Introduction Monogamy is rare in mammals. About 3-5% percent of mammals are considered monogamous, while 90% of avian species are monogamous (Choleris, Pfaff and Kavaliers, 2013). One may wonder why there are different mating systems in different species. This paper will discuss the role of hormones in relation to monogamy and polygamy. Research by Elmen and Oring (1977) has found that one of the reasons for differing mating systems is the ability of a segment of the population to monopolize the access of others to potential mates. Polygamous species are more likely to have one sex freed from parental duties. This would allow the parent exempted from parental responsibility to spend more time and energy on intrasexual competition for resources and mates to maximize its reproductive fitness. It is common for the offspring of polygamous species to require minimal parental care. One example is birds with precocial young. Single parents also require abundant food resources to be able to raise their young properly. The receptivity of females plays a part in determining whether a species is monogamous. The more time it takes to court and mate with the female, the more likely the species is monogamous. The chances of offspring survival can be maximized when both mates remain together. High degree of within-pair synchronization enables both parents to provide higher quality care to their young. Longer mate fidelity increases the adaptivity of mates to share the burden of parental