive, if good mates are hard to find, and if strong pair bonding between parents is highly beneficial to raising the young and/or other social constructs. True monogamy is exceptionally rare. Males may mate with other females to increase the number of offspring they can have, and females may mate with a newfound male who is more fit . These encounters are called extra pair copulations.
Animal Attraction: The Many Forms of Monogamy in the Animal Kingdom. Lily Whiteman. NSF.gov. https://www.nsf.gov/discoveries/disc_summ.jsp?cntn_id=126932
Krasnec, M. O., Cook, C. N. & Breed, M. D. (2012) Mating Systems in Sexual Animals. Nature Education Knowledge 3(10):72 http://www.nature.com/scitable/knowledge/library/mating-systems-in-sexual-animals-83033427
“animal social behaviour”. Encyclopædia Britannica. Encyclopædia Britannica Online.
Encyclopædia Britannica Inc., 2016. Web. 01 Oct. 2016